A view of the Milky Way galaxy in microwaves is captured by the European Space Agency's Planck satellite. Credit: ESA/NASA/JPL-Caltech.{{fairuse}}

Astronomy specifically focused at the microwave portion of the electromagnetic spectrum is microwave astronomy.

## Microwaves

Optics involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[1] Optics usually describes the behavior of visible, ultraviolet, and infrared light. Because light is an electromagnetic wave, other forms of electromagnetic radiation such as X-rays, microwaves, and radio waves exhibit similar properties.[1]

Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.[2] This broad definition includes both [ultra high frequency] UHF and [extremely high frequency] EHF (millimeter waves), and various sources use different boundaries.[3] In all cases, microwave includes the entire [super high frequency] SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with [radio frequency] RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).

Microwave frequency bands
Letter Designation Frequency range Wavelength range Typical uses
L band 1 to 2 GHz 15 cm to 30 cm military telemetry, GPS, mobile phones (GSM), amateur radio
S band 2 to 4 GHz 7.5 cm to 15 cm weather radar, surface ship radar, and some communications satellites (microwave ovens, microwave devices/communications, radio astronomy, mobile phones, wireless LAN, Bluetooth, ZigBee, GPS, amateur radio)
C band 4 to 8 GHz 3.75 cm to 7.5 cm long-distance radio telecommunications
X band 8 to 12 GHz 25 mm to 37.5 mm satellite communications, radar, terrestrial broadband, space communications, amateur radio
Ku band or P band 12 to 18 GHz 16.7 mm to 25 mm satellite communications
K band 18 to 26.5 GHz 11.3 mm to 16.7 mm radar, satellite communications, astronomical observations, automotive radar
Ka band 26.5 to 40 GHz 5.0 mm to 11.3 mm satellite communications
Q band 33 to 50 GHz 6.0 mm to 9.0 mm satellite communications, terrestrial microwave communications, radio astronomy, automotive radar
U band 40 to 60 GHz 5.0 mm to 7.5 mm
V band 50 to 75 GHz 4.0 mm to 6.0 mm millimeter wave radar research and other kinds of scientific research
W band 75 to 110 GHz 2.7 mm to 4.0 mm satellite communications, millimeter-wave radar research, military radar targeting and tracking applications, and some non-military applications, automotive radar
F band 90 to 140 GHz 2.1 mm to 3.3 mm SHF transmissions: Radio astronomy, microwave devices/communications, wireless LAN, most modern radars, communications satellites, satellite television broadcasting, DBS, amateur radio
D band 110 to 170 GHz 1.8 mm to 2.7 mm EHF transmissions: Radio astronomy, high-frequency microwave radio relay, microwave remote sensing, amateur radio, directed-energy weapon, millimeter wave scanner

## Planetary sciences

"Other important applications of gyrotrons are high-power microwave sources include high resolution radar ranging and imaging in atmospheric and planetary science as well as deep-space and specialized satellite communications and RF drivers for next-generation high-gradient linear accelerators".[4]

## Colors

WMAP 3-year Power spectrum of CMB is compared to recent measurements of BOOMERanG, CBI, VSA and ACBAR. Credit: NASA/WMAP Science Team.

The figure at the right "shows the three-year WMAP spectrum compared to a set of recent balloon and ground-based measurements that were selected to most complement the WMAP data in terms of frequency coverage and l range. The non-WMAP data points are plotted with errors that include both measurement uncertainty and cosmic variance, while the WMAP data in this l range are largely noise dominated, so the effective error is comparable. When the WMAP data are combined with these higher resolution CMB measurements, the existence of a third acoustic peak is well established, as is the onset of Silk damping beyond the 3rd peak."[5]

## Theoretical microwave astronomy

"Within the WMAP frequency range, it is difficult to distinguish between a primordial CMB spectrum and a thermal SZ [Sunyaev-Zeldovich (SZ) fluctuations] spectrum, so we adopt the Komatsu & Seljak (2002) model for the SZ power spectrum and marginalize over the amplitude as a nuisance parameter."[5]

## Entities

Fluctuations in the cosmic microwave background are shown in this COBE all-sky image. Credit: NASA.

Compare the COBE all-sky CMB at right with the map from WMAP in the background section.

"The cosmic microwave background fluctuations are extremely faint, only one part in 100,000 compared to the 2.73 degree Kelvin average temperature of the radiation field."[6]

## Sources

This is an artist's impression of Aquarius above the Earth's atmosphere. Credit: NASA.
The diagram shows the many natural sources, besides salinity, that contribute to the microwave radiation at L-band frequencies (approximately 1 gigahertz). Credit: NASA.

"Many natural sources, besides salinity, contribute to the microwave radiation at L-band frequencies (approximately 1 gigahertz) measured by satellites. Correcting the influence of these natural sources is a key to obtaining Aquarius’ accurate salinity measurements."[7]

"Aquarius is dedicated to making precise measurements of ocean salinity over months and years, providing important new information for climate studies. It will produce monthly maps of the surface salinity of the global ocean with a 93-mile (150-kilometer) resolution and an accuracy of 0.2 practical salinity units, which is equal to about one-eighth teaspoon of salt in a gallon of water. (Practical salinity is a scale used to describe the concentration of dissolved salts in seawater, nearly equivalent to parts per thousand.) The mission is to make these measurements continuously for at least three years."[7]

"The radiometers on Aquarius measure the microwave emissions from the sea surface at 1.4 gigahertz in the L-band portion of the electromagnetic spectrum. This energy, which is measured as an equivalent temperature called the "brightness temperature" in Kelvin, has a direct correlation to surface salinity."[7]

## Electromagnetics

The image shows charged particles moving along the galaxy's magnetic field. Credit: ESA/NASA/JPL-Caltech.{{fairuse}}

The image above center shows the magnetic field of the Milky Way galaxy via charged particles moving along it.

## Weak forces

The Gravity Recovery and Climate Experiment (GRACE) mission uses a microwave ranging system to accurately measure changes in the speed and distance between two identical spacecraft flying in a polar orbit about 220 kilometers (140 mi) apart, 500 kilometers (310 mi) above Earth. The ranging system is sensitive enough to detect separation changes as small as 10 micrometres (approximately one-tenth the width of a human hair) over a distance of 220 kilometers.[8]

As the twin GRACE satellites circle the globe 15 times a day, they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, a gravity anomaly, it is pulled slightly ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft then passes the anomaly, and slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.

## Emissions

The "discovery of the anomalous dust-correlated microwave emission (AME) in the galaxy [was] by Leitch et al (1997) [18] [Characteristics include]

1. the AME constitutes a foreground emission to cosmic microwave background (CMB) radiation. [...]
2. it provides a window into the properties of small grains, which play crucial roles for the physics and chemistry of the ISM.
3. [It is a] diffuse and localized AME"[9]

"In the case of electric dipole radiation, the associated fluctuation in angular momentum is due to absorption of and decays stimulated by microwave photons (dominated by Cosmic Microwave Background (CMB) photons in the diffuse ISM)."[9]

"The [warm ionized medium] WIM is characterized by a large gas temperature T ≈ 8000 K, and a fully ionized gas at low density, nH+ ≈ 0.1 cm-3. Collisions with ions provide the dominant excitation mechanism. Grains are mostly negatively charged due to the high rate of sticking collisions with high-velocity electrons. For a coronene molecule, the characteristic time between ion collisions and the characterstic rotational damping time at the peak angular momentum τrot = √ττed turn out to be comparable6, of order a few years."[9]

The "peak emissivity is enhanced by about 23% for the WIM [and only 11 % for the warm neutral medium (WNM)], although the peak frequency remains unchanged."[9]

"A more important effect on the spectrum is that of increasing the characteristic internal temperature Tω, which makes the grains wobble rather than simply spin about their axis of greatest inertia."[9]

For triaxiality there is an "additional enhancement of the peak frequency and total power by up to the same factors (~ 30 % and 2, respectively) for a large internal relaxation temperature and highly elliptical grains."[9]

## Absorptions

"The 111 → 110 rotational transition of formaldehyde (H2CO) [occurs] in absorption in the direction of four dark nebulae. The radiation ... being absorbed appears to be the isotropic microwave background".[10] One of the dark nebulae sampled, per SIMBAD is TGU H1211 P5.

## Scatterings

"Radio waves propagated through the atmosphere are affected by a wide variety of scattering and fading effects due to local anomalies and movements both of neutral and of charged particles."[11]

"Electrons and ions react much more vigorously with radio waves than do neutral species; and electrons, because of their much smaller mass, interact much more vigorously than other ions. Atmospheric radio propagation is, therefore, almost entirely the study of electron-wave-field interactions."[11]

There "is a broad maximum of the rate of absorption of the radio wave; this occurs in the day-time in the standard broadcast band in temperate latitudes and is due to ionization and collision in the D region of the ionosphere."[11]

"At higher frequencies, attenuation decreases with the square of the frequency; the radio wave may be able to pass through the D layer with relatively little absorption and be reflected, with little absorption, in the E or F regions. The least-absorbed frequency is the highest completely reflected from the ionosphere."[11]

"Still higher frequencies may be scattered forward from irregularities in neutral-molecule densities in the troposphere, or in electron densities in the ionosphere. These scattering mechanisms greatly attenuate the forward-scattered radio wave, but these higher frequencies have compensating advantages: less congestion in the radio spectrum and higher directivity achievable with relatively small antennas."[11]

## Backgrounds

This is a detailed, all-sky picture of the microwave background created from nine years of WMAP data. Credit: NASA / WMAP Science Team.
This graph shows the power density spectrum of the extragalactic or cosmic gamma-ray background (CGB). Credit: pkisscs@konkoly.hu.

[B]ackground radiation may simply be any radiation that is pervasive, whether ionizing or not. A particular example of this is the cosmic microwave background radiation, a nearly uniform glow that fills the sky in the microwave part of the spectrum; stars, galaxies and other objects of interest in radio astronomy stand out against this background.

The image at the top shows the "detailed, all-sky picture of the infant universe created from nine years of WMAP data. The image reveals 13.77 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. The signal from the our Galaxy was subtracted using the multi-frequency data. This image shows a temperature range of ± 200 microKelvin."[12]

In the figure at right, CUVOB stands for the cosmic ultraviolet and optical background.

The diffuse extragalactic background light (EBL) is all the accumulated radiation in the Universe due to star formation processes, plus a contribution from active galactic nuclei (AGNs). This radiation covers the wavelength range between ~ 0.1-1000 microns (these are the ultraviolet, optical, and infrared regions of the electromagnetic spectrum). The EBL is part of the diffuse extragalactic background radiation (DEBRA), which by definition covers the overall electromagnetic spectrum. After the cosmic microwave background, the EBL produces the second-most energetic diffuse background, thus being essential for understanding the full energy balance of the universe.

"The observations were made using two arrays of radio telescopes – the Cosmic Background Interferometer (CBI) in Chile and the Very Small Array (VSA) in Tenerife. The experiments have produced the sharpest measurements ever of the temperature variations in the cosmic microwave background. These variations trace the fluctuations in the distribution of primordial matter that seeded the formation of large-scale structure in the universe."[13]

## Cosmic rays

Notation: let the symbol GZK represent Greisen-Zatsepin-Kuzmin.

Based on interactions between cosmic rays and the photons of the cosmic microwave background radiation (CMB) ... cosmic rays with energies over the threshold energy of 5x1019 eV interact with cosmic microwave background photons ${\displaystyle \gamma _{\rm {CMB}}}$ to produce pions via the ${\displaystyle \Delta }$ resonance,

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow p+\pi ^{0},}$

or

${\displaystyle \gamma _{\rm {CMB}}+p\rightarrow \Delta ^{+}\rightarrow n+\pi ^{+}.}$

## Neutrals

Pions produced in this manner proceed to decay in the standard pion channels—ultimately to photons for neutral pions, and photons, positrons, and various neutrinos for positive pions. Neutrons decay also to similar products, so that ultimately the energy of any cosmic ray proton is drained off by production of high energy photons plus (in some cases) high energy electron/positron pairs and neutrino pairs.

## Protons

The pion production process begins at a higher energy than ordinary electron-positron pair production (lepton production) from protons impacting the CMB, which starts at cosmic ray proton energies of only about 1017eV. However, pion production events drain 20% of the energy of a cosmic ray proton as compared with only 0.1% of its energy for electron positron pair production. This factor of 200 is from two sources: the pion has only about ~130 times the mass of the leptons, but the extra energy appears as different kinetic energies of the pion or leptons, and results in relatively more kinetic energy transferred to a heavier product pion, in order to conserve momentum. The much larger total energy losses from pion production result in the pion production process becoming the limiting one to high energy cosmic ray travel, rather than the lower-energy light-lepton production process.

## Mesons

The pion production process continues until the cosmic ray energy falls below the pion production threshold. Due to the mean path associated with this interaction, extragalactic cosmic rays traveling over distances larger than 50 Mpc (163 Mly) and with energies greater than this threshold should never be observed on Earth. This distance is also known as GZK horizon.

## Beta particles

"The attenuation of photons in the microwave background via the process

${\displaystyle \gamma +\gamma (3^{o}K)\rightarrow e^{+}e^{-}}$

is strongly energy dependent, with a minimum attenuation length of ≈ 7 kpc around 2.5 PeV, as determined by the threshold for e+e- production (Gould and Schreder, 1966; Jelley, 1966)."[14]

## Electrons

A "PeV energy photon cannot deliver information from a source at the edge of our own galaxy because it will annihilate into an electron [positron] pair in an encounter with a 2.7 Kelvin microwave photon before reaching our telescope."[15]

"In general, energetic photons above a threshold E given by

${\displaystyle 4E\epsilon \sim (2m_{e})^{2},}$

where E and ε are the energy of the high-energy and background photon, respectively. [This] implies that TeV-photons are absorbed on infrared light, PeV photons on the cosmic microwave background and EeV photons on radio-waves".[15]

"Each [optical module] OM contains a 10 inch [photo-multiplier tube] PMT that detects individual photons of Cerenkov light generated in the optically clear ice by muons and electrons moving with velocities near the speed of light."[15]

"Radio Cerenkov experiments detect the Giga-Hertz pulse radiated by shower electrons produced in the interaction of neutrinos in ice."[15]

"Above a threshold of ≃ 1PeV, the large number of low energy(≃ MeV ) photons in a shower will produce an excess of electrons over positrons by removing electrons from atoms by Compton scattering. These are the sources of coherent radiation at radio frequencies, i.e. above ∼ 100MHz."[15]

## Gamma rays

Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely energetic explosions that have been observed in distant galaxies. They are the most luminous electromagnetic events known to occur in the universe. Bursts can last from ten milliseconds to several minutes, although a typical burst lasts 20–40 seconds. The initial burst is usually followed by a longer-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet, optical, infrared, microwave and radio).[16]

## Submillimeters

Submillimetre astronomy or submillimeter astronomy is the branch of observational astronomy that is conducted at submillimetre wavelengths of the electromagnetic spectrum. Astronomers place the submillimetre waveband between the far-infrared and microwave wavebands, typically taken to be between a few hundred micrometres and a millimetre. Using submillimetre observations, astronomers examine molecular clouds and dark cloud cores with a goal of clarifying the process of star formation from earliest collapse to stellar birth.

This image shows the early planetary radar at Pluton, USSR, 1960. Credit: Rumlin.

Radar astronomy is a technique of observing nearby astronomical objects by reflecting microwaves off target objects and analyzing the echoes. This research has been conducted for six decades. Radar astronomy differs from radio astronomy in that the latter is a passive observation and the former an active one. Radar systems have been used for a wide range of solar system studies. The radar transmission may either be pulsed or continuous.

Several satellites have served as observatories for radio waves and specifically for microwaves. The Radio Astronomy Explorer (RAE) 1 is launched into orbit on July 4, 1968, around Earth, while the [Explorer 49] RAE 2 is launched on June 10, 1973, around the Moon.

The COBE is launched into Earth orbit on November 18, 1989. The WMAP is launched on June 30, 2001, into orbit at the [Lissajous orbit] Lagrange 2 location. Both satellites have aboard detectors designed to perform microwave astronomy, as these are limited to only the microwave band.

## Superluminals

"We propose a method for estimating the composition, i.e. the relative amounts of leptons and protons, of extragalactic jets which exhibit X-ray bright knots in their kpc scale jets. The method relies on measuring, or setting upper limits on, the component of the Cosmic Microwave Background (CMB) radiation that is bulk-Comptonized by cold electrons in the relativistically flowing jet. These measurements, along with modeling of the broadband knot emission that constrain the bulk Lorentz factor Γ of the jets, can yield estimates of the jet power carried by protons and leptons. We provide an explicit calculation of the spectrum of the bulk-Comptonized (BC) CMB component and apply these results to PKS 0637–752 and 3C 273, two superluminal quasars with Chandra – detected large scale jets."[17]

## Liquid objects

Number of days in January 2016 when surface melt was detected from passive microwave satellite observations are shown. Credit: Julien Nicolas, The Ohio State University.{{fairuse}}

"Passive microwave satellite observations [in the image on the right] indicate that surface melt occurred during one or more days over a broad sector of West Antarctica (termed Ross sector hereafter) in January 2016, with up to 15 melt days over parts of the eastern Ross Ice Shelf and Siple Coast."[18]

"January 2016 was one of the three largest melt events in the Ross sector since 1978 (second behind 1991–92 for [melt index (MI) (melt area weighted by duration of the melting)] MI, and a virtual tie for first with January 2005 for melt extent)."[18]

"The satellite observations were corroborated on the ground by a number of automatic weather stations (AWSs) that recorded near-surface temperatures near or above 0 °C for several consecutive days during 10–21 January [...]. The onset of the melt event on 10 January was accompanied by an abrupt temperature increase at WAIS Divide and Byrd, in central West Antarctica. The temperature time series from these two sites highlight roughly two phases: Phase 1 (10–14 January), during which the temperatures were at their warmest; and Phase 2 (15–21 January), during which the temperatures gradually decreased towards their pre-event levels. The transition from Phase 1 to Phase 2 is characterized by a shift of the melt pattern towards the Transantarctic Mountains apparent in the AWS temperature time series and in the sequence of daily melt maps [...]."[18]

"[R]ain was witnessed by a field party on 12 January (Dr Huw Horgan, Victoria University of Wellington, personal communication)."[18]

## Rocky objects

This map of the Milky Way shows the distribution of interstellar dust across the galaxy as seen by the Planck space observatory. Credit: ESA/NASA/JPL-Caltech.

The Askaryan effect is the phenomenon whereby a particle traveling faster than the phase velocity of light in a dense dielectric (such as salt, ice or the lunar regolith) produces a shower of secondary charged particles which contain a charge anisotropy and thus emits a cone of coherent radiation in the radio or microwave part of the electromagnetic spectrum. It is similar to the Cherenkov effect.

"The red [image at center shows] the heat coming from dust throughout the Milky Way galaxy. Planck can capture this thermal light even though the dust is extremely cold — about minus 420 Fahrenheit (minus 251 Celsius)."[19]

## Astrochemistry

"The detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[20] "H2CO is the first organic polyatomic molecule ever detected in the interstellar medium".[20]

## Hydrogens

The hydrogen line, 21 centimeter line or HI line refers to the electromagnetic radiation spectral line that is created by a change in the energy state of neutral hydrogen atoms. This electromagnetic radiation is at the precise frequency of 1420.40575177 [megahertz] MHz, which is equivalent to the vacuum wavelength of 21.10611405413 cm in free space. This wavelength or frequency falls within the microwave radio region of the electromagnetic spectrum, and it is observed frequently in radio astronomy, since those radio waves can penetrate the large clouds of interstellar cosmic dust that are opaque to visible light.

## Ions

This image shows the microwave light from charged particle interactions around the galaxy. Credit: ESA/NASA/JPL-Caltech.

Charged particle interactions around the galaxy are shown in the image above using microwaves as detected by the Planck satellite.

## Molecules

This all-sky image shows the distribution of carbon monoxide (CO). Credit: ESA/Planck Collaboration.{{fairuse}}

"This all-sky image [above center] shows the distribution of carbon monoxide (CO), a molecule used by astronomers to trace molecular clouds across the sky, as seen by Planck."[21]

## Compounds

So far the effect has been observed in silica sand,[22] rock salt,[23] and ice[24].[25]

## Atmospheres

A plot of the zenith atmospheric microwave transmission on the summit of Mauna Kea in the gigahertz range at a precipitable water vapor level of 0.001 mm is shown. Credit: Westeros91.

At the right is a plot of the zenith atmospheric microwave transmission on the summit of Mauna Kea, Earth, in the gigahertz range at a precipitable water vapor level of 0.001 mm.

## Spectrometers

The graph is of the cosmic microwave background spectrum measured by the FIRAS instrument on the COBE satellite, the most-precisely measured black body spectrum in nature,[26] the error bars are too small to be seen even in enlarged image, and it is impossible to distinguish the data from the theoretical curve. Credit: .

[C]osmic microwave background (CMB) radiation (also CMBR, CBR, MBR, and relic radiation) is thermal radiation filling the observable universe almost uniformly.[27]

Precise measurements of cosmic background radiation are critical to cosmology, since any proposed model of the universe must explain this radiation. The CMBR has a thermal black body spectrum at a temperature of 2.725 K,[28] which peaks at the microwave range frequency of 160.2 GHz, corresponding to a 1.873 mm wavelength. This holds if measured per unit frequency, as in Planck's law. If measured instead per unit wavelength, using Wien's law, the peak is at 1.06 mm corresponding to a frequency of 283 GHz.

"The differential 850-μm counts are well described by the function

${\displaystyle n(S)=N_{0}/(a+S^{3.2}),}$

where ${\displaystyle S}$ is the flux in mJy, ${\displaystyle N_{0}}$ = 3.0 × 104 per square degree per mJy, and ${\displaystyle a}$ = 0.4 − 1.0 is chosen to match the 850-μm extragalactic background light."[29]

## Sun

The quiet Sun at 4.6 GHz imaged by the VLA with a resolution of 12 arcsec, or about 8400 km on the surface of the Sun. Credit: NRAO.

At right is a radio image of the Sun at 4.6 GHz. "The brightest discrete radio source is the Sun, but it is much less dominant than it is in visible light. The radio sky is always dark, even when the Sun is up, because atmospheric dust doesn't scatter radio waves, whose wavelengths are much longer than the dust particles."[30]

"The quiet Sun at 4.6 GHz imaged by the [Very Large Array] VLA with a resolution of 12 arcsec, or about 8400 km on the surface of the Sun. The brightest features (red) in this false-color image have brightness temperatures ~ 106 K and coincide with sunspots. The green features are cooler and show where the Sun's atmosphere is very dense. At this frequency the radio-emitting surface of the Sun has an average temperature of 3 x 104 K, and the dark blue features are cooler yet. The blue slash crossing the bottom of the disk is a feature called a filament channel, where the Sun's atmosphere is very thin: it marks the boundary of the South Pole of the Sun. The radio Sun is somewhat bigger than the optical Sun: the solar limb (the edge of the disk) in this image is about 20000 km above the optical limb."[30]

"The microwave radiation from the sun was observed during the partial eclipse of July 9, 1945."[31]

## Earth

"In the winter of 1931 Karl G. Jansky1 of the Bell Telephone Laboratoies was making studies of the direction of arrival of high-frequency atmospheric static with a radio receiver tuned to a frequency of 20.5 x 106 cycles/sec. He discovered a faint source of static whose direction slowly changed throughout the day, and had approximately the same direction every day at the same time. He began an intensive study of this phenomenon, and determined that the variation of azimuth of the unknown source coincided with that of the sun. He continued his observations over a period of several months, and found that as the sun moved eastward, the direction from which the signal was coming remained fixed on the celestial sphere.2 By an ingenious method he determined its approximate right ascension and declination, and showed that they coincided roughly with the direction in which astronomers placed the centre of our galactic system. His papers contain the first published evidence for the existence of extra-terrestrial radiation at radio-frequencies."[32]

"Jansky's observations dramatically opened up the new "microwave region" -- a whole new section of the electromagnetic spectrum to which the earth's atmosphere is transparent. […] The microwave region is limited on the high frequency side by the absorptions of various molecules in the atmosphere, and on the low frequency side by the ionosphere, which absorbs or reflects electromagnetic vibrations of frequencies lower than a critical value."[32]

"[E]xtra-terrestrial microwave radiation has been detected from four sources:

1. The Milky Way emits radiation in frequencies between 20 mc./sec and 480 mc./sec.
2. The sun has measurable radiation between the limits of 20 mc./sec. and 30,000 mc./sec.
3. The moon6 at full phase radiates at 24,000 mc./sec., as though it were at a temperature near 0°C.
4. The Andromeda nebula7 (the nearest galaxy comparable in size with our own) gives faint indications of radiation at 162 mc./sec. Most observers have reported negative results from the planets and some of the brighter stars."[32]

## Greenland ice sheets

(a) The probability is for of a pixel melting at least as many times as observed during the 1995, 1998 and 2002 melt seasons given the last 25 years of melt observations. (b) Melt extent is for 2002: Pixels are color coded for number of melt days during the season. (c) Slopes of the trend lines are fit to the areas observed to melt between April and November from 1979 to 2003. Credit: K. Steffen, S. V. Nghiem, R. Huff, and G. Neumann.
Half-decade records for ETH/CU Camp station: (a) Top panel is for QSCAT backscatter, (b) middle panel for QSCAT diurnal signature, and (c) bottom panel for air temperature measured at the AWS site. Credit: K. Steffen, S. V. Nghiem, R. Huff, and G. Neumann.
QSCAT melt maps are shown on the climatological peak-melt day (1 August). Red color represents current active melt areas, light blue is for areas that have melted but currently refreeze, white is for areas that will melt later, and magenta is for areas that do not experience any melt throughout the melt season. The dark blue color surrounding Greenland is the ocean mask. Credit: K. Steffen, S. V. Nghiem, R. Huff, and G. Neumann.
QSCAT maps of number of melt days (violet to red for 1 to 31 days) in 2000–2003 with the overlaid black contours representing melt extent derived from PM data are shown. Credit: K. Steffen, S. V. Nghiem, R. Huff, and G. Neumann.

"Active and passive microwave satellite data are used to map snowmelt extent and duration on the Greenland ice sheet. The passive microwave (PM) data reveal the extreme melt extent of 690,000 km2 in 2002 as compared with an average extent of 455,000 km2 from 1979–2003."[33]

"Several PM-based melt assessment algorithms [Mote and Anderson, 1995; Abdalati and Steffen, 1995] are applicable to Scanning Multi-channel, Microwave Radiometer (SMMR) and Special Sensor Microwave/Imager (SSM/I) instruments providing near-continuous coverage since 1979. The PM data as gridded brightness temperatures on polar stereographic grids (25 km resolution) [used] are from the National Snow and Ice Data Center [Maslanik and Stroeve, 2003], containing daily data spanning 25 melt seasons from 1979 to 2003."[33]

In the image at the right, (a) "shows the probabilities of the observed melt behavior on the Greenland ice sheet for several large melt years and indicates the extreme melt anomaly observed in northeastern Greenland in 2002."[33]

"Prior to 2002, both 1995 and 1998 were extreme melt years in terms of maximum areal extent and total melt. During 1995 melt was dominated by a high frequency of melt along the western margin of the ice sheet. During 1998 melt was spatially diverse with slightly more melt than usual in the northeast and southwest. However, the high frequency melt in 2002 in the northeast and along the western margin is unprecedented in the PM record with a log likelihood of occurrence that is 35% lower than the previous record melt anomaly in 1991."[33]

(c) "depicts the magnitude of the increasing trends in melt extent on a daily basis over the last 25 years. Although there is a large amount of inter-annual variability in melt extent on a given day, 56 days show statistically significant (alpha = 0.1) increasing trends in melt area."[33]

"Melt along the west coast was extensive during 2002 but not atypical for large melt years. However melt in the north and northeast was highly irregular both in terms of extent and frequency. Nearly 3,000 km2[(b)] were classified as melting during 2002 that had not previously melted during any other year between 1979 and 2003."[33]

The figure at the left "presents QSCAT backscatter and diurnal signatures, and ETH/CU AWS air temperature."[33] Half-decade records for ETH/CU Camp station: (a) Top panel is for QSCAT backscatter, (b) middle panel for QSCAT diurnal signature, and (c) bottom panel for air temperature measured at the AWS site.[33]

At the lower right QSCAT melt maps are shown on the climatological peak-melt day (1 August). Red color represents current active melt areas, light blue is for areas that have melted but currently refreeze, white is for areas that will melt later, and magenta is for areas that do not experience any melt throughout the melt season. The dark blue color surrounding Greenland is the ocean mask.

"QSCAT mapping can reveal details of the spatial pattern of surface melt evolution in time. There are large variabilities in melt extent and melt timing over different regions. [The figure at tje lower right] confirms that 2002 has the most extensive areal melt. In 2002, the northeast quadrant of the Greenland ice sheet, extending well into the dry snow zone, experienced at least some melt where melt never happened before (from satellite data records to date). Since the beginning of the QSCAT data record (July 1999), the smallest spatial extent of melt occurred in 2001, and melt extent was similar for years 2000 and 2003."[33]

"To provide a direct comparison of PM and QSCAT results, we overlay results for PM melt extent and QSCAT number of melt days in [the figure at the lower left] for years 2000–2003. PM XPGR melt extent is approximately confined to QSCAT melt areas experiencing 2 weeks or more of melting time [the figure at the lower left]. QSCAT melt areas outside of the PM melt extent represent the surface that has less melt corresponding to about 15 melt days or less. This is consistent with the relationship of relative melt strength measured by active and passive data as discussed above. Note that such areas can total up to a large region in year 2002. Surface albedo can reduce considerably once the snow melts for a period of 2 weeks. The albedo reduction may significantly impact the surface heat balance and thus change the mass balance. The large number of melt days around the northern perimeter of the ice sheet, which is shown as the narrow dark-red band in north Greenland in the 2003 map was an anomalous feature [the figure at the lower left]. This band was wider as defined by the PM melt extent in 2002 than in 2003. However, there were more QSCAT melt days in the 2003 northern melt band."[33]

"The comparison reveals that the PM cross-polarized gradient algorithm classifies melt more conservatively than the scatterometer algorithm. The active microwave identifies melt approximately up to two weeks more than the PM at higher elevation in the percolation zone toward the dry snow zone [the figure at the lower left]. Both methods (active and passive microwave) consistently identify melt areas that have a melt duration of at least 10–14 days. The longer snowmelt duration can be sufficient to decrease surface albedo and affect surface heat and mass balance."[33]

## Moon

This image shows the variations in the lunar gravity field as measured by NASA's Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012. Credit: NASA/JPL-Caltech/MIT/GSFC.

"On October 19, 1945, at 10:25 P.M., E.S.T., the effective black-body temperature of the nearly full moon was measured to be 292° K."[31]

"Very precise microwave measurements between two spacecraft, named Ebb and Flow, were used to map gravity with high precision and high spatial resolution. The field shown resolves blocks on the surface of about 12 miles (20 kilometres) and measurements are three to five orders of magnitude improved over previous data. Red corresponds to mass excesses and blue corresponds to mass deficiencies. The map shows more small-scale detail on the far side of the moon compared to the nearside because the far side has many more small craters."[34]

"Twin NASA probes orbiting Earth's moon have generated the highest resolution gravity field map of any celestial body. The new map, created by the Gravity Recovery and Interior Laboratory (GRAIL) mission, is allowing scientists to learn about the moon's internal structure and composition in unprecedented detail. Data from the two washing machine-sized spacecraft also will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved."[34]

"The gravity field map reveals an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters. Data also show the moon's gravity field is unlike that of any terrestrial planet in our solar system."[35]

""What this map tells us is that more than any other celestial body we know of, the moon wears its gravity field on its sleeve," said GRAIL Principal Investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge. "When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains.""[35]

"According to Zuber, the moon's gravity field preserves the record of impact bombardment that characterized all terrestrial planetary bodies and reveals evidence for fracturing of the interior extending to the deep crust and possibly the mantle. This impact record is preserved, and now precisely measured, on the moon. The probes revealed the bulk density of the moon's highland crust is substantially lower than generally assumed. This low-bulk crustal density agrees well with data obtained during the final Apollo lunar missions in the early 1970s, indicating that local samples returned by astronauts are indicative of global processes."[35]

""With our new crustal bulk density determination, we find that the average thickness of the moon's crust is between 21 and 27 miles (34 and 43 kilometres), which is about 6 to 12 miles (10 to 20 kilometres) thinner than previously thought," said Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris. "With this crustal thickness, the bulk composition of the moon is similar to that of Earth. This supports models where the moon is derived from Earth materials that were ejected during a giant impact event early in solar system history.""[35]

"The map was created by the spacecraft transmitting radio signals to define precisely the distance between them as they orbit the moon in formation. As they fly over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly."[35]

""We used gradients of the gravity field in order to highlight smaller and narrower structures than could be seen in previous datasets," said Jeff Andrews-Hanna, a GRAIL guest scientist with the Colorado School of Mines in Golden. "This data revealed a population of long, linear gravity anomalies, with lengths of hundreds of kilometres, crisscrossing the surface. These linear gravity anomalies indicate the presence of dikes, or long, thin, vertical bodies of solidified magma in the subsurface. The dikes are among the oldest features on the moon, and understanding them will tell us about its early history.""[35]

## Jupiter

Details in radiation belts close to Jupiter are mapped from measurements that NASA's Cassini spacecraft made. Credit: NASA Jet Propulsion Laboratory (NASA-JPL).

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 [Gigahertz] GHz to as low as 3 [Kilohertz] kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers.

"Details in radiation belts close to Jupiter are mapped from measurements that NASA's Cassini spacecraft made of radio emission from high-energy electrons moving at nearly the speed of light within the belts."[36]

"The three views show the belts at different points in Jupiter's 10-hour rotation. A picture of Jupiter is superimposed to show the size of the belts relative to the planet. Cassini's radar instrument, operating in a listen-only mode, measured the strength of microwave radio emissions at a frequency of 13.8 gigahertz (13.8 billion cycles per second or 2.2 centimeter wavelength). The results indicate the region near Jupiter is one of the harshest radiation environments in the solar system."[36]

"From Earth-based radio telescopes, the telltale radio emissions would be swamped out by heat-generated radio emissions from Jupiter's atmosphere, but Cassini was close enough to Jupiter in January 2001 to differentiate between the emissions from the radiation belts and those from the atmosphere."[36]

"The belts appear to wobble as the planet turns because they are controlled by Jupiter's magnetic field, which is tilted in relation to the planet's poles."[36]

## Titan

"Due to advances in infrared/microwave astronomy and to the settlement of space missions to outer planets, recently the attention has shifted towards larger molecules, mostly of organic nature. Due to the tremendous complexity of its chemistry, the Titan’s ionosphere is the most pertinent example showing the importance of good chemical models for the interpretation of Cassini data. Heavy ions with masses over 100 amu have been detected in significant amounts into the Titan’s ionosphere below 1200 km [1]. Possible chemical structures include PAHs, nitrile aromatic polymers [2], fullerenes [3] and polyphenyls [4] and such heavy particles have been proposed to act as seeds for aerosols formations [5]."[37]

## Uranus

Photometry over the course of half a Uranian year (beginning in the 1950s) has shown regular variation in the brightness in two spectral bands, with maxima occurring at the solstices and minima occurring at the equinoxes.[38] A similar periodic variation, with maxima at the solstices, has been noted in microwave measurements of the deep troposphere begun in the 1960s.[39] Stratospheric temperature measurements beginning in the 1970s also showed maximum values near the 1986 solstice.[40] The majority of this variability is believed to occur owing to changes in the viewing geometry.[41]

Detailed analysis of the visible and microwave data revealed that the periodical changes of brightness are not completely symmetrical around the solstices, which also indicates a change in the meridional albedo patterns.[42] Finally in the 1990s, as Uranus moved away from its solstice, Hubble and ground based telescopes revealed that the south polar cap darkened noticeably (except the southern collar, which remained bright),[43] while the northern hemisphere demonstrated increasing activity,[44] such as cloud formations and stronger winds, bolstering expectations that it should brighten soon.[45] This indeed happened in 2007 when the planet passed an equinox: a faint northern polar collar arose, while the southern collar became nearly invisible, although the zonal wind profile remained slightly asymmetric, with northern winds being somewhat slower than southern.[46]

## Interstellar medium

"The invention of microwave spectroscopy and the subsequent development of microwave astronomy has revealed two great régimes of interstellar chemistry: the dense molecular clouds and the circumstellar shells."[47]

## IK Tauri

"For both distributions, there is a dust shell of radius about 100 mas, or diameter of 200 mas, and two more or less discrete shells separated by about 240 mas. This approximately regular spacing is what produces a hump in the visibility curve in the (6-8.5) x 105 rad-1 range, though in somewhat different positions for the two years. It can be seen from Figure 4 that the prominent and well-defined outer dust shell expanded between 1992 and 1993. One should not expect that each dust shell emitted has exactly the same velocity, but if the motion of the outer shell is taken as 20.5 km s-1, an average of the 22 km s-1 velocity measured by CO line emission of material surrounding the star (Knapp & Morris 1985) and of 18.7 km s-1 for OH masers (Bowers et al. 1989), then the stellar distance can be obtained from the observed motion. The displacement of the outer shell is 17 mas between average dates of 1992 August 24 and 1993 September 14. From these values, the stellar distance is calculated to be 265 pc. This is in good agreement with the estimates of 270 and 220 pc by Knapp & Morris (1985) and Le Sidaner & Le Bertre (1996), respectively. Other estimates in the literature for the distance to IK Tau vary from 240 to 500 pc (Knapp & Morris 1985). If the velocity of the outer shell is indeed close to the average of OH and CO gas measured by microwave astronomy, the distance of 265 pc should be approximately correct and should confirm the distance estimates that have arrived at comparable values, rather than others that are substantially different."[48]

## Messier 106

Messier 106 is one of the brightest and nearest spiral galaxies to our own. Credit: NASA, ESA, the Hubble Heritage Team (STScI/AURA), and R. Gendler (for the Hubble Heritage Team).{{free media}}

M106 has a water vapor megamaser (the equivalent of a laser operating in microwave instead of visible light and on a galactic scale) that is seen by the 22-GHz line of ortho-H2O that evidences dense and warm molecular gas that give M106 its characteristic purple color.[49] Water masers are useful to observe nuclear accretion disks in active galaxies, enabling the first case of a direct measurement of the distance to a galaxy, thereby providing an independent anchor for the cosmic distance ladder.[50][51] M106 has a slightly warped, thin, almost edge-on Keplerian disc which is on a subparsec scale that surrounds a central area with mass 4 × 107 M.[52]

## MCG+01-38-005

The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). Credit: NASA Hubble Space Telescope.{{free media}}

"The [microwave] detection of interstellar formaldehyde provides important information about the chemical physics of our galaxy. We now know that polyatomic molecules containing at least two atoms other than hydrogen can form in the interstellar medium."[20]

"Phenomena across the universe emit radiation spanning the entire electromagnetic spectrum — from high-energy gamma rays, which stream out from the most energetic events in the cosmos, to lower-energy microwaves and radio waves."[53]

"Microwaves, the very same radiation that can heat up your dinner, are produced by a multitude of astrophysical sources, including strong emitters known as masers (microwave lasers), even stronger emitters with the somewhat villainous name of megamasers, and the centers of some galaxies. Especially intense and luminous galactic centers are known as active galactic nuclei. They are in turn thought to be driven by the presence of supermassive black holes, which drag surrounding material inwards and spit out bright jets and radiation as they do so."[53]

"The two galaxies shown here, imaged by the Hubble Space Telescope, are named MCG+01-38-004 (the upper, red-tinted one) and MCG+01-38-005 (the lower, blue-tinted one). MCG+01-38-005 is a special kind of megamaser; the galaxy’s active galactic nucleus pumps out huge amounts of energy, which stimulates clouds of surrounding water. Water’s constituent atoms of hydrogen and oxygen are able to absorb some of this energy and re-emit it at specific wavelengths, one of which falls within the microwave regime. MCG+01-38-005 is thus known as a water megamaser!"[53]

"Astronomers can use such objects to probe the fundamental properties of the universe. The microwave emissions from MCG+01-38-005 were used to calculate a refined value for the Hubble constant, a measure of how fast the universe is expanding. This constant is named after the astronomer whose observations were responsible for the discovery of the expanding universe and after whom the Hubble Space Telescope was named, Edwin Hubble."[53]

## Recent history

The image portrays a brief history of the detection of the microwave background. Credit: NASA.

The recent history period dates from around 1,000 b2k to present.

"Penzias and Wilson discovered the remnant afterglow from the Big Bang and were awarded the Nobel Prize for their discovery. COBE first discovered the patterns in the afterglow. WMAP will bring the patterns into much better focus to unveil a wealth of information about the history and fate of the universe."[54]

in the figure at the right, specifically the top left (TL) is the "Penzias and Wilson microwave receiver - 1965"[54], (TR) a "Simulation of the sky viewed by Penzias and Wilson's microwave receiver - 1965"[54], (ML) "COBE Spacecraft, Painting - 1992"[54], (MR) "COBE's view of early universe- 1992"[54], (BL) "WMAP Spacecraft, Computer Rendering - 2001"[54], and (BR) "Simulated WMAP view of early universe"[54].

## Sciences

The submillimeter, millimeter, and microwave spectral line catalog is "a computer-accessible catalog of submillimeter, millimeter, and microwave spectral lines in the frequency range between 0 and 10 000GHz (ie wavelengths longer than 30μm)."[55]

## Clocks

This chart shows the increasing accuracy of NIST (formerly NBS) atomic clocks. Credit: National Institutes of Standards and Technology (NIST), USA.

An atomic clock is a clock device that uses an electronic transition frequency in the microwave, optical, or ultraviolet region[56] of the electromagnetic spectrum of atoms as a frequency standard for its timekeeping element. Atomic clocks are the most accurate time and frequency standards known, and are used as primary standards for international time distribution services, to control the wave frequency of television broadcasts, and in global navigation satellite systems such as GPS.

## Low noise amplifiers

The QUIET module is a pseudo-correlation receiver comprising low noise amplifiers, phase shifters, detector diodes, and passive components. Credit: Immanuel Buder and the QUIET collaboration.
These are schematic views of a QUIET cryostat shown with a 91 element array of W-band modules. Credit: Immanuel Buder and the QUIET collaboration.
An illustration of the QUIET telescope on the Cosmic Background Imager mount. Credit: Immanuel Buder and the QUIET collaboration.
The site for the QUIET telescopes is the Chajnantor scientific reserve in Chile. Credit: Immanuel Buder and the QUIET collaboration.
The QUIET cosmic microwave background (CMB) polarization telescope has its dome lowered at the Llano de Chajnantor Observatory in Atacama, Chile. Credit: Joezuntz.

At right is an image of the QUIET module, a pseudo-correlation receiver comprising low noise amplifiers, phase shifters, detector diodes, and passive components. On the left is the first QUIET module which includes the "low noise amplifiers[, an] InP monolithic microwave integrated circuit (MMIC) high electron mobility transistor (HEMT) amplifiers."[57] The upper right shows "an earlier prototype 90 GHz module. The modules are 1.25 x 1.14."[57] The lower right is "the interior of a (2 x 2) 40 GHz module."[57]

"Both Q and U are measured simultaneously by a single QUIET module with the introduction of an appropriate optical element: a circularly polarized orthomode transducer (OMT)."[57]

At second right are schematic views "of a QUIET cryostat shown with a 91 element array of W-band modules."[57]

"The horns are held at ≈ 20 K and shielded from 300K radiation by a radiation shield (shown only in the figure on the left) held at ≈ 80 K on the top and sides and the aluminum plate (also 80K) on the bottom."[57]

The third right image is of the QUIET telescope. "The 2m design [...] accommodates 400 W-band receivers or 100 Q-band receivers, with each mirror machined as a single piece. [The] three 2m systems, [are] accommodated on the CBI platform as indicated in [the third right image]. The design uses two reflectors of approximately equal size in a crossed arrangement, and is known as a side fed Cassegrain (or crossed Dragone system). The primary mirror is parabolic and the secondary is a concave hyperboloid. By correctly selecting the angle between the two reflectors, a system that has a wide field of view with minimal cross-polarization results."[57]

"The site [in the image on the left] for the QUIET telescopes [in the second left image with its dome down] is the Chajnantor scientific reserve in Chile, at an altitude of 5080 m. This site is recognized as one of the best in the world for millimeter and submillimeter astronomy. The site belongs to the state of Chile and is leased to the international Atacama Large Millimeter Array project (ALMA)."[57]

## Detectors

"The transmutation products are germanium acceptors (Ga), donors (As) and double donors (Se). It is evident that changing the relative isotopic composition directly affects the relative dopant concentrations. Growing germanium crystals consisting only of a mixture of 70Ge and 74Ge isotopes and using [Neutron Transmutation Doping] NTD allows the formation of a continuous series of crystals doped form purely p-type (70Ge 100%) to purely n-type (74Ge 100%) with all the possible compensation ratios between these two extremes. Our group has used NTD with natural Ge to form highly sensitive thermal detectors operating in the Kelvin and milliKelvin temperature range16) in a large number of far infrared and microwave astronomy and astrophysics experiments.17–19)"[58]

## Spacecraft

This is an exploded view of the Juno spacecraft. Credit: .
The diagram shows where the instruments aboard Juno are attached. Credit: .
An artist's impression of Juno near Jupiter. Credit: .

"The [Juno] spacecraft is built around a hexagonal cylinder bus measuring 3.5 m in diameter by 3.5 m high. Three solar panel wings extend from alternate sides of the hexagon giving a total diameter of approximately 20 m. A high gain antenna is mounted on top of the bus, with instruments mounted on the deck and propellant, oxygen, and pressurant tanks mounted within. At the center of the top deck is a 0.8 x 0.8 x 0.6 m titanium "vault" which houses the spacecraft avionics and critical systems to protect them from the severe jovian radiation environment. The vault has a mass of 150 kg and walls up to over a cm in thickness. Power is provided by ultra triple junction GaAs solar cells, covered with thick glass for radiation shielding, which are grouped into 11 solar panels, four on two of the wings and three on the other. (The end of the third wing is a boom structure holding science instruments.) The solar panels will produce a total of 18 kW at Earth and 400 W initially at Jupiter. The science payload comprises ten instruments: the Jovian Auroral Distributions Experiment (JADE), the Jupiter Energetic-particle Detector Instrument (JEDI), the Ultraviolet Spectrograph (UVS), the JunoCam, the Jovian Infrared Auroral Mapper (JIRAM), the Plasma Waves Instrument (Waves), the Microwave radiometer (MWR), the Fluxgate Magnetometer (FGM), the Advanced Stellar Compass (ASC), the Scalar Helium Magnetometer (SHM), and the Gravity Science experiment."[59] Bold added.

## Microwave telescopes

The Planck telescope was launched in 2009 to observe the Cosmic Microwave Background Radiation. Credit: ESA.

"The basic scientific goal of the Planck mission is to measure [cosmic microwave background] CMB anisotropies at all angular scales larger than 10 arcminutes over the entire sky with a precision of ~2 parts per million. The model payload consists of a 1.5 meter off-axis telescope with two focal plane arrays of detectors sharing the focal plane. Low frequencies will be covered by 56 tuned radio receivers sensitive to 30-100 GHz, while high frequencies will be covered by 56 bolometers sensitive to 100-850 GHz."[60]

## Wilkinson microwave anisotropy probe

This is a spacecraft diagram of WMAP. Credit: NASA.

"The Wilkinson Microwave Anisotropy Probe (WMAP) is a Medium-class Explorer (MIDEX) mission designed to elucidate cosmology by producing full-sky maps of the cosmic microwave background (CMB) anisotropy."[5]

## Explorer 66

This is a diagram of Explorer 66, the COBE spacecraft. Credit: NASA.

The Cosmic Background Explorer (COBE) has aboard a differential microwave radiometer (DMR) labeled in the diagram at right.

## The E and B experiment

The E and B Experiment (EBEX) will measure the cosmic microwave background radiation of a part of the sky during two sub-orbital (high altitude) balloon flights. It is an experiment to make large, high-fidelity images of the CMB polarization anisotropies. By using a telescope which flies at over 42,000 metres high, it is possible to reduce the atmospheric absorption of microwaves to a minimum. This allows massive cost reduction compared to a satellite probe, though only a small part of the sky can be scanned and for shorter duration than a typical satellite mission such as WMAP.

EBEX was launched on 29 December, 2012, near McMurdo Station in Antarctica.[61][62]

"EBEX is meant to hone in on one specific feature of the CMB light that's been predicted, but never seen — a signature called B-type polarization, thought to have been produced by the gravity waves created by the universe's extremely rapid infant expansion, which happened even before the CMB light was released."[63]

## Explorer 49

Several satellites have served as observatories for radio waves and specifically for microwaves.

Explorer 49 [is] a 328 kilogram satellite launched on June 10, 1973 for longwave radio astronomy research. It had four 230-meter long X-shaped antenna elements, which made it one of the largest spacecraft ever built. Explorer 49 was placed into lunar orbit to provide radio astronomical measurements of the planets, the sun, and the galaxy over the frequency range of 25 kHz to 13.1 MHz.

## Cosmic Anisotropy Telescope

This is a stitched panorama of the Cosmic Anisotropy Telescope (CAT) enclosure at the Mullard Radio Astronomy Observatory, Cambridgeshire in June 2014. Credit: Cmglee.

The Cosmic Anisotropy Telescope (CAT), built in the mid 1990s, was the first interferometer to measure fluctuations in the cosmic microwave background (CMB). Its first results, published in 1996, were the highest resolution CMB detection at that time, and showed that the rise in fluctuation power towards scales of ~1 degree (l ~ 200) measured by the Saskatoon experiment were matched by a decline in power at smaller angles (l = 500-700), thus showing the existence of the long-predicted acoustic peak in the CMB power spectrum.

## Relict-2

"A Russian microwave astronomy satellite called Relict-2 was the first one proposed to use a Sun-Earth L2 orbit in about 1990."[64]

## Orbital platforms

Skylab is an example of a manned observatory in orbit. Credit: NASA.

Skylab included an Apollo Telescope Mount, which was a multi-spectral solar observatory. Numerous scientific experiments were conducted aboard Skylab during its operational life, and crews were able to confirm the existence of coronal holes in the Sun. The Earth Resources Experiment Package (EREP), was used to view the Earth with sensors that recorded data in the visible, infrared, and microwave spectral regions.

## Sun-synchronous orbital rocketry

Diagram shows the orientation of a Sun-synchronous orbit (green) in four points of the year. A non-sun-synchronous orbit (magenta) is also shown for reference. Credit: Brandir.
The photograph shows a full-size model of ERS-2. Credit:Poppy.
The ERS-2 is carried into a sun-synchronous polar orbit by an Ariane 4 similar to the one imaged. Credit: NASA.

A Sun-synchronous orbit (sometimes called a heliosynchronous orbit[65]) is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given Earth latitude at the same local mean solar time. The surface illumination angle will be nearly the same every time. This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths (e.g. weather and spy satellites) and for other remote sensing satellites (e.g. those carrying ocean and atmospheric remote sensing instruments that require sunlight). For example, a satellite in sun-synchronous orbit might ascend across the equator twelve times a day each time at approximately 15:00 mean local time. This is achieved by having the osculating orbital plane precess (rotate) approximately one degree each day with respect to the celestial sphere, eastward, to keep pace with the Earth's movement around the Sun.[66]

The uniformity of Sun angle is achieved by tuning the inclination to the altitude of the orbit ... such that the extra mass near the equator causes the orbital plane of the spacecraft to precess with the desired rate: the plane of the orbit is not fixed in space relative to the distant stars, but rotates slowly about the Earth's axis. Typical sun-synchronous orbits are about 600–800 km in altitude, with periods in the 96–100 minute range, and inclinations of around 98° (i.e. slightly retrograde compared to the direction of Earth's rotation: 0° represents an equatorial orbit and 90° represents a polar orbit).[66]

European remote sensing satellite (ERS) was the European Space Agency's first Earth-observing satellite. It was launched on July 17, 1991 into a Sun-synchronous polar orbit at a height of 782–785 km.

ERS-1 carried an array of earth-observation instruments that gathered information about the Earth (land, water, ice and atmosphere) using a variety of measurement principles. These included:

• RA (Radar Altimeter) is a single frequency nadir-pointing radar altimeter operating in the Ku band.
• ATSR-1 (Along-Track Scanning Radiometer) is a 4 channel infrared radiometer and microwave sounder for measuring temperatures at the sea-surface and the top of clouds.
• SAR (synthetic aperture radar) operating in C band can detect changes in surface heights with sub-millimeter precision.
• Wind Scatterometer used to calculate information on wind speed and direction.
• MWR is a Microwave Radiometer used in measuring atmospheric water, as well as providing a correction for the atmospheric water for the altimeter.

To accurately determine its orbit, the satellite included a Laser Retroreflector. The Retroreflector was used for calibrating the Radar Altimeter to within 10 cm.

Its successor, ERS-2, was launched on April 21, 1995, on an Ariane 4, from ESA's Guiana Space Centre near Kourou, French Guiana. Largely identical to ERS-1, it added additional instruments and included improvements to existing instruments including:

• GOME (Global Ozone Monitoring Experiment) is a nadir scanning ultraviolet and visible spectrometer.
• ATSR-2 included 3 visible spectrum bands specialized for Chlorophyll and Vegetation

## Aqua

Aqua (EOS PM-1, 2002-022A) was launched on 4 May 2002 from Vandenberg AFB Space Launch Complex 2W. Credit: NASA.{{free media}}
Aqua carries six state-of-the-art instruments in a near-polar low-Earth orbit. Credit: NASA.{{free media}}

"Aqua [...] is a NASA Earth Science satellite mission named for the large amount of information that the mission is collecting about the Earth's water cycle, including evaporation from the oceans, water vapor in the atmosphere, clouds, precipitation, soil moisture, sea ice, land ice, and snow cover on the land and ice. Additional variables also being measured by Aqua include radiative energy fluxes, aerosols, vegetation cover on the land, phytoplankton and dissolved organic matter in the oceans, and air, land, and water temperatures."[67]

"It continues transmitting high-quality data from four of its six instruments, AIRS [Atmospheric Infrared Sounder], AMSU [Advanced Microwave Sounding Unit], CERES [Clouds and the Earth's Radiant Energy System], and MODIS [Moderate Resolution Imaging Spectroradiometer], and reduced quality data from a fifth instrument, AMSR-E [Advanced Microwave Scanning Radiometer-EOS]. The sixth Aqua instrument, HSB [Humidity Sounder for Brazil — VHF band], collected approximately nine months of high quality data but failed in February 2003."[67]

"Aqua follows a kind of polar orbit known as a Sun-synchronous orbit, which means it crosses the equator at the same local time during each pass. Aqua’s orbit ascends from south to north during the daylight hours, crosses near the North Pole, circles around Earth’s nighttime side, and crosses near the South Pole to return to the daytime side."[68]

## Hypotheses

1. Once literature searching has brought you to the state of the science, it is time to prepare an original research effort. Control groups and proof of concept are tools that can help to verify a model approach to an astronomical phenomenon.

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