Solar System, technical/Comets

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Comets are celestial objects with a nucleus of ice and rock and, when near the Sun or another star, a "tail" of gas and dust particles which points away from the star.

Development status: this resource is experimental in nature.
Educational level: this is a secondary education resource.
Educational level: this is a tertiary (university) resource.
Educational level: this is a research resource.
Type classification: this is an article resource.
Resource type: this resource contains a lecture or lecture notes.
Subject classification: this is an astronomy resource.


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Notation: let the symbol Def. indicate that a definition is following.

Notation: let the symbols between [ and ] be replacement for that portion of a quoted text.


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To help with definitions, their meanings and intents, there is the learning resource theory of definition.

Def. evidence that demonstrates that a concept is possible is called proof of concept.

The proof-of-concept structure consists of

  1. background,
  2. procedures,
  3. findings, and
  4. interpretation.[1]

The findings demonstrate a statistically systematic change from the status quo or the control group.


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"While it is arguable that any entities are controlling the sky, entities may be assigned as a first approximation in the theory of cause and effect. ... An entity that produces comet-like objects may exist. The Sun emits visual radiation that may reflect off a comet's tail. The coronal cloud in close proximity to the Sun also emits X-rays that produce visual fluorescence from gases in a comet's coma and tail."

X-ray astronomy

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"Apart from the Sun, the known X-ray emitters now include planets (Venus, Earth, Mars, Jupiter, and Saturn), planetary satellites (Moon, Io, Europa, and Ganymede), all active comets, the Io plasma torus, the rings of Saturn, the coronae (exospheres) of Earth and Mars, and the heliosphere."[2] Bold added.

Ultraviolet astronomy

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Oxygen has three emission lines in the ultraviolet common in comets at 130.22, 130.49, and 130.60 nm from O I.[3]

"[T]he presence of the [oxygen] green line [in comets] can still be questioned, unless the 2972 Å trans-auroral line [1S - 3P] is detected (Herbig, 1976)."[4] "The transitions involved ... in the spectrum of the oxygen atoms in a cometary atmosphere" include 295.8 and 297.2 nm, 98.9 nm (a triplet), and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm.[4]

"Ultraviolet spectra of comets show strong emission in the hydrogen Lyman-α line, the O I 130.2 nm resonance lines, and the OH Α 2Π-Χ 2Σ+ bands."[3]

"Discovery of the S2 molecule in comets came from UV spectroscopy of the comet IRAS – Araki – Alcock ( 1983d) which passed close to the Earth [59]. ... Emission from S2 was shown to be confined to a small region ( < 100 km) around the nucleus. Outside this region, S2 is destroyed. ... its presence in comet Hyakutake [60] suggests it is ubiquitous and only its narrow survival zone close to the nucleus inhibits regular detection."[3]

"The transitions involved (allowed and forbidden) in the spectrum of the oxygen atoms in a cometary atmosphere" are 557.7 nm, 630.0 and 636.4 nm, 295.8 and 297.2 nm, 98.9 nm (a triplet), 799.0 nm, 844.7 nm, and 1304 nm (a triplet), 102.7 nm (a triplet) and 1128.7 nm.[4]

"Measurements of the spatial distribution of the hydroxyl radical in cometary atmospheres [may be] made by observations of ultraviolet emission at 309 nm ... The distribution depends upon the velocities of the parent water molecules from which OH is produced by photodissociation and on the lifetime of OH ... The ultraviolet data ... yield a lifetime of OH at 1 AU from the Sun for Comet Bennett (1970 LI) of 2(+1,-1)105 sec (Keller and Lillie, 1974), for Comet Kobayashi-Berger-Milon (1975 IX) a lifetime of 2.3(+1.5,-1.3)105 sec (Festou, 1981b), for Comet Kohoutek (1973 XII) a lifetime of 2(+2,-0.7)105 sec (Blamont and Festou, 1974; Festou, 1981b), and for Comet Bradfield (1979X) a lifetime between 5 x 104 and 1.6 x 105 sec (Weaver et al., 1981 a)."[5]

"Measurements of hydrogen Lyman alpha emission from comets indicate the presence of two populations of hydrogen atoms, one with a velocity of about 20 km sec-1, the second with a velocity of about 8 km sec-1 ... the high-velocity component [may arise] from photodissociation of H2O and the low-velocity component from photodissociation of OH"[5].

Red astronomy

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Armada of "mini-comets" is left behind by Comet LINEAR as observed by the Hubble Space Telescope and the 2.2-meter telescope in Hawaii. Credit: NASA, University of Hawaii, H. Weaver (John Hopkins University).

"My first thought was Hubble Space Telescope does it again! We caught the fish! This is amazing, very exciting, very neat."[6]

"Actually, I would have been more amazed if Hubble saw no pieces ... They just had to be there. The amount of heat available from sunlight just isn't enough to boil away something the size of a mountain in so short a time".[7]

"On July 27th, ground-based observers had lost sight of the bright core of the comet and were suggesting that the nucleus had totally disintegrated into a pile of dust. ... On Weaver's screen was at least a half dozen "mini-comets" with tails, resembling the shower of glowing fireballs from an aerial firework. They are clustered in the lance-head tip of an elongated stream of dust seen from a ground-based telescope."[8]

"The λλ6300, 6363 Auroral red doublet of [OI] has been measured on digital sky-subtracted spectra of nine cometary nuclei ... The cometary oxygen lines are confined to their nuclear source, so that small apertures include much of the oxygen emission, particularly for small comets with Δ ≳ 1.0 AU."[9]

Coronal cloud

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Comet Lovejoy is detected in STEREO/SECCHI's EUVI-A imager's 17.1-nm wavelength. Credit: STEREO/SECCHI image courtesy NASA/NRL.

"[C]omets, such as that of 1843, have approached the sun with enormous velocities within the region of the prominences without suffering disruption or retardation."[10]

The coronal cloud in close proximity to the Sun also emits X-rays that produce visual fluorescence from gases in a comet's coma and tail.

At left is Comet Lovejoy as detected in STEREO/SECCHI's EUVI-A imager's 17.1-nm wavelength. "The comet is clearly visible racing away from the Sun, leaving a wiggly-tail in its wake! Why the wiggles? We're not sure -- we need to start studying that when we get all of the spacecraft data from STEREO-B this weekend. However, we think there may some kind of helical motion going on, or perhaps there's a projection affect and we're seeing tail material magnetically "clinging" to coronal loops and moving with them. There are other possibilities too, though, and we will certainly investigate those! We should have equivalent images from the STEREO-A spacecraft which we will also get this weekend. When we pair these together, and throw in the SDO images too, we should be able to get an incredibly unique 3-D picture of how this comet is reacting the the intense coronal heat and magnetic loops."[11]


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The sodium tail of Mercury is mapped out during the MESSENGER's first flyby on January 14, 2008. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.

At right is a diagram of Mercury's sodium tail. "As the MESSENGER spacecraft approached Mercury, the UVVS field of view was scanned across the planet's exospheric "tail," which is produced by the solar wind pushing Mercury's exosphere (the planet's extremely thin atmosphere) outward. This figure, recently published in Science magazine, shows a map of the distribution of sodium atoms as they stream away from the planet (see PIA10396); red and yellow colors represent a higher abundance of sodium than darker shades of blue and purple, as shown in the colored scale bar, which gives the brightness intensity in units of kiloRayleighs. The escaping atoms eventually form a comet-like tail that extends in the direction opposite that of the Sun for many planetary radii. The small squares outlined in black correspond to individual measurements that were used to create the full map. These measurements are the highest-spatial-resolution observations ever made of Mercury's tail. In less than six weeks, on October 6, 2008, similar measurements will be made during MESSENGER's second flyby of Mercury. Comparing the measurements from the two flybys will provide an unprecedented look at how Mercury's dynamic exosphere and tail vary with time."[12]

Interplanetary medium

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Def. "[t]hat part of outer space between the planets of a solar system and its star"[13] is called interplanetary space.

Def. "the material which fills the solar system and through which all the larger solar system bodies such as planets, asteroids and comets move"[14] is called an interplanetary medium.

"It is found that near 1 AU, the dominant group of the local geometrical cross section changes."[15] Approximately 80 % of interplanetary dust is cometary at R ~ 0.8 AU.


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Most meteors are actually pieces of rock that have broken off a comet and continue to orbit the Sun. The Earth travels through the comet debris in its orbit. As the small pieces enter the Earth's atmosphere, friction causes them to burn up.


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For elongated dust particles in cometary comas an investigation is performed at 535.0 nm (green) and 627.4 nm (red) peak transmission wavelengths of the Rosetta spacecraft's OSIRIS Wide Angle Camera broadband green and red filters, respectively.[16] "In the green, the polarization of the pure silicate composition qualitatively appears a better fit to the shape of the observed polarization curves".[16] "[B]ut they are characterized by a high albedo."[16] The silicates used to model the cometary coma dust are olivene (Mg-rich is green) and the pyroxene, enstatite.[16]


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"This vast electric mass must have a great electric repulsion through vacant space, and it lends probability to my position that it drives away from the sun the tails of comets and our zodiacal light and aurora borealis."[17]

"Electricity alone can repel electricity. ... the direction of the comets' tails is but the interaction between the sun and the comets, the same as the action between a charged prime conductor and a charged pith ball of an electric machine."[17]


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"An image of comet Hale-Bopp (C/1995 O1) in soft x-rays reveals a central emission offset from the nucleus, as well as an extended emission feature that does not correlate with the dust jets seen at optical wavelengths."[18]


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X-ray emission from Hyakutake is seen by the ROSAT satellite. Credit: NASA.

"One of the great surprises of Hyakutake's passage through the inner Solar System was the discovery that it was emitting X-rays [image at left], with observations made using the ROSAT satellite revealing very strong X-ray emission.[19] This was the first time a comet had been seen to do so, but astronomers soon found that almost every comet they looked at was emitting X-rays. The emission from Hyakutake was brightest in a crescent shape surrounding the nucleus with the ends of the crescent pointing away from the Sun."[20]

Comet Lulin

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This is a composite image of Comet Lulin. X-ray emission is in red. Credit: .

"Comet Lulin was passing through the constellation Libra when Swift imaged it on January 28, 2009. The image at right merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the Sun."[21]

"NASA's Swift Gamma-ray Explorer satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind -- a fast-moving stream of particles from the sun -- interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.[22]"[21]

Comet West

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Comet West is photographed on March 6, 2006. Credit: Jlsmicro.
Visual photograph of Comet West in early March 1976 shows red gases coming off the comet's head. Credit: Peter Stättmayer (Munich Public Observatory) and ESO.

At left is an image of Comet West. "Comet West was a stunning sight in the predawn sky of March, 1976, bright with a tall and broad dust tail. ... [T]he comet [was] discovered on photographs taken in August 1975 by Richard West of the European Southern Observatory ... Comet West passed perihelion on February 25, 1976, at a distance of 0.20 a.u. [and] had reached about magnitude -3 at perihelion. Several observers saw it telescopically in daylight, and John Bortle observed it with the naked eye shortly before sunset. ... The following morning, March 7, ... It was brilliant, with a head as bright as Vega (which was nearly overhead) and a huge tail, about 20 degrees tall, straight near the bottom and bending to the left in its upper reaches. The comet quickly faded during March".[23]

Halley's comet

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This is a photograph taken in 1910 during the passage of Halley's comet. Credit: The Yerkes Observatory.

"The 1910 approach [of Halley's comet], which came into naked-eye view around 10 April[24] and came to perihelion on 20 April,[24] was notable for several reasons: it was the first approach of which photographs exist, and the first for which spectroscopic data were obtained.[25] Furthermore, the comet made a relatively close approach of 0.15AU,[24] making it a spectacular sight. Indeed, on 19 May, the Earth actually passed through the tail of the comet.[26][27] One of the substances discovered in the tail by spectroscopic analysis was the toxic gas cyanogen,[28] which led astronomer Camille Flammarion to claim that, when Earth passed through the tail, the gas "would impregnate the atmosphere and possibly snuff out all life on the planet."[29] His pronouncement led to panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.[30] In reality, as other astronomers were quick to point out, the gas is so diffuse that the world suffered no ill effects from the passage through the tail.[29]"[31]

"It is quite possible that [faint streamers preceding the main tail and lying nearly in the prolonged radius vector] may have touched the Earth, probably between May 19.0 and May 19.5, [1910,] but the Earth must have passed considerably to the south of the main portion of the tail [of Halley's comet]."[32]

Of the other planets of the solar system, Mercury, Mars, Jupiter, Saturn, Uranus, and Neptune, none has apparently produced as much drama and excitement recently on Earth among some of the intelligent life forms as Halley's comet.

A magnetohydrodynamics (MHD) and chemical comet-coma model is applied to describe and analyze the plasma flow, magnetic field, and ion abundances in Comet Halley.[33] A comparison of model results is made with the data from the Giotto mission.[33]

"In the second dominant group of ions we generally see more discrepancies in the model and the HIS data".[33]

The principal application of the dominant group concept is to the ion density measurements at or within 1500 km of the comet nucleus, where "the model abundances for the light ions, up to 21 amu, are in very good agreement with the 1500 km observations."[33]

The comparison between model and measurements "generally becomes worse as one considers higher molecular masses and greater distance from the [comet] nucleus."[33]


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This ultraviolet image of Jupiter is taken by the Wide Field Camera of the Hubble Space Telescope. Credit: NASA/Hubble Space Telescope Comet Team.

The image at lower right "shows Jupiter's atmosphere at a wavelength of 2550 Angstroms after many impacts by fragments of comet Shoemaker-Levy 9. The most recent impactor is fragment R which is below the center of Jupiter (third dark spot from the right). This photo was taken 3:55 EDT on July 21, about 2.5 hours after R's impact. A large dark patch from the impact of fragment H is visible rising on the morning (left) side. Proceding to the right, other dark spots were caused by impacts of fragments Q1, R, D and G (now one large spot), and L, with L covering the largest area of any seen thus far. Small dark spots from B, N, and Q2 are visible with careful inspection of the image. The spots are very dark in the ultraviolet because a large quantity of dust is being deposited high in Jupiter's stratosphere, and the dust absorbs sunlight."[34]

Oort cloud

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Here, the presumed distance of the Oort cloud is compared to the rest of the Solar System using the orbit of Sedna. Credit: NASA / JPL-Caltech / R. Hurt.
Sedna, a possible inner Oort cloud object, is a discovery in 2003. Credit: NASA.

Def. a "roughly spherical region of space from 50,000 to 100,000 astronomical units (approximately 1 light year) from the sun; supposedly the source of most comets"[35] is called an Oort cloud.

"The Oort cloud ... or the Öpik–Oort cloud[36] ... is a hypothesized spherical cloud of comets which may lie roughly 50,000 AU, or nearly a light-year, from the Sun.[37] This places the cloud at nearly a quarter of the distance to Proxima Centauri, the nearest star to the Sun. ... The outer limit of the Oort cloud defines the cosmographical boundary of the Solar System and the region of the Sun's gravitational dominance.[38]"[39]

Cometary capture

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In addition to sources of radiation, there are sources of objects such as the Oort cloud, the Kuiper belt, and the asteroid belt. These may have been formed at the beginning of the solar system or be a product or partial product of the solar binary consisting of the Sun and Jupiter. Such a solar binary may serve to establish an upper limit for interstellar cometary capture.

Comet-like objects

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This ultraviolet-wavelength image mosaic, taken by NASA's GALEX, shows a comet-like "tail" stretching 13 light years across space. Credit: NASA.
The red light depicts nitrogen emission ([N II] 658.4 nm); green, hydrogen (H-alpha, 6563A); and blue, oxygen (5007A). These are "cometary knots" in the Helix nebula. Credit: NASA Robert O Dell Kerry P. Handron Rice University, Houston Texas.

In the centered image is an ultraviolet-wavelengthmosaic, taken by NASA's GALEX, which shows a comet-like "tail" stretching 13 light years across space.

At right is an image gaseous objects ("cometary knots") discovered in the thousands. These knots are imaged with the Hubble Space Telescope while exploring the Helix nebula, the closest planetary nebula to Earth at 450 light-years away in the constellation Aquarius. Although ground-based telescopes have revealed such objects, astronomers have never seen so many of them. The most visible knots all lie along the inner edge of the doomed star's ring, trillions of miles away from the star's nucleus. Although these gaseous knots appear small, they're actually huge. Each gaseous head is at least twice the size of our solar system; each tail stretches for 100 billion miles, about 1,000 times the distance between the Earth and the Sun. The image was taken in August 1994 with Hubble's Wide Field Planetary Camera 2. The red light depicts nitrogen emission ([NII] 658.4 nm).

Planetary astronomy

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"Energetic photons, ions and electrons from the solar wind, together with galactic and extragalactic cosmic rays, constantly bombard surfaces of planets, planetary satellites, dust particles, comets and asteroids."[40]

"The advantages of radar in planetary astronomy result from (1) the observer's control of all the attributes of the coherent signal used to illuminate the target, especially the wave form's time/frequency modulation and polarization; (2) the ability of radar to resolve objects spatially via measurements of the distribution of echo power in time delay and Doppler frequency; (3) the pronounced degree to which delay-Doppler measurements constrain orbits and spin vectors; and (4) centimeter-to-meter wavelengths, which easily penetrate optically opaque planetary clouds and cometary comae, permit investigation of near-surface macrostructure and bulk density, and are sensitive to high concentrations of metal or, in certain situations, ice."[41]


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Suisei (the Japanese name meaning `Comet') was launched on August 18, 1985 into heliocentric orbit to fly by Comet P/Halley. Credit: NASA.

"Suisei [at left] began UV observations in Nov. 1985, generating up to 6 images/day. The spacecraft encountered Comet P/Halley at 151,000 km on sunward side during March 8, 1986, suffering only 2 dust impacts."[42]

"Suisei took a lot of UV pictures of the neutral hydrogen corona around the comet [Comet P/Halley]".[43]

See also

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  1. Ginger Lehrman and Ian B Hogue, Sarah Palmer, Cheryl Jennings, Celsa A Spina, Ann Wiegand, Alan L Landay, Robert W Coombs, Douglas D Richman, John W Mellors, John M Coffin, Ronald J Bosch, David M Margolis (August 13, 2005). "Depletion of latent HIV-1 infection in vivo: a proof-of-concept study". Lancet 366 (9485): 549-55. doi:10.1016/S0140-6736(05)67098-5. Retrieved 2012-05-09. 
  2. Anil Bhardwaj, Ronald F. Elsner, G. Randall Gladstone, Thomas E. Cravens, Carey M. Lisse, Konrad Dennerl, Graziella Branduardi-Raymont, Bradford J. Wargelin, J. Hunter Waite Jr., Ina Robertson, Nikolai Østgaard, Peter Beiersdorfer, Steven L. Snowden, Vasili Kharchenko (June 2007). "X-rays from solar system objects". Planetary and Space Science 55 (9): 1135-89. doi:10.1016/j.pss.2006.11.009. Retrieved 2013-05-23. 
  3. 3.0 3.1 3.2 S.R. Federman, David L. Lambert (May 2002). "The need for accurate oscillator strengths and cross sections in studies of diffuse interstellar clouds and cometary atmospheres". Journal of Electron Spectroscopy and Related Phenomena 123 (2-3): 161-71. Retrieved 2013-01-20. 
  4. 4.0 4.1 4.2 M. C. Festou and P. D. Feldman (November 1981). "The Forbidden Oxygen Lines in Comets". Astronomy & Astrophysics 103 (1): 154-9. 
  5. 5.0 5.1 Ewine F. Van Dishoeck and A. Dalgarno (September 1984). "The Dissociation of OH and OD in Comets by Solar Radiation". Icarus 59 (3): 305-13. doi:10.1016/0019-1035(84)90104-0. Retrieved 2013-01-21. 
  6. Harold Weaver (August 7, 2000). "Hubble Discovers Missing Pieces of Comet Linear". Baltimore, Maryland USA: Hubblesite - Newscenter. Retrieved 2013-05-02.
  7. Carey Lisse (August 7, 2000). "Hubble Discovers Missing Pieces of Comet Linear". Baltimore, Maryland USA: Hubblesite - Newscenter. Retrieved 2013-05-02.
  8. Ray Villard and Michael Purdy (August 7, 2000). "Hubble Discovers Missing Pieces of Comet Linear". Baltimore, Maryland USA: Hubblesite - Newscenter. Retrieved 2013-05-02.
  9. Hyron Spinrad (December 1982). "Observations of the red auroral oxygen lines in nine comets". Publications of the Astronomical Society of the Pacific 94 (12): 1008-16. doi:10.1086/131101. Retrieved 2013-01-21. 
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  11. Karl Battams (December 2, 2011). "The Great "Birthday Comet" of 2011, Chapter 2: Survival". Washington, DC, USA: Naval Research Laboratory. Retrieved 2013-07-07.
  12. Sue Lavoie (August 26, 2008). "PIA11076: Exploring Mercury's "Tail"". Pasadena, California USA: NASA/JPL. Retrieved 2013-07-09.
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  15. Hiroshi Ishimoto (June 1998). "Collisional evolution and the resulting mass distribution of interplanetary dust". Earth, Planets, and Space 50 (6): 521-9. 
  16. 16.0 16.1 16.2 16.3 I. Bertini, N. Thomas, and C. Barbieri (January 2007). "Modeling of the light scattering properties of cometary dust using fractal aggregates". Astronomy & Astrophysics 461 (1): 351-64. doi:10.1051/0004-6361:20065461. Retrieved 2011-12-08. 
  17. 17.0 17.1 Jacob Ennis (August 1878). "Electricity and the Solar System". Astronomical Register 16: 255-6. 
  18. Vladimir A. Krasnopolsky, Michael J. Mumma, Mark Abbott, Brian C. Flynn, Karen J. Meech, Donald K. Yeomans, Paul D. Feldman, Cristiano B. Cosmovici (September 5, 1997). "Detection of Soft X-rays and a Sensitive Search for Noble Gases in Comet Hale-Bopp (C/1995 O1)". Science 277 (5331): 1488-91. doi:10.1126/science.277.5331.1488. PMID 9278508. Retrieved 2013-05-21. 
  19. J Glanz (1996). "Comet Hyakutake Blazes in X-rays". Science 272 (5259): 194–0. doi:10.1126/science.272.5259.194. 
  20. "Comet Hyakutake". Wikipedia (San Francisco, California: Wikimedia Foundation, Inc). March 12, 2013. Retrieved 2013-05-21. 
  21. 21.0 21.1 "Astrophysical X-ray source". Wikipedia (San Francisco, California: Wikimedia Foundation, Inc). June 12, 2012. Retrieved 2012-06-28. 
  22. F Reddy. "NASA's Swift Spies Comet Lulin".
  23. Tony Hoffman. "Comet West: The Great Comet of 1976". Earthlink. Retrieved 2013-05-02.
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  26. Ian Ridpath (1985). "Through the comet's tail". Revised extracts from A Comet Called Halley by Ian Ridpath, published by Cambridge University Press in 1985. Retrieved 2011-06-19.
  27. Brian Nunnally (May 16, 2011). "This Week in Science History: Halley's Comet". pfizer: ThinkScience Now. Retrieved 2011-06-19.
  28. "Yerkes Observatory Finds Cyanogen in Spectrum of Halley's Comet". The New York Times. 8 February 1910. Retrieved 15 November 2009.
  29. 29.0 29.1 "Ten Notable Apocalypses That (Obviously) Didn't Happen". Smithsonian magazine. 2009. Retrieved 14 November 2009.
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  32. Heber D. Curtis (June 1910). "Photographs of Halley's Comet made at the Lick Observatory". Publications of the Astronomical Society of the Pacific 22 (132): 117-30. 
  33. 33.0 33.1 33.2 33.3 33.4 R. Wegmann, H.U. Schmidt, W.F. Huebner, and D.C. Boice (November 1987). "Cometary MHD and chemistry". Astronomy and Astrophysics 187 (1-2): 339-50. 
  34. Space Telescope Image of Fragment BDGLNQ12R Impacts.jpg "File:Hubble Space Telescope Image of Fragment BDGLNQ12R Impacts.jpg". Wikimedia Commons (San Francisco, California: Wikimedia Foundation, Inc). October 31, 2010. Space Telescope Image of Fragment BDGLNQ12R Impacts.jpg. Retrieved 2012-07-20. 
  35. "Oort cloud". Wiktionary (San Francisco, California: Wikimedia Foundation, Inc). June 16, 2013. Retrieved 2013-08-04. 
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  37. Alessandro Morbidelli (2006). "Origin and dynamical evolution of comets and their reservoirs of water ammonia and methane". arXiv:astro-ph/0512256.
  38. "Kuiper Belt & Oort Cloud". NASA Solar System Exploration web site. NASA. Retrieved 2011-08-08.
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  40. Theodore E. Madey, Robert E. Johnson, Thom M. Orlando (March 2002). "Far-out surface science: radiation-induced surface processes in the solar system". Surface Science 500 (1-3): 838-58. doi:10.1016/S0039-6028(01)01556-4. Retrieved 2012-02-09. 
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Further reading

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{{Astronomy resources}}