Comets and Asteroids

Jasmine Grondin, Katherine Jamieson, Sandrew Martins, Crystal Montoya, Madison Ogilvy, Victoria Oster

Since humans have looked at the night sky, we have wondered about the mysteries surrounded in darkness. Some of the fascinating aspects of the cosmos are comets, asteroids and the paths they follow through our solar system. Many similarities and differences exist between asteroids and comets. One way to highlight these differences is to use well-known asteroids and comets to help compare and contrast asteroids and comets. The discussion of these objects aims to answer how asteroids and comets are different based on various properties, including physical appearance, orbit, chemical composition, and impact on other objects in the solar system.

Asteroids

Asteroids are the irregularly shaped, pitted, atmosphere-less rocky remains of the solar system’s creation some 4.6 billion years ago.^1,2 Occasionally referred to as minor planets, asteroids are found primarily in the Asteroid Belt located between Mars and Jupiter. The sizes of these minor planets can range from a diameter of over 500 kilometres to less than 10 metres. These asteroids move on elliptical orbits, and some (more than 150) have been shown to have companion moons.¹

Asteroid names are commonly associated with an accompanying number that indicates what order they were found in. For example, 1 Ceres was the first asteroid to be discovered, 4 Vesta was the fourth asteroid to be discovered, 16 Psyche was the 16th asteroid to be discovered, and so on.³ The Asteroid belt, home of the majority of asteroids, is discussed below giving a better idea of where these objects are located. The first asteroids identified were named after goddesses in classical mythology. However, because so many were discovered this was no longer possible and so their naming practices have become regulated by the International Astronomical Union (IAU). This system allows the discoverer of the object to suggest a name which is then judged by a committee that determines its suitability.³

Composition

Asteroids come in three compositions classes C-,S-, and M-types. These compositional differences relate to their distance from the Sun during their formation.¹ Some asteroids were exposed to high temperatures after their formation and partially melted causing the denser materials to sink to the core and the lighter materials to rise to the surface, a process we now call differentiation.¹

C-type: (chondrite) are most common and are found to be dark in appearance, and comprised of a mixture of silicate rocks and clay.¹

S-type: (stony) are made of Silicate and Nickel-Iron.¹

M-type: (metallic) are made of Nickel-Iron.¹

Classification

Asteroids have three main classifications, which are determined by their location within the solar system. These are the main Asteroid Belt, Trojans, and Near Earth Asteroids.

Main Asteroid Belt: This location contains approximately 1.1-1.9 Million Asteroids.¹

Trojans: Asteroids that share an orbit with a planet but do not collide with it due to their location at stable positions where gravitational forces balance, called Lagrangian points.¹

Near Earth Asteroids: These are Asteroids with orbits that pass close to the Earth. Those that cross paths with the earth’s orbit are referred to as ‘Earth-crossers’.¹

Comets

Although sometimes associated with asteroids, comets have important differences which will explained more in depth. Comets are often described as large ‘snowballs’ of frozen gas, rock, and dust that were left after the creation of the Solar System some 4.6 billion years ago. These space snowballs reside primarily in the Oort Cloud which is located approximately 100,000 Astronomical Units (AU) from the Sun,⁴ or else in the Kuiper Belt which is located beyond Neptune. To put this distance into perspective, one Astronomical Unit (AU) is the mean distance between the Earth and the Sun, putting the Oort Cloud at approximately 100,000 times that distance.⁴ The Oort Cloud is a theoretical cloud said to exist beyond the Sun’s heliosphere.⁵ First hypothesized in 1932 by Estonian astronomer Ernst Öpik, the concept for this spherical cloud was revisited in 1950 by Jan Oort.⁵ Although the existence of the Oort cloud has not been verified through direct observation, it is widely accepted in the scientific community.⁵ It is from this Oort Cloud that many short and long-period comets are believed to have originated.⁵ While not able to sustain life themselves, comets are hypothesized to have brought organic compounds and water to Earth through a series of collisions.^4,6

Comets, when orbiting too close to the Sun, begin to heat up forming their own atmosphere or coma. The heat then expands the coma as the frozen gases are released which causes the characteristic ‘tail’. While many of these comets orbit at a safe distance from the Sun, there are some that collide with the Sun or get so close that they evaporate.^4,6

Classification

Long-period comets: primarily from the Oort Cloud, these can take up to millions of years to orbit the Sun.⁴

Short-period comets: Comets pushed into orbits so they are closer to the Sun and have a significantly shorter orbital period of ~200 years.⁴ (Note that an orbital period is just the object’s equivalent to a year.)

Comet and Asteroid Evolution

Comets and asteroids have undergone three types of evolution since their formation: gravitational encounters, collisions, and thermal processes.⁷

Gravitational encounters occur when a comet or asteroid encounters a large body (planet) and its gravitational effects cause a change in the orbit of the comet or asteroid. This can result in an ejection from the solar system or a collision with another object. It is this gravitational force that is thought to have caused the population of the hypothetical Oort cloud.⁷

Collisions occur and alter the size and number of comets and asteroids.⁷

Thermal processes can alter the physical, chemical, and geological characteristics of the bodies. An excellent example is the production of the comet’s ‘tail’ when it penetrates the inner solar system and heated by the Sun.⁷

 

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When considering how asteroids and comets are different based on various properties, including physical appearance, orbit, chemical composition, and impact on other objects in the solar system, it is beneficial to outline the difference between these two objects by looking at specific examples of each. This serves to not only highlight the differences and similarities within each object type, but also to outline the differences and similarities between asteroids and comets. However, before coming to that we will first review what is known about the Asteroid belt and the Kuiper belt in general, to provide context for discussion about specific comets and asteroids within our solar system.

The Asteroid Belt

The Asteroid Belt, found between the planets Mars and Jupiter, is comprised primarily of asteroids, although there is some speculation that a small collection of comets, or objects showing comet like activity, also reside within the belt.⁸ These comets or objects showing comet like activity have been observed through both 1-2m or 8-10m telescopes in Hawaii, Chile, and Taiwan.⁸ This ‘main belt’, as it is commonly known, is comprised of the largest population of asteroids. The belt contains approximately 5 X 10^-4 Earth masses of material, together that is about enough to form a single body with a mass that is roughly 1/20 of Earth’s moon’s.⁹ The mass of all the asteroids in the belt weighs only three times the mass of Ceres, the largest asteroid.¹⁰ Along with being smaller in mass, the asteroids in the Asteroid Belt move close to the ecliptic (the path of the Sun through the sky)¹¹ and are between 2.3 and 3.3 AU from the Sun.¹²

Pioneer 10 was the first spacecraft to fly through the Asteroid Belt. On July 15^th, 1972 Pioneer 10 entered the Asteroid Belt and emerged in February 1973 after completing its 435 kilometer trip through the belt.¹³

The Kuiper Belt

The Kuiper Belt is a region of the solar system that exists past Neptune, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It has similarities to the asteroid belt, in that it contains numerous small bodies that are remnants from the solar system’s formation.¹⁴ After the discovery of Pluto by Clyde Tombaugh in 1930, astronomers began to discuss the possible existence of a population of objects in the outer solar system beyond Neptune.

The composition of the objects in the Kuiper Belt are speculated to be made of different forms of ice and hydrocarbons. Hydrocarbons are the simplest organic compounds, containing only hydrogen and carbon. Also, varying in colours from light blue to deep red, it is difficult to definitively say what the composition is because of how far away the objects are from the Earth.¹⁴

New Horizons is a satellite that launched on January 19, 2006. In February 2007 it travelled past Jupiter and then flew by Pluto and its moons in July 2015. The spacecraft is expected to head farther into the Kuiper Belt to examine one or more other Kuiper Belt objects. Sending a spacecraft on this long journey is helping us to answer basic questions about the surface properties, geology, interior makeup, and atmospheres on these bodies.¹⁵

This video provides additional information about the Kuiper Belt.

Now that we have provided a context and location for these objects within our solar system, let us explore in detail several asteroids and comets.

Vesta

Vesta was discovered on March 29, 1807, by Heinrich Wilhelm Olbers of Germany.¹⁶ Olbers gave the honor of naming the new asteroid to German mathematician Carl Friedrich Gauss, who had computed its orbit using a complex mathematical formula. Gauss named it Vesta after the goddess of the hearth and household in Roman mythology.¹⁷ The asteroid’s official name is “4 Vesta” because it was the fourth asteroid discovered.¹⁶

NASA’s Dawn Spacecraft, launched in 2007, extensively studied Vesta from July 16th, 2011 to September 5th, 2012.¹⁷ The purpose of this mission was to comprehensively map Vesta before moving on to investigate the dwarf planet Ceres in 2015.¹⁸ It is from this mission that we know so much about the surface, composition, and early history of Vesta.

Vesta is the second most massive asteroid in the main belt between Mars and Jupiter.   It was from the Dawn mission that Vesta was discovered to have formed early, within 1 to 2 million years of the birth of the Solar System.¹⁷ This explains why the asteroid has separated into crust, mantle, and core similar to Earth. The object was heated so much by the short-lived radioactive material that was incorporated into bodies that formed around this time that it melted causing denser material to sink into the core and everything else to rise, a process known as differentiation.¹⁷ Vesta is about 578 x 560 x 458 kilometers.¹⁶ The asteroid is almost spherical with the Rheasilvia crater, a giant crater 460 kilometers across and 13 kilometers deep, at the asteroid’s south pole.¹⁶ The massive collision with another asteroid that created the Rheasilvia crater gouged out one percent of Vesta’s mass, blasting over one-half million cubic miles of debris into space.¹⁶ A class of meteorites, such as Howardite, Eucrite, and Giogenite, which are spectroscopically identical to Vesta are thought to be fragments of the asteroid that created the Rheasilvia crater. In fact, scientists believe that about five percent of all meteorites we find on Earth are a result of this single crash.¹⁹ Another huge crater is Veneneia, which is about 400 km in diameter. An extensive system of troughs encircles Vesta’s equatorial region, the largest named Divalia Fossa.¹⁷

As mentioned above, unlike most known asteroids, Vesta has separated into crust, mantle and core.¹⁷ As a side note, not all asteroids are differentiated since many are very small, the examples considered here all are thought to have been differentiated because they are some of the largest ones. Vesta may once have been geologically active, with lava flowing on its surface when it was young.¹⁹ It appears to have a surface of basaltic rock (frozen lava), which oozed out of the asteroid’s presumably hot interior shortly after its formation 4.5 billion years ago and has remained largely intact ever since.¹⁶ Telescopic observations reveal mineralogical variations across its surface.¹⁶ Geologists interpreted the patterns in the mineralogical image to suggest that Vesta probably melted all the way through early in its history, which was later verified by the Dawn mission in 2011-2012. Vesta has a small positive gravity anomaly, meaning it exhibits more gravity than expected, indicating that the material there is denser, perhaps originating from deep within the asteroid.¹⁹ Vesta has one of the largest brightness ranges observed on any rocky body in our solar system. The bright materials on Vesta seem to be native rocks and the dark material is thought to have been deposited by other asteroids crashing into Vesta. From the Dawn team’s examination of the asteroid they found that likely about 300 dark asteroids hitting Vesta would be enough to wrap it in the dark material.¹⁷ Vesta has a rotation period of once every 5 hours, 20 minutes.¹⁶

Ceres

 

The Sicilian monk Giuseppe Piazzi discovered the first asteroid on January 1, 1801; it was later named Ceres after the Roman goddess of the harvest.¹⁹ Astronomers know the object as “1 Ceres” because it was the very first minor planet discovered.¹⁶ Ceres was initially classified as a planet and later classified as an asteroid.²⁰ Ceres has now been reclassified as a dwarf planet because it has enough gravitational strength to squeeze itself into a spherical shape but not enough to have swept up or cleared away the rest of the asteroids in the main belt.¹⁹

Our information about Ceres is mostly determined through observations from images taken from the Hubble telescope and Herschel telescope, infrared images taken by the Keck telescope, and from the NASA spacecraft Dawn when it entered orbit around Ceres in early 2016. This NASA spacecraft is expected to remain there until the conclusion of the Dawn mission. One of the main goals of Dawn’s orbit of Ceres is to determine the chemical elements on the surface, as well as various other physical properties of the asteroid, using its gamma-ray and neutron detector and a variety of other instruments.¹⁸

Ceres is 950 km in diameter (approximately the size of Texas²⁰) and orbits within the Asteroid Belt.¹⁹ Ceres comprises nearly 25 percent of the asteroid belt’s total mass.²⁰ Ceres’ nearly spherical body has a differentiated interior, meaning that it has denser material at the core and lighter minerals near the surface. Astronomers believe that water ice may be buried under Ceres’ crust because its density is less than that of the Earth’s crust, and because the dust-covered surface bears spectral evidence of water-bearing minerals.¹⁶ The density of Ceres and reflection spectra of its surface indicate it is composed of materials containing carbon as well as ice.¹⁹ Ceres could even possess frost-covered polar caps. Astronomers estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres’ water is expected to be in the form of water ice located in its mantle.¹⁶ Observations of Ceres with the Herschel infrared space telescope revealed water vapour venting from two regions, creating a very thin atmosphere.¹⁹ Ceres rotates once every 9 hours, 4.5 minutes.¹⁶

The following video explores many of the features of Ceres’ surface. The detailed look at Ceres was made possible by NASA’s Dawn spacecraft.

Psyche

16 Psyche is an asteroid found in the asteroid belt. Named after Psyche from Greek mythology, this asteroid was discovered on March 17, 1852, by the Italian astronomer Annibale de Gasparis.²¹ Psyche is the most massive M-type asteroid discovered.²²

Psyche is one of the largest asteroids in the asteroid belt. It contains just under 1% of the entire mass of the asteroid belt and has a diameter of 253.16 kilometers.²² Psyche seems to be ellipsoidal shaped with an axial tilt of about 95 degrees.²³

Psyche has an orbital period of 4.99 Earth years and has an average orbital speed of 17.34 km/h.²⁴

Psyche has been determined to be an M-type asteroid as radar observations of it have shown that it has a high radar reflectivity and high density. When compared with the reflectivity of various metals the comparison indicates that the asteroid contains a fairly pure iron-nickel composition.²⁵ However, unlike other M-type asteroids, Psyche does not show signs of having water or water-bearing materials on the surface.²⁶

Due to Psyche’s almost completely metallic body, similar to the Earth’s own metallic core, and to the fact that this is the only metallic core-like body discovered in the Solar System, it is sometimes speculated that the asteroid is a survivor of a protoplanet which has become differentiated. The hypothesis is that violent collisions stripped the outer layers off of Psyche leaving only the metallic interior.²² As of yet this theory remains quite speculative – most of the information we have about Psyche comes from radar observations from Earth, helping determine the shape and surface properties.

In 2014, a mission to Psyche was proposed to NASA and on September 30, 2015, was selected as one of the five semi-finalists for the Discovery Program. This proposed mission would send a robotic spacecraft to orbit Psyche for six months. The goal would be to obtain further information about topography, surface features, gravity, and magnetism.^22,27

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As you can see from the discussion on Vesta, Ceres, and Psyche asteroids tend to be large objects comprised of metal and rock. As you will learn, this differs in many ways from comets. Let us now explore two comets from within our solar system.

Halley’s Comet

In 1986, when space exploration was at its political height, four probes were sent into space to explore the coma of Halley’s comet. Probes were sent from Russia, Japan, and Europe.²⁸ The Giotto mission, using a European space probe, was a mission designed to take photos and collect information about the volatility of the coma, and determine the composition of particles in the coma.²⁹ The probe was very successful as evidence was found explaining what matter is in the dust trail of a comet.²⁸ This mission also helped scientists to differentiate between a meteor and a comet.²⁸ Prior to the discoveries of Giotto, most of the knowledge was shaped from speculation. What is known about Halley’s comet has set a precedent for what is known today about comets.

Halley’s Comet is possibly the most well known comet.²⁸ Halley’s comet’s namesake is after the British scientist, Edmund Halley, who published the periodicity of Halley’s Comet in 1705. Halley was able to apply Newton’s principles to the orbit of the comet.³⁰ When Halley calculated his observations he was able to determine the orbital cycle, which is approximately a 76 year orbit.²⁸ Halley’s comet is the first observed comet determined to be a short-term comet, meaning that its orbital period is shorter than 200 years. There are texts which trace the origin of observation for Halley’s orbit to 200 BC with Chinese Astronomers, and to 400-500 BC with Greek Astronomers.³¹ Though Halley’s Comet was observed by many people throughout history, only the orbital estimates of Edmund Halley (in 1682) received recognition. It is estimated that that Halley became a short-term comet about 200, 000 years ago.³² Halley’s comet is unique because it is a short-term comet with a retrograde orbit. Scientists are uncertain how Halley’s comet entered into its orbit. A current theory is that Halley’s comet may have been in the Oort Cloud and through gravitational attraction, been pulled closer to the sun. ³² From gravitational influence, Halley’s comet began its retrograde orbit. Halley’s Comet is classified as a short-period comet because it has an orbit which is less than 200 years.³³ Halley’s Comet’s orbit is called retrograde because it orbits opposite to the direction of the planetary orbits around the Sun.³¹ The last time Halley was close to Earth was in 1986 and it is predicted to return in 2061.³²

Halley is a smaller comet. Its dimensions are approximately 16 x 8 x 8 kilometers in dimension. Its form has been compared to a peanut shape.²⁸ Though Halley’s comet is smaller, in comparison to other comets, as stated earlier, its tail can stretch very far.³⁴

The following video shows the five exploratory spacecraft that have studied Comet Halley: Giotto, Vega 1 and 2, Planet A, and Sakigaki.

19P/Borrelly Comet

One of the comets that moves through our solar system is Borrelly’s comet, which was discovered by Alphonse Louis Nicolas Borrelly in 1904. Alphonse Borrelly described this comet as having a diameter of 2 arc minutes and a tail of 10 arc minutes.³⁵ Borrelly’s comet has an orbital period of 6.91 years which, when compared to short-term comets with an average of 200 years, is extremely fast.³⁵ When looking at comets with long-period orbital periods, which can be thousands of years, it is clear that Comet Borrelly has an extremely short orbital path.

In 2001, NASA’s Deep Space 1 probe (DS1) mission completed a successful flyby navigation of Comet Borrelly’s rocky, icy core. This is special because Comet Borrelly is one of the few comets that has had a successful study of its nucleus. The information discovered about the nucleus was determined through instruments onboard DS1 measuring the composition of the gases around the nucleus.³⁶ What surprised scientists is that the nucleus shows a variety of terrains and surface textures.³⁶

Another interesting fact of Borrelly is that it showed scientists that the nucleus of a comet is not central in relation to the solar winds. It makes sense to think that the nucleus should be located at the centre of the comet, but it is surprising to find that this is not often the case. Often the nucleus is off to one side shooting jets of material. As the comet gets closer to the sun, some of the material comes off of the nucleus which creates a coma. This coma material is what forms a cloud-like tail which allows us to see it from Earth.³⁷

Borrelly has shown scientists that many of these comets, like Halley’s comet above share similar characteristics. Not only do they have similar movements, but they seem to be made up of similar icy material. Despite these similarities, there are certainly differences among comets such as their size and shape.

Conclusion

From looking at both comets and asteroids you can begin to see similarities emerge. One main similarity found between these two object types is that they both contain rock. Both comets and asteroids range in size and shape, however it is important to note that one object type is not uniformly larger than the other–the individual sizes of comets and asteroids are highly variable. However, the largest asteroids are bigger than the largest comets. This may be due to differences in orbits where comets may pass closer to the sun prompting them to partially melt, creating their characteristic ‘tail’.   Another similarity between these two object types is the ways in which information about them is discovered. Telescopes, radar observations, and unmanned spacecraft missions such as Pioneer 10, New Horizons, Dawn, and DS1 have aided in learning more about both asteroids and comets.

The examples above also highlight some of the differences that exist between comets and asteroids. Asteroids tend to be comprised of varying ratios of both metal and rock, some have more rock like Vesta and Ceres, while others, like Psyche, are more metallic. Comets, however, are primarily comprised of ice, dust, and rock. Asteroids tend to maintain their size over time since they follow more circular orbits far from the Sun, while comets travel closer to the Sun causing them to get smaller each time they pass by it. This is because each time they pass by the Sun the heat melts the ice on them a little bit causing debris to be released and the comet’s tail to be formed. This release of debris can be the cause of meteor showers on Earth.

Radar and unmanned spacecraft missions have aided in the understanding of how asteroids and comets are different based on various properties, including physical appearance, orbit, chemical composition, and impact on other objects in the solar system. It is through these means that we have been able to differentiate between asteroids and comets as well as determine similarities and differences within each object type.

 

References

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² C.R. Chapman, J.G. Williams and W.K. Hartmann. Ann. Rev. Astron. Astrophys 16, 33 (1978).

³ The International Astronomical Union. How are Minor Planets Named?, WWW Document, (http://www.minorplanetcenter.net/iau/info/HowNamed.html).

⁴ NASA. Comets: In Depth, WWW Document, (http://solarsystem.nasa.gov/planets/comets/indepth).

⁵ M Williams, What is the Oort Cloud?, WWW Document, (http://www.universetoday.com/32522/oort-cloud/)

⁶ J. A. Fernandez. Comets: Nature, Dynamics, Origin, and their Cosmogonical Relevance, (Springer Netherlands), pp. 39-76.

⁷ C. R, Chapman. Reviews of Geophysics and Space Physics 21 (2), 196-197 (1983).

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¹⁰ F. E, DeMeo, et al., The Compositional Structure of the Asteroid Belt, WWW Document, (http://arxiv.org/abs/1506.04805).

¹¹ M.A. Seeds and D.E. Backman. Foundations of Astronomy, 6th Ed. (Cengage Learning, Boston, MA, 2016).

¹² James Evans, Astronomy, WWW Document, (http://www.britannica.com/science/astronomy#ref910481).

¹³ NASA, Pioneer 10: In Depth, WWW Document, (http://solarsystem.nasa.gov/missions/pioneer10/indepth).

¹⁴ Williams, Matt, What is the Kuiper Belt?, WWW Document, (http://www.universetoday.com/107598/what-is-the-kuiper-belt/).

¹⁵ Dunbar, Brian. Talbert, Tricia, New Horizons: The First Mission to the Pluto System and the Kuiper Belt, WWW Document, (https://www.nasa.gov/mission_pages/newhorizons/main/index.html).

¹⁶ B. Dunbar. NASA, Dawn – Ceres and Vesta. WWW Document,(http://www.nasa.gov/mission_pages/dawn/ceresvesta/index.html).

¹⁷ B. Dunford, and K. McKissick. 4 Vesta – In Depth | Planets – NASA Solar System Exploration. WWW Document, (http://solarsystem.nasa.gov/planets/4vesta/indepth).

¹⁸ NASA, Dawn: In Depth, WWW Document, (http://solarsystem.nasa.gov/missions/dawn/indepth).

¹⁹ M. A. Seeds and D. E. Backman. “Chapter 25 – Meteorites, Asteroids, and Comets.” The Solar System. 9th ed. (Cengage Learning, 2014) pp. 349-350, 360, 363-66-367.

²⁰ NASA, Ceres: Overview, WWW Document, (http://solarsystem.nasa.gov/planets/ceres).

²¹ L.D. Schmadel, Dictionary of Minor Planet Names, 6th Ed.(Springer Science & Media Business Media, Germany, 2012), pp. 14

²² M. Williams, A Mission to a Metal World: The Psyche Mission, WWW Document, (http://www.universetoday.com/122764/mission-to-the-metal-world-the-psyche-mission/)

²³ M. Kaasalainen, Icarus 159 (2), 269 (2002)

²⁴ JPL, 16 Psyche, WWW Document, (http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=16)

²⁵ S.J. Ostro, Astronomical Society of the Pacific, Publications 97 877 (1985)

²⁶ E. Merényi, Icarus 129 (2), 421 (1997)

²⁷ L. T. Elkins-Tanton; et al., 45th Lunar and Planetary Science Conference.LPI Contribution No. 1777 pp. 1253.

²⁸ Halley’s Comet Armada: “Comet Halley Returns-Voyager Uranus Flyby” 1986 NASA,( https://www.youtube.com/watch?v=oBq3-ZqW4Ac)

²⁹ D R Williams, Giotto, WWW Document, (http://nssdc.gsfc.nasa.gov/planetary/giotto.html)

³⁰ M Williams, Universe Today, What is Halley’s Comet (http://www.universetoday.com/48991/halleys-comet/)

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³² P Weissman, Halley’s Comet. WWW Document, (http://www.britannica.com/topic/Halleys-Comet).

³³ Halley’s Comet: Once in a Lifetime (1986), Nova

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³⁵ G. Kronks, 19P/Borrelly, WWW Document (http://cometography.com/pcomets/019p.html)

³⁶ NASA, Comet Borrelly Revealed, WWW Docmuent (http://science.nasa.gov/science-news/science-at-nasa/2001/ast25sep_1/)

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