Comets: The Delivery System What'sNEW

Isaac Newton

The objection that life can't survive in space needs examination. A serious problem for Svante Arrhenius's theory in 1908 was that spores in empty space would be subject to radiation damage, especially in the vicinity of a star. In 1978, Hoyle and Wickramasinghe observe in Lifecloud (1) that if a cloud of bacterial matter were dense enough, the inner contents would be protected from radiation by the outer layers. Other scientists have recently observed that a coating of dust only half a micrometer thick would adequately protect a bacterium from ultraviolet radiation in space (2-4).

Hoyle and Wickramasinghe also discuss another means of space travel which solves the radiation problem: comets. The idea that comets could bring to Earth the ingredients for life is not completely new. Among others, Isaac Newton endorsed it. "Newton considered the continual arrival of cometary material to be essential for life on Earth" (5).

Some Basic Facts about Comets

Comets, as astronomer Fred Whipple figured out, are made largely of ice. Much of the ice in comets is frozen water, but ices of other compounds such as carbon monoxide and carbon dioxide are also present. And comets contain, we have recently learned, a large amount of more complex organic compounds. These organic compounds may be limited to a mixture of molecules such as the original Miller - Urey experiment was able to produce, or they may be even more closely related to life; we can't be sure from here, yet. In the interior of a comet, under layers of opaque organic material, viable cells would be shielded from radiation. Of course, freezing slows or stops metabolism, so cells could exist there in suspended animation.

A few larger comets such as Halley's comet have orbits that bring them as close to the sun as Earth is. Even fewer comets, called "sungrazers," actually strike the sun, or pass so close that they are destroyed by it. Most comets reside at distances far beyond that of Pluto, in orbits not confined to the plane in which the planets' orbits lie. They are so numerous that the total mass of comets in solar orbit may be as great as the total mass of the planets. Slight gravitational disturbances caused by the outer planets or neighboring stars can change a comet's orbit completely, steering some closer to the sun, others completely away.

When a comet nears the sun, some of its surface material boils off, making the comet's "tail." This process usually begins somewhere between the orbits of Jupiter and Mars. Some of the discharged material is gas, some of it is dust. Each makes a different kind of cometary tail. Dust and larger debris left by comets remain for a while in solar orbit. Earth often passes through the orbits of cometary debris, causing meteor showers such as the Perseid meteor shower around August 10 every year, when we cross the orbit of comet Swift-Tuttle.

Today, thousands of tons of cometary dust, debris and larger fragments fall to Earth every year. Starting in the late 1960's, U.S. military intelligence observers doing surveillance against enemy missile attacks began to observe and photograph comets and other objects as big as thirty to fifty meters in diameter exploding in the upper atmosphere. From 1975 to 1992, 136 such objects were observed — about eight per year. That information was kept classified until 1993-1994 (5.5). It's worth remembering that four billion years ago, when life on Earth first appeared, the number of comets nearing the sun was hundreds or thousands of times greater than it is now (6).

The study of comets today is rich with surprises. For example, comet Hyakutake, which was easily visible to the naked eye in March, 1996, was first discovered by a Japanese amateur astronomer using binoculars. Astronomers were surprised to learn "Hyakutake contains abundant ethane and methane, compounds never before confirmed in comets" (7-9). On March 27, 1997, NASA announced that a year-long study using Hubble and several Earth-based telescopes shows that the trace ices in the nucleus of comet Hale-Bopp are somehow segregated from water-ice (10). And on April 21, 1997, astronomers on the Canary Islands reported that Hale-Bopp has a third tail of a kind never seen before; it is composed of sodium gas (11). Following so many new findings, comet theorists are completely rethinking how comets are formed and what they contain. Perhaps in the process they should consider biological causes for some of the unexpected phenomena. For example, on Earth, ethane comes from methane, and methane is made from carbon dioxide by bacteria. This process could happen on comets as well.

Comets Reaching Earth

Christopher Chyba, Paul Thomas, Leigh Brookshaw and Carl Sagan wrote a study of this problem, published in Science in 1990, entitled "Cometary Delivery of Organic Molecules to the Early Earth" (12). They carefully calculate the heat generated by high speed impacts with Earth, and then conclude that life's building blocks (not whole cells) could arrive intact. It is reasonable to extend their conclusion to cells, by expanding the scope of their study. Chyba and his coauthors in 1990 admittedly do not examine the case of a comet exploding before impact. However most comets, indeed most large meteoroids of any type except iron ones would explode before impact. In 1992 Chyba and Sagan (13) did address the explosion of comets in the atmosphere and found that for the delivery of intact organic compounds at least, this method of transfer was far more effective than comets that collide with the surface.

The best known atmospheric explosion of a meteoroid happened eight kilometers above Tunguska in central Siberia on June 30, 1908. The explosion flattened the forest for roughly 15 kilometers in every direction. The object was most likely an asteroid, perhaps 60 meters in diameter, because a comet would have exploded higher in the atmosphere. Our knowledge of this event is indirect because no one investigated the site until twenty years after the explosion (14). A similar atmospheric explosion, again over Siberia, occurred in 1947. We know that atmospheric explosions before impact by comets and asteroids are common. An explosion in the air would be much gentler than a collision with either Earth's hard surface or the ocean. Matter on the trailing side of a comet exploding in the atmosphere would be significantly slowed by the jolt. And matter located there would also be the best protected from the heat generated during atmospheric entry prior to the explosion.

In March 1965, an object estimated at 7 - 8 meters in diameter exploded 30 kilometers over Revelstoke, Canada. This time investigators arrived promptly and recovered many fragments a few millimeters in size. Most of these were not altered by heat, proving that a plausible delivery mechanism for cells exists (15).

In a new development, on May 28, 1997, NASA announced observations that comets as large as houses — "thousands per day" — actually break up and are destroyed at 600 to 15,000 miles above Earth. Dr. Lewis A. Frank, the principal investigator for NASA's Polar spacecraft instruments, described their descent as a "relatively gentle 'cosmic rain'." (16-16.2)

Middle-Sized Objects

There are, as we see, several plausible methods of delivering cells in comets from space to Earth. Of course, our case for the presence of bacteria in comets would be stronger if we could actually find on Earth fossilized bacteria in a meteorite that came from a comet. Is this possible? Not all meteorites were necessarily ever embedded in comets. Most meteorites appear to be fragments of more solid objects such as asteroids or even other planets. However, a minority of meteorites, about 4 percent, are different from the rest. They are called carbonaceous chondrites. They contain a much higher percentage of carbon, from .35 percent to 4.8 percent versus a mean of .1 percent for ordinary meteorites. Their chemical composition makes them more susceptible to destruction by heat; the proportion of them entering the atmosphere may be as high as 50 percent (18). We know from the recovered fragments that the object that exploded over Revelstoke, for example, was a carbonaceous chondrite.

Among astronomers there are several theories about how carbonaceous chondrites came to be different (19). Recently scientists have determined that "salt-rich fluids analogous to terrestrial brines" flowed through the veins found in them (20), and that they were formed very early in the development of the solar system (21). The most accepted theory today is that they are remnants of spent comets.

Nagy

It is well known that carbonaceous chondrites contain a variety of organic compounds, including some of life's amino acids. Even more interesting is the evidence that, according to some scientists, several of them contain fossils resembling cells. In 1961, George Claus and Bartholomew Nagy (22) reported finding things resembling biological cells in two carbonaceous chondrites, the Orgueil and Ivuna meteorites. After discussion with other scientists, they concluded, "we are of the opinion that the organized elements are microfossils apparently indigenous to the meteorite parent body" (23). Harold Urey, of the famous experiment, accepted their conclusion (24, 25). At that time, Urey believed that the meteorites had come from the moon and were evidence of life or former life there. Consequently, NASA decided that the astronauts of the 1969 Apollo 11 and Apollo 12 missions to the moon would have to endure three weeks of quarantine in Houston upon their return.

By 1975, however, Nagy was no longer sure that the objects he and Claus had seen were extraterrestrial microfossils. In 1982, now looking at traces of amino acids from the Murchison meteorite, Nagy was cautious and noncommittal as to where they came from.

Photo by Hans D. Pflug

On the morning of September 28, 1969, a carbonaceous chondrite fell near Murchison, Victoria, Australia. The parent object disintegrated in the air and scattered fragments over an area of about five square miles. Scientists from NASA analyzed some of the fragments and determined that they contained organic molecules which "seem to have been formed before the meteorites reached the Earth" (26). The researchers took care to assure that the organic contents were not Earthly contaminants.

Nearly ten years later, German geologist and paleontologist Hans Dietrich Pflug also examined fragments of the Murchison meteorite. Using a new and difficult technique with the fragments, he isolated and photographed some startling things. The photographed forms resemble fossilized cells and virus particles. Pflug considered the arguments for and against Earthly contamination as the source for the fossils and is convinced that the fossils came from space (27). But he is officially noncommittal as to whether they are actually what they look like — cells and virus particles. "There is no convincing evidence that the forms are more than 'organized elements,' possibly some kind of prebiotic structures," he now says. Today the consensus is that all such fossils are Earthly contaminants, but the case is not settled. More research is needed.

Recently, new and better methods to conduct this kind of research have been developed by NASA researchers examining the meteorite from Mars designated ALH 84001. Fortunately, similar methods are now being applied to the Murchison meteorite, and the preliminary results are startling. Pictures made available in July, 1997, show microscopic forms in Murchison that look very lifelike (27.5).

A fossil from the Murchison meteorite and a bacterium recovered in the high atmosphere, by Hans D. Pflug

Dust and Spores

Hoyle and Wickramasinghe calculated that a particle the size of a typical bacterium might get briefly heated to 500 degrees C. They then cite studies in which E. coli were subjected to close to twenty seconds of flash heating of up to 700 degrees C and survived (30). E. coli do not normally live in a hot environment; they live in our intestines. There are archaebacteria, however, that thrive at well above the boiling point of water, and many bacteria form heat-resistant spores. It is reasonable to assume that the sporulated forms of some of these could survive flash heating even better than normal E. coli can.

An array of cellular equipment that could help bacteria survive sudden heating is their set of "heat-shock" proteins. According to the textbook The Molecular Biology of the Gene (31):

"In almost all kinds of cells subject to heat shock, certain proteins (about 17 in E. coli) begin to be made much faster than usual.... Remarkably, some heat-shock proteins of widely different species are closely related; in fact, there are even similarities between those of bacteria and those of eukaryotes.... [W]e have no idea what function they have in common that is essential to save cells from the rigors of a sudden temperature rise...."

The response time of heat-shock proteins is less than a minute in the eukaryotic cells that have been observed for this response. In prokaryotic cells the response would be much quicker, because transcription is simpler and translation begins immediately. The writers of the quoted text do not consider that these proteins could help cells survive the heat of atmospheric entry. But why couldn't they?

After being released as dust by a comet, before entering the atmosphere, bacteria would be subject to radiation damage for a short while at least. Interestingly, certain bacteria, such as Deinococcus radiodurans, can survive a dose of nuclear radiation 3,000 times stronger than the dosage that would kill a human (32). There has never been radiation that strong on Earth. If life originated here from nonliving chemicals, and if evolution works according to the darwinian paradigm, the evolution of that survival capability is hard to explain. In fact, Dr. Kenneth Minton, one of the scientists who studied these bacteria, said, "Deinococcus radiodurans would be a good vector for panspermia" (33).

Anomalous Behavior by Some Comets

M. K. Wallis and Wickramasinghe (39) proposed another explanation; namely, that a cycle of surface freezing, compression, and cracking may cause compressed liquid or gas to squirt from a comet's interior. A third possibility, which Hoyle and Wickramasinghe have considered, is that cellular metabolism may be under way on some comets. This metabolism could generate gasses that could, directly or indirectly, account for the brightening. The recent discovery of abundant methane and ethane in the coma of comet Hyakutake (40) strengthens this possibility. In any case, the anomalous brightening of these comets needs explaining. The presence of liquid water or even life on the comets could help explain it. More research would help here as well.

What'sNEW (Also see Comet Rendezvous)

12 Jan 2010: Comets and the Origin of Life, by Wickramasinghe, Wickramasinghe and Napier.
Alien Gases in Our Atmosphere by Phil Berardelli, ScienceNOW Daily News, 10 Dec 2009.
9 Oct 2009: An asteroid covered with frozen water and organic compounds....
18 Aug 2009: NASA scientists have discovered glycine... in samples of comet Wild 2.
Comets: Cosmic Life Preservers by Jonathan Fahey, Forbes.com, 5 Aug 2009.
Crashing comets not likely the cause of Earth's mass extinctions by Vince Stricherz, University of Washington News, 30 Jul 2009.
30 Jul 2009: Comets contained oceans of liquid water during the first million years of their formation.
D. Hutsemekers et al., "New constraints on the delivery of cometary water and nitrogen to Earth from the 15N/14N isotopic ratio" [abstract], arXiv:0906.5221v1 [astro-ph.EP], online 29 Jun 2009.
9 Jun 2009: Formic acid ...has been found at record levels on [the Tagish Lake] meteorite....
Richard W. Court and Mark A. Sephton, "Meteorite ablation products and their contribution to the atmospheres of terrestrial planets: An experimental study using pyrolysis-FTIR" [abstract], doi:10.1016/j.gca.2009.03.006, p 3512-3521 v 73, Geochimica et Cosmochimica Acta, 1 Jun (online 19 Mar) 2009. Also see commentary:
Meteorite bombardment may have made Earth more habitable, says study, by Colin Smith, EurekAlert!, 1 Jun 2009.
NASA Study Shows Asteroids May Have Accelerated Life on Earth, by Dwayne Brown and Jim Scott, NASA News Release 09-111, 20 May 2009.
Why Comets Are Dirty, by Phil Berardelli, ScienceNOW Daily News, 14 May 2009.
We Owe It All to Comets, American Friends of Tel Aviv University, 28 Apr 2009.
17 Mar 2009: Predominantly left-handed amino acids were found in six out of six meteorites....
Comet Dust Reveals Unexpected Mixing of Solar System, by Jill Sakai, the University of Wisconsin-Madison, 18 Sep 2008.
A web story about the Desaguadero Meteorite is the subject of a Reply from Ron McGhee, 15 Mar 2008.
14 Mar 2008: Meteorites with the highest observed concentrations of amino acids have been examined at the Carnegie Institution.
5 Mar 2008: A carbonaceous meteorite collected in Antarctica contains amino acids that are predominately left-handed.
Peru meteorite may rewrite rule books, by Maggie Fox, News in Science, the Australian Broadcasting Corporation, 12 Mar 2008.
Mixing It Up in the Early Solar System, by Richard A. Kerr, ScienceNOW Daily News, 25 Oct 2007. "But Ciesla performed more computationally demanding, two-dimensional simulations. In these, most nebular material still flows inward toward the sun; but near the midplane, the inward flow ceases or even reverses, letting the outward diffusion of material to the comet-forming region proceed."
24 Oct 2007: The Desaguadero Meteorite was an ordinary H4/5 chondrite.
20 Sep 2007: A meteorite caused illness in Peru?
20 Aug 2007: Comets are much more likely than Earth to have hosted the origin of life....
Janaki Tara Wickramasinghe, The Role of Comets in Panspermia, PhD Thesis, Cardiff University, 25 May 2007.
4 Apr 2007: How did life on Earth originate?
Edward M. Drobyshevski, "Stardust findings favor not only the planetary origin of comets but the underlying close-binary cosmogony of the Solar system as well" [abstract], astro-ph/0702601, arXiv.org, 22 Feb 2007. "...Findings of the Stardust mission that brought to the Earth dust from the 81P/Wild 2 coma.... are at odds with the widely accepted condensation/sublimation comet paradigm. They fit rather well to the approach assuming ejection of nuclei of short-period comets from moon-like bodies of the type of Galilean satellites in rare (six to seven events in 4.5 aeons) global explosions of their massive icy envelopes."
Dust around nearby star like powder snow, by Robert Sanders, UC Berkeley News, 8 Jan 2007.
1 Dec 2006: It's not from around here. It's from somewhere else. — Mike Zolensky talking abut the Tagish Lake meteorite.
Pre-life Molecules Are Present in Comets, University of Michigan, 21 Jul 2006.
Fossil Meteorite Unearthed From Crater, Linda M. V. Martel, Planetary Science Research Discoveries, 14 Jun 2006.
9 May 2006: Organic matter in meteorites is extraterrestrial and ancient.
24 Apr 2006: Pristine samples of the Tagish Lake meteorite now belong to Canadian curation and research facilities.
13 Mar 2006: Fire and ice — the first examined particles returned from comet Wild 2 by NASA's Stardust mission.
15 Jan 2006: Stardust landed safely.
22 Dec 2005: Building blocks of life orbiting a Sun-like star were observed by astronomers using NASA's Spitzer Space Telescope.
Organic chemistry in meteorites, comets and the interstellar medium, abstract by Oliver Botta, International Space Science Institute, presented at IAU Symposium 231, 2 Sep 2005.
4 Nov 2005: Hyperion's craters look like vents to us.
3 Oct 2005: More about Deep Impact comes from an interview with Jessica Sunshine.
8 Sep 2005: Deep inside comet Tempel 1 there are lots of organic compounds.
Comet Hale-Bopp, photographed March 1997, Astronomy Picture of the Day, 22 May 2005.
NASA's Deep Impact spacecraft spots its quarry, Jet Propulsion Laboratory, 27 Apr 2005.
11 Apr 2005: There's an unexplained brown stain in the Genesis science canister.
Comet strike surprisingly more likely, Cardiff University, 26 Oct 2004.
Rob Sheldon emails about evidence for liquid water on comets, 25 Oct 2004.
Scientists Say Comet Smashed Into Southern Germany In 200 BC [posted on SpaceDaily], Agence France-Presse, 15 Oct 2004.
Associated Press, "Comet Research Pioneer Whipple Dies" [text], The New York Times, 31 Aug 2004.
2 Aug 2004: Microfossils in a meteorite.
17 Jun 2004: Mass spectrometer on Stardust sees organic matter at comet Wild 2.
...Comet Wild 2 unlike any other body in solar system, EurekAlert!, 17 June 2004.
Did comets flood Earth’s oceans? re: Ptolemy, an instrument on Rosetta’s Philae lander, European Space Agency, 16 June 2004.
Comet Hale-Bopp Over Indian Cove, Astronomy Picture of the Day, 20 Apr 2004.
4 Feb 2004: A small comet impact caused sixth century global chill?
3 Jan 2004: Stardust sailed safely by comet Wild 2.
Eberhard Grün, Bo Å. S. Gustafson, Stan Dermott and Hugo Fechtig, eds., Interplanetary Dust, Springer-Verlag, Berlin, Heidelberg, New York, 2001.
Carbon clue implies comets orbit other stars, NewScientist.com , 12 Dec 2003.
2003, November 4: Amino acids in meteorites....
2003, October 3: NASA will explore comets for clues about the origin of life.
Optical Detection of Anomalous Nitrogen in Comets, ESO Press Release 25/03, European Southern Observatory, 12 Sep 2003.
Too few icy rocks to explain comets, NewScientist.com, 3 Sep 2003.
Alan Stern, "The evolution of comets in the Oort cloud and Kuiper belt" [abstract], p 639-642 v 424 Nature, 7 Aug 2003.
Catching a Comet’s Tail in the Earth’s Upper Atmosphere, News Release J03-80, Johnson Space Center, 10 Jul 2003.
Very Large Meteorite Fell Down in Siberia, Pravda, 18 Mar 2003.
2002, December 11: Tasgish Lake meteorite has unfamiliar organic globules.
2002, November 4: Bacteria can survive high-speed impacts.
2002, September 29: Ice falls come from atmosphere, not space
First Evidence For Early Meteorite Bombardment Of Earth, SpaceDaily, 26 Jul 2002.
Rare space rock 'a gem' re: Tagish Lake meteorite, BBCNews, 22 Jul 2002.
Kenneth Chang, "Abundance of Cometlike Objects With Moons Stuns Experts" [text], The New York Times, 23 Apr 2002.
Aged and Wounded Spacecraft Photographs Comet, Space.com "Special Report" with links to half-a-dozen stories about Comet Borrelly, Sep - Nov 2001.
Comet's black heart revealed, by Jeff Hecht, New Scientist, 30 Nov 2001. "Complex organics are the best suggestion for the dark surface material."
Serving Up Meteorites on Ice, re: meteorites in general, by Linda M.V. Martel, Hawai'i Institute of Geophysics and Planetology, 7 Nov 2001.
Carbonaceous Clues to the Early Solar System, re: Tagish Lake meteorite, by Leslie Mullen, NASA's Astrobiology Institute, [publicised] 12 Oct 2001.
Electa Draper, "Maybe it was meteors: Wildfire cluster probed" [text], Denver Post, 9 October 2001.
2001, September 29: Deep Space 1 photos comet Borrelly.
Sandra Pizzarello et al., "The Organic Content of the Tagish Lake Meteorite" [abstract], p 2236-2239 v 293 Science, 21 September 2001.
2001, August 31: Tagish Lake meteorite may be a D asteroid
Meteorites Don't Pop Corn, by Toni Phillips, Science@NASA, 27 July 2001. "A fireball that dazzled Americans on July 23rd was a piece of a comet or an asteroid, scientists say."
2001, July 11: A dying star boils the comets around it.
William J. Broad, "Military Warning System Also Tracks Bomb-Size Meteors" [text], The New York Times, 29 May 2001.
2001, May 17: Hubble Space Telescope's pictures of comet LINEAR...
As cool as the cosmos, by Duncan Steel, The Guardian, 3 May 2001. "Just-fallen meteorites... are cold."
Do asteroids contain seeds of life?, ENN.com, 27 April 2001.
Cosmic Impacts Punctuated Human Evolution, CCNet, 17 April 2001.
2001, April 6: Tagish Lake meteorite contains few amino acids.
Was Johnny Appleseed a Comet?, Science@NASA, 5 April 2001 (Polypeptides produced in collision experiments).
Understanding Life's Impact, SpaceDaily.com, 4 April 2001 (Water and organic chemicals delivered by comets.)
2001, March 30: A giant comet with a very large orbit....
Deep Space 1 Loads up for Trek to Comet, JPL, NASA, 20 March 2001.
2001, March 19: Did a chunk of ice fall to Earth from a comet?
2001, March 7: Comet Hale-Bopp is still actively shedding gas and dust....
2001, February 26: Non-biological chemistry cannot produce the organic molecules in comets.
Deep Space 1 Mission Log, by Marc Rayman, [updated] 15 February 2001.
2001, January 23: Japanese scientists have found 3,554 meteorites in Antarctica.
Meteors come in with a bang, by Philip Ball, Nature Science Update, 5 January 2001.
2000, November 27: Leonid meteor reveals spectral signature of bacteria?
2000, November 14: Organics in Leonid Meteors.
2000, October 13: More about Tagish Lake meteorite in Science.
2000, August 31: Analysis of the Tagish Lake meteorite will be methodical.
Hot X-rays from a Cold Comet, Science@NASA, 23 August 2000.
2000, July 19: Comets could have formed at different times during the evolution of the solar nebula, and may reveal their age by the structure of their dust grains.
2000, July 11: A revived Deep Space 1 mission will visit comet Borrelly.
It came out of the sky re: Tagish Lake meteorite, by Duncan Steel, The Guardian, 29 June 2000.
2000, June 12: Salt in Zag meteorite suggests planets formed early in a warm wet place.
2000, June 12: Argon in Hale-Bopp ...
2000, June 1: Tagish Lake Meteorite is "equivalent to a sample-return space mission."
Kenneth Chang, "Meteorite Offers a Primordial Peek" re: Tagish Lake [text], The New York Times, 2 May 2000.
2000, March 28: Shallow craters... cover the coastal plain of the southeastern United States.
2000, March 18: Pristine samples of January's Yukon meteorite (Tagish Lake).
2000, March 8: A carbonaceous asteroid was observed passing Earth.
2000, January 25: Yukon Meteor Blast (Tagish Lake meteorite)...
How Comets Might Seed Planets, by Robert Roy Britt, Space.com, 24 January 2000.
1999, November 29: Tunguska explosion probably caused by a comet.
Life From a Dirty Snowball, NASA's Astrobiology Institute, November 1999.
1999, November 1: Comets' nuclei can be blacker than coal.
Harvest of the heavens starts with a sludgeball on Earth by James Woodford, The Sydney Morning Herald, 24 September 1999.
The house of comets by Wil Milan, Explorezone.com.
1999, September 7: Liquid water in another meteorite.
1999, August 27: Liquid water found in a meteorite.
Robert S. Boyd, "Earth battered through history by comets," San Jose Mercury News, 17 August 1999.
1999, August 13: Stardust Update
At age 92 astronomer Fred Whipple is joining NASA space team to explore three comets in 2002 — Cornell University News, 26 July 1999.
1999 July 7: A mission to excavate the interior of a comet has been selected by NASA's Discovery Program.
ESA's Rosetta comet chaser unveiled in London — European Space Agency Info Note 09-99, 1 July 1999.
Champollion Mission to Be Canceled — SpaceViews, 29 June 1999.
Return to Tunguska, by David Whitehouse, BBC News Online, 28 June 1999.
1999, May 28: Stardust is experiencing electrical problems.
1999, February 6: Stardust to be launched today.
1998, October 22: Comet Hale-Bopp shows amazingly strong activity 6.7 AU from the Sun.
1998, September 8: Fossilized magnetotactic bacterium identified in the Orgueil meteorite.
1998, March 21: There appear to be two distinct classes of Kuiper-belt objects, as distinguished by color.

References

Related Reading

M. E. Bailey, S. V. M. Clube and W. M. Napier, The Origin of Comets. Oxford: Pergamon Press 1990.
William J. Broad, "The Comet's Gift: Hints of How Earth Came to Life" p B9,B12 The New York Times April 1, 1997.
William J. Broad, "Spotlight On Comets In Shaping of Earth" p B7,B11 The New York Times June 3, 1997.
Cristiano Batalli Cosmovici, Stuart Bowyer and Dan Werthimer, eds., Astronomical and Biochemical Origins and the Search for Life in the Universe: Proceedings of the 5th International Conference on Bioastronomy, IAU Colloquium No. 161, Capri, July 1-5, 1996, Editrice Compositori 1997. Note especially:

Christopher F. Chyba, "Catastrophic impacts and the Drake equation" p 157-164.

Michael J. Mumma, "Organics in comets" p 121-142.

Juan Orò and Cristiano B. Cosmovici, "Comets and life on the primitive Earth" p 97-120.
Sara Schechner Genuth, Comets, Popular Culture, and the Birth of Modern Cosmology. Princeton University Press 1997. (Lots about Isaac Newton.)
Roland Meier et al., "Deuterium in Comet C/1995 01 (Hale-Bopp): Detection of DCN" p 1707-1710 v 279 Science. 13 March 1998. "The observed values... imply a kinetic temperature above 30 K in the ... interstellar cloud that formed the solar system."
Bartholomew Nagy, Carbonaceous Meteorites, Elsevier Scientific Publishing Company 1975.
William R. Newcott, "The Age of Comets" p 94-109 v 192 n 6 National Geographic, December, 1997.
Paul J. Thomas, Christopher F. Chyba and Christopher P. McKay, eds., Comets and the Origin and Evolution of Life. Springer-Verlag, New York, Inc. 1997.

Related Websites

The Leonid Sample Return Mission from Space Science News at Marshall Space Flight Center, 16 November 1998.
Chemical Measurements of Comet Hyakutake Suggest a New Class of Comets: News release 96-108 from NASA JPL.
Meteorites and Comets: Organic Matter and Exobiological Hypotheses, 1834-Present: a large, categorized, annotated bibliography in "The Net Advance of Physics" by N. Redington and K.R. Keck at MIT.
SpaceViews: Comet Hale Bopp. SpaceViews is a monthly publication of the Boston chapter of the National Space Society.
Stratospheric Dust: part of Planetary Materials Curation at NASA JSC.
NASA researchers: Comet shower triggered life on Earth by Don Knapp on CNN Interactive, April 17, 1997.
Comets, maintained by Ron Baalke, JPL, NASA.
Comets, one of Bill Arnett's The Nine Planets, an excellent resource.
The Comet Hale-Bopp Home Page at NASA's Jet Propulsion Laboratory, Pasadena, CA, contains more than 3,200 images, including one obtained by a California astronomer the night after the comet was discovered in July 1995.
The Near-Live Comet Watching System at NASA Headquarters, Washington, DC, contains more than 1,300 images. Astronomers from Australia, Asia, Europe and the Americas have submitted images, which have been captured by everything from professional observatory equipment to the backyard gear of an amateur astronomer. The archives include photographs of the comet over San Francisco; Dublin, Ireland; and Genoa, Italy.
UMass Astronomers Report Comets May Have Introduced Interstellar Chemicals to Earth by Arthur Clifford, University of Massachusetts Amherst, 9 October 1996.
SKY Online's Comet Page by Sky Publishing Corporation, 1998.