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Voyages to Comets
Editor's Note: This is the 17th in a series of essays
on exploration by Steven J. Dick.
Voyages to
Comets
Among the more adventurous voyages of exploration are those whose
destinations are comets. Unlike planets, comets can be dangerous. They can be
dangerous to our home planet, having impacted the Earth in the past, perhaps as
recently as 1908 in Siberia, the famous "Tunguska event." They can also be
dangerous if approached by spacecraft. As their highly eccentric orbits bring
them closer to the Sun, comet nuclei may become active, often spewing jets of
material, forming a coma surrounding the nucleus, and a tail that appears to
spread across the sky. Occasionally comets are perturbed and collide with other
objects; numerous comets have been observed diving into the Sun, and in 1994 a
fragmented comet Shoemaker-Levy 9 smashed into Jupiter.
Although
millions of miles away, historically the appearance of a comet brought fear and
trepidation among those who believed they foretold plague and doom. They were
also sometimes used for political purposes. When a comet appeared for seven days
in July 44 BC, a few months after Julius Caesar's assassination, many Romans
believed Caesar was ascending to the region of the immortal gods, a view of
deification that his successor Augustus did nothing to discourage. Gradually,
comets were seen as more benign, and most people are familiar with recent
appearances like Comet Halley in 1986 and Comet Hale-Bopp in 1997. Even the
latter, however, induced 39 members of the Heaven's Gate cult to commit suicide
in San Diego, in the hope that their souls would hitch a ride on a spaceship
they believed accompanied the comet.
| "Whether only passing through the comet's coma or
tail, capturing images of the comet nucleus, or actually returning
samples, humanity's intrepid robotic explorers have added to our knowledge
in the true spirit of exploring the unknown."
| By the 1970s quite a bit was known about
comets based on telescopic observations from the ground. Already in the 1950s
Harvard astronomer Fred Whipple had theorized that they were "icy conglomerates"
composed mostly of ice with a mixture of dust, a model the press dubbed "dirty
snowball." At about the same time astronomer Jan Oort postulated they came from
a cloud of comets surrounding the solar system. Comets were also known to harbor
complex organic molecules, thereby making them of great interest to those who
study the origin of life on Earth. And, they were believed to be primordial
bodies whose chemistry and physics could unlock the secrets of the formation of
the solar system. But the real nature of comets remained unknown until they
could be visited up close and personal.
Amazingly, over the last two
decades a variety of spacecraft have done just that - voyaged to 6 of these
denizens of the heavens. After completing its initial mission in 1982, the
International Sun-Earth Explorer 3 was renamed the International Cometary
Explorer (ICE) and sent to comet Giacobini-Zinner. It did so with very little
remaining fuel, exploiting the gravitational instabilities of the Sun-Earth and
Earth-Moon "Lagrangian points," and passing less than 100 miles over the Apollo
11 landing site as it received a lunar gravitational assist. ICE passed first
through the plasma tail of the comet, some 26,550 kilometers behind the comet's
nucleus, and on September 11, 1985 came within 7,800 kilometers of the nucleus.
Due to the nature of its original mission, ICE carried no cameras, but measured
the particles and fields surrounding the comet.
ICE was only a prelude
to the flotilla of spacecraft sent to rendezvous the following year with the
most famous comet in history, comet Halley. (Today comets are named after their
discoverers; in the case of Halley's Comet, Edmond Halley was the first to
calculate its orbit). Appearances of Halley have been recorded at least since
240 BC, and its 1066 passage was recorded in the Bayeux Tapestry. Halley's comet
has a period of 76 years, the measure of a human lifespan, and when it returned
to the Earth's vicinity in March, 1986 it was greeted by the "Halley Armada".
The Halley Armada consisted of two Russian spacecraft (Vega 1 and 2),
two Japanese spacecraft (Sagigake and Suisei), and the European Space Agency's
Giotto. ICE itself would observe Halley from a comfortable distance of more than
10 million kilometers, while the Japanese craft ventured within a million
kilometers and the two Vega spacecraft came within 1000 kilometers (about 660
miles). Giotto would come closest of all, so close that navigators were worried
about damage to the spacecraft.
Giotto's encounter with
Comet Halley provided the first ever opportunity to take images of a comet
nucleus. The images were obtained with the Halley Multicolor Camera on Giotto.
Credit: ESA
Giotto was named after the medieval Italian painter
Giotto di Bondone, who had observed the comet during its 1301 passage and
depicted it as the Star of Bethlehem in his painting "Adoration of the Magi." On
March 14, 1986 Giotto passed within 600 km of Halley's nucleus. Two hours before
close encounter, Giotto began to be hit by thousands of dust impacts. The
spacecraft survived, and altogether more than 2000 images were returned from
Giotto before its camera was destroyed. The images showed Halley's rough and
porous nucleus to be a dark peanut-shaped body, 15 km long and 7 to 10 km wide.
On the sunlit side three outgassing jets were seen. Of the volume of material
ejected by Halley, 80% was water, 10% carbon monoxide, and 2.5% a mix of methane
and ammonia. Halley's nucleus was blacker than soot, suggesting that dust was
dominant rather than ice. It seemed Halley was more of a "snowy dirtball" rather
than a dirty snowball.
Giotto was reawakened in 1990, and after a
gravitational assist from the Earth (the first by a spacecraft coming from deep
space), came within 200 kilometers of the nucleus of Comet Grigg-Skjellerup in
July, 1992. Although no images were taken because the camera was destroyed
during the Halley encounter, other instruments indicated the comet was much less
dusty than Halley. Those instruments detected a somewhat higher magnetic field
than at Halley, and unusual magnetic waves.
On September 22, 2001,
eleven days after terrorist events shook the world, an American spacecraft known
as Deep Space 1 flew with 1,350 miles of the nucleus of Comet Borrelly,
capturing images of the second comet nucleus after Halley, this one about 8
kilometers long and 4 kilometers wide. A series of jets were again observed
emanating from the surface.
In recent years space scientists have been
even more ambitious. In 1999 NASA launched a comet sample return mission known
as Stardust, the fourth in its Discovery series of spacecraft. In January 2004
the spacecraft flew within 149 miles of the nucleus of comet Wild 2, collected
samples of comet dust, and stored them in a return capsule. After a roundtrip
journey of some 2.88 billion miles, the capsule is scheduled to return to Earth
with its precious sample on January 15, 2006. Along with interstellar dust also
collected during the journey, the samples will be analyzed at NASA's Johnson
Space Center.
Deep Impact, another NASA Discovery mission, brought yet
another approach to comet exploration - impacting a comet and studying the
subsequent debris for clues to the origin of the solar system. After a journey
of 171 days and 268 million miles, on July 3, 2005 the Deep Impact flyby
spacecraft released it 820 lb impactor on a course for Comet Tempel 1. The
following day it impacted the comet's 14 kilometer-long nucleus at 23,000 miles
per hour, producing a spectacular flash of light and a crater of undetermined
depth. Analysis of the ejection plume showed large amounts of organic material,
confirming that during its history the Earth might have been infused with
organics from similar comets. In addition, images from three cameras showed what
appear to be impact craters, never before seen on a comet and of unknown origin.
Other data indicates that the nucleus is extremely porous, a fluffy structure
weaker than powdered snow.
Composite image of
Comet Tempel 1, with data from Deep Impact's three cameras. Arrows a and b point
to large, smooth regions. The impact site is indicated by the third large arrow.
The scale bar is 1 km and the two arrows above the nucleus point to the sun and
the rotational axis of the nucleus. Credit: NASA/JPL/UMd
Whether
only passing through the comet's coma or tail (Giacobini-Zinner and
Grigg-Skjellerup), capturing images of the comet nucleus (Halley, Borrelly,
Tempel 1 and Wild 2), or actually returning samples (Wild 2), humanity's
intrepid robotic explorers have added to our knowledge in the true spirit of
exploring the unknown.
Along with the successes have come failures, most
notably NASA's CONTOUR mission. Launched in July, 2003, little more than a month
later communication was lost with the spacecraft as its motor fired to send it
on its way to comet Encke. Only pieces of debris were detected. The Mishap
Investigation Board identified four possible causes, any one of them
catastrophic. We can sympathize with those who spent years working on this
mission, only to see it end up in pieces. Their only consolation is one known to
explorers throughout history: along with the prospect of success comes the risk
of failure.
Space explorers will keep trying. In March 2004 the European
Space Agency launched Rosetta on a ten year journey to Comet
67P/Churyumov-Gerasimenko. In another example of an incredible journey planned
by space navigators, Rosetta flew by the Earth last month, will fly by Mars in
February, 2007, and will make two more Earth flybys before reaching its
destination in early 2014. Unlike past missions, Rosetta includes an orbiter
that will allow long-term mapping of the comet, and, in the ultimate adventure,
a lander that will touchdown in November 2014, if all goes well.
Steven
J. Dick
Further Reading
Deep Impact Website. http://www.nasa.gov/mission_pages/deepimpact/main/index.html
Giotto Website. http://sci.esa.int/science-e/www/area/index.cfm?fareaid=15
Kronk, Gary W. Cometography (Cambridge: Cambridge University
Press, 1999 and 2003). See also website http://cometography.com/
Ramsey, John and Lewis Licht.
The Comet of 44 BC and Caesar's Funeral Games (Atlanta, Ga., Scholars
Press: 1997)
Sagan, Carl and Ann Druyan. Comet (London: Guild
Publishing, 1985).
Schechner, Sara J. Comets, Popular Culture, and
the Birth Modern Cosmology (Princeton: Princeton University Press, 1997)
Stardust Web Site. http://www.nasa.gov/stardust and http://stardust.jpl.nasa.gov/
Yeomans, Donald K.
Comets: A Chronological History of Observation, Science, Myth and
Folklore (New York:John Wiley and Sons, 1991).
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