Instruments aboard a spacecraft that will be launched next year to explore two, and perhaps three or more, comets in the solar system will for the first time provide a "fingerprint" of the surface of cometary nuclei, giving the first firm evidence of the composition of the icy, rocky objects.
About 50 of the world's leading comet experts, meeting at the Harvard Smithsonian Center for Astrophysics in Cambridge, Mass., recently, were told that the spacecraft's infrared imaging spectroscopy will map the composition of the nucleus of comet Encke at a resolution of 100 meters to 200 meters (109 to 218 yards), detailed enough to see craters and other large geologic features and to determine their composition.
Comet Encke will be the first target of NASA's Cornell University-led Comet Nucleus Tour (CONTOUR), scheduled for launch July 1, 2002. In a report prepared for the meeting, James Bell, Cornell assistant professor of astronomy and one of the scientists responsible for the spectrometer on the close-up imager, noted that the surface resolution of Encke's nucleus by the CONTOUR spectrometer will be even better than that obtained by the infrared spectrometer on the Near Earth Asteroid Rendezvous spacecraft during its recent orbital mission to asteroid 433 Eros. "The CONTOUR spacecraft will come within about 100 to 160 kilometers (62 to 100 miles) of the nucleus, although the exact distance is still in doubt because we don't know the orbital position of the nucleus with extreme precision," said Bell.
The imaging instrument, called the CONTOUR remote image/spectrograph, also will send back digital-camera images of Encke's nucleus. The camera will capture the images as the spacecraft speeds through the comet's dusty, gaseous head, called the coma, at 28 kilometers (about 17 miles) a second in November 2003. Joseph Veverka, Cornell professor of astronomy and principal investigator on the $155 million mission, noted at the Cambridge meeting that "success" will be defined as obtaining digital images of the nucleus showing automobile-size details, such as rocks, about 4 meters (4 yards) across. Encke, first discovered 225 years ago, is about 8 kilometers (5 miles) long and has an average radius of about 2.5 kilometers (1.5 miles). It orbits the sun once every 3.2 years, and its most recent apparition from Earth was last year. It is unique in that it has been observed from Earth on 56 of its apparitions, more than any other comet, including Halley.
Encke will not be the only comet on CONTOUR's agenda. In June 2006 the spacecraft is scheduled to encounter Comet Schwassmann-Wachmann 3 and, possibly, Comet d'Arrest in 2008. These targets are so-called "Jupiter family" comets because they are thought to have had their orbital periods shortened by previous gravitational encounters with the giant planet. The science team hopes it also might be possible to visit other kinds of comets, particularly primitive members of the so-called "dynamically young" family that are in long elliptical orbits and might be making one of their first close passes by the sun.
Cornell senior research associate and science team member Peter Thomas noted at the meeting that during the 30-minute flyby of the nucleus, the spacecraft's instruments "will be able to obtain detailed compositional measurements of gas and dust in the near-nucleus environment." The comet's coma is a vast but extremely thin atmosphere, approaching the size of the sun, consisting of gas and debris thrown off the nucleus as it orbits the sun. The peak of this shedding of material is reached as the comet approaches the sun, and all the spacecraft's flybys will occur when the target comet is near this point in its solar orbit.
The scientific team will be particularly searching the coma for evidence of curious particles previously detected in interstellar clouds by Jochen Kissel, a comet researcher at the Max-Planck-Institute for Extraterrestrial Physics in Garching, Germany. Kissel made his discovery in data sent back by NASA's Stardust mission, which will reach comet Wild 2 in 2004. The mission is using the same dust analyzer as will be carried by the CONTOUR. Said Veverka, "The particles have a completely weird composition and don't seem to have minerals in them but seem to be made of chains of carbon-hydrogen and oxygen-nitrogen, like polymers. But there isn't any polymer with that kind of composition that we are normally familiar with."
There is an indication, said Veverka, that some particles might have weathered the massive meltdown of material when the sun and planets were formed from interstellar dust and clouds. "The question now is, have any of these particles been preserved in comets? We have to get close enough to a comet to find out." Although Encke has been much studied from ground-based observatories, little is known about its composition, which is why the comet experts gathered to exchange information on the object. Most assumptions about Encke, the researchers agreed, are drawn from data gathered by the European Space Agency's Giotto spacecraft, which visited comet Halley in 1986. Much of what astronomers know about comets "comes from the one object we've come close to, comet Halley," noted Casey Lisse, an astronomer at the University of Maryland. However, the CONTOUR images from Encke will be 25 times higher resolution than those from Halley.
Indeed, the most that the astronomers at the meeting could agree on was that Encke, some 30 million miles from Earth, is an extremely elongated "icy dirt ball" with a density, size, shape and rotation that defy precise analysis. Veverka wryly noted that the conflicting information about the comet is such that the mission will "not be dependent on any prejudices."
And science team member Anita Cochran, a research scientist at the McDonald Observatory, the University of Texas, ruefully concluded that "we started off by saying we didn't know, and we just made up things from there."
The complex journey of CONTOUR is shown in a computer simulation video, made for NASA by recent Cornell graduate Dan Maas, who previously produced a video for the 2003 Mars Rover mission. The new video can be seen on the CONTOUR web site at http://www.contour2002.org/