Deep Space 1 Provides Unprecedented Images of a Comet’s Surface

By Keith Cowing
September 25, 2001
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NASA released the most detailed pictures ever taken of a comet’s surface today. The images were taken by the Deep Space 1 spacecraft last Saturday as it flew a scant 1,350 miles above the surface of Comet Borrelly at a speed of 36,900 mph. As a result, our knowledge of cometary surface details, chemical composition, and interaction with the space environment was substantially enhanced.

° Real Video animation of DS-1 encounter with Comet Borrelly, SpaceRef.TV

As successful as this flyby has already proven to be, it almost didn’t happen – more than once, in fact. According to Marc Rayman, Deep Space 1 project manager, “Deep Space 1 was not even built to explore comets.” Noting that a number of problems had arisen on the spacecraft over the years, Rayman, undaunted by the prospect of a comet encounter “if it isn’t impossible it isn’t worth doing.”

After long- distance modification of the spacecraft’s camera to fill in for a failed star tracker, the spacecraft continued on its mission. Other issues with guidance and software also threatened to curtail the mission. Undaunted, the spacecraft managed to recover from each setback and continue on. As soon as it became clear that the spacecraft had survived its encounter with Comet Borrelly, cheers and waving flags instantly appeared in the control room.

Once the spacecraft’s main mission was completed, an extended mission – one that included a comet encounter was put into place. DS-1 is equipped with sensors which Rayman said allowed it to “see, smell, and hear the cometary environment”. These instruments were all brought to bear last Saturday.

With a resolution of 45 meters per pixel these images offer the most detailed view of any comet. As shown in the black and white images flashed on the screen the comet is approximately 8 km long and perhaps 4 km wide. The comet is shaped something like a bent hotdog.

Larry Soderblom, from the U.S. Geological Survey, explained that the surface of comets has been shown to be comprised of a variety of ices and organic materials – much of which is rather dark and similar in many ways to “soot”.

In examining the images, Soderblom expressed surprise at the variations in brightness among surface features – all of which are covered with “soot” of one type or another. “This is not just shadows but differences in chemical composition.” He said. In describing the comet’s surface, Soderblom said that it is “seems to have elevated mesas – and peaks.” He noted that the “dark areas are actually on the top of peaks – not where shadows would be.”

Adjacent to the rough elevated areas are large smooth areas. These areas have been correlated with the locations on the comet where erosive jets are emanating. This activity would seem to form the depressed “saddles” that are seen in these smooth areas. When questioned by reporters, Soderblom agreed that the position of a jet in the middle of the comet, with erosion happening on either side, could conceivably lead to a fracture in the comet – something comets are known to do with some regularity.

In the coming weeks data collected by the suite of instruments on the spacecraft will be correlated with the visual features on the comet – with more surprises certain to follow.

Editor’s note: In the days leading up to this encounter, NASA seemed to be going out of its way not to raise expectations that the mission would be successful in capturing images of the comet – indeed, there was repeated doubt expressed that the spacecraft would even survive the pummeling it would receive as it flew through the cloud of dust and gas surrounding the comet.

NASA also seemed to go out of its way to claim that the DS-1 project did not have cometary studies as part of its objectives. According to Marc Rayman, Deep Space 1 project manager, “Deep Space 1 was not even built to explore comets.”

While the prime mission of DS-1 was to test a series of risky technologies – among them an ion propulsion system and a semi-autonomous navigation system, it was tasked at its inception to study comets. This is abundantly evident in noting the two comets pictured in the mission’s logo, in artist’s conceptions of the mission, and in the original mission requirements laid out in AO: 97-OSS-01.

While the spacecraft did not have the shielding one would expect a close flyby mission to have, it was intended to study comets at close range none the less. As stated in AO: 97-OSS-01 (see below) DS-1 was supposed to study the “Relationship of nucleus surface features to coma dust “jets.” This is something that requires a certain proximity to a comet in order to accomplish.

NASA’s precautionary PR not withstanding, the success of DS-1 is most certainly not to be overlooked. The spacecraft’s multiple accomplishments are clear evidence of the ingenuity and persistence of its creators and operators – accomplishments for which they should be proud. Indeed, the mission would have been a success years ago even if it had never encountered Comet Borrelly. Given the cost of this mission and the scientific payback it has delivered, this is clearly one of the best bargains NASA has enjoyed in many years.

Related Links

  • DS-1 home page

  • 25 September 2001: Aging NASA Spacecraft Captures Best-ever View of Comet’s Core, NASA JPL

  • 24 September 2001: Comet Results briefing set for Tuesday, NASA JPL

  • 24 September 2001: Deep Space 1 Mission Log Mission Update: 23 Sep 2001 , NASA JPL

  • 24 August 2001: Deep Space 1 Mission Encounters Major Problems, SpaceRef

  • 17 September 2001: Deep Space Mission Log 17 Sep 2001, NASA JPL

    “When the probe enters the coma, it will be subjected to a fusillade of high-speed debris from the comet. Unlike Giotto, DS1 was not built to encounter a comet. As the lowest cost mission into the solar system yet undertaken by NASA, no resources could be devoted to anything other than the prime mission objectives, so it carries no shielding. “

  • 19 September 2001: Veteran spacecraft attempts to earn extra credit at comet

    “Since Deep Space 1 wasn’t built to go to a comet, it does not carry a protective shield.”

  • 26 February 1997: Deep Space 1 Scientific and Technical Objectives, AO: 97-OSS-01, Science Investigations on the New Millennium Deep Space One (DS1) Mission

    “Scientific goals for Comet P/West-Kohoutek-Ikemura, to the extent feasible, include:

    • Gross physical properties of the nucleus: dimensions, shape, surface morphology, albedo, heterogeneity, and estimates for mass, volume, density, spin state, etc.
    • Coma features, plasma properties, and solar wind interaction: brightness profiles of coma, jet-like features (“jets”) of dust and gas, brightness changes along and perpendicular to dust “jets,” plasma boundaries, plasma flux, etc.
    • Relationship of nucleus surface features to coma dust “jets.”
    • Composition of the coma/tail plasma: elemental, molecular, isotopic, dust-to-gas mass ratio.”

  • SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.