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Stardust-NExT charts new territory on comet Tempel-1
Posted: 16 February 2011

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After a five-and-a-half year wait since the Deep Impact spacecraft flung a probe into Comet Tempel-1, Stardust-NExT last night revealed the crater that had been obscured from view by the debris ejected in the blast. Significant changes were also seen on the comet's surface as a result of its orbit around the Sun, and new territory was imaged for the first time.

"Was the mission 100 percent successful?" said Stardust-NExT's principal investigator Joe Veverka in last night's press conference, who was happy to report: "No, it was 1,000 percent successful!"

All four science objectives were achieved in the flyby, which saw the spacecraft swoop to within 178 kilometres of the comet's nucleus at closest approach. Those objectives were to identify changes since Tempel-1 looped around the Sun, image the Deep Impact crater site, image areas not previously seen before, and to analysis the dust and debris surrounding the comet.

Smooth terrain first identified by Deep Impact in 2005 (top, and close up in middle image), has been eroded by up to 30 metres since its loop around the Sun (indicated by the yellow lines).Three depressions have also merged together over time (box), caused by the evaporation of volatile substances. Image: NASA/JPL-Caltech/University of Maryland/Cornell.

The mission was the first to revisit a comet nucleus after it had passed around the Sun, and the effects of this passage were clearly seen. In the smooth region first identified by Deep Impact, some 20-30 metres worth of erosion has since taken place along its boundary, and three pits seen previously have eroded into just one pit. This is an expected result of evaporation and sublimation of ices and gas as the comet's temperature rises during its approach towards the Sun. During this phase, gases erupt out of the surface carrying particles of ice and dust, and while some material jets into space, other components flow across the surface.

As for the crater, which was produced by slamming a coffee-table sized probe into the comet at around 10 kilometres per second, it closely matched predictions, measuring approximately 150 metres in diameter. The image also revealed a mound of material in its centre, most likely formed when material ejected in the impact fell back into the crater.

After a five and a half year wait the crater left by the Deep Impact mission was finally revealed by Stardust-NExT to be approximately 150 metres wide with a mound of material in its centre. The arrows in the "after" picture (right) identify the rim of the crater. Image: NASA/JPL-Caltech/University of Maryland/Cornell.

"The crater was more subdued than perhaps we first thought, but it was the size we expected, which is consistent with the ejecta pattern we observed, and what we'd expect in an oblique, 30 degree impact," said cratering expert Pete Shultz of Brown University. "A lot of material came back down and the crater partly buried, or healed, itself. The cometary nucleus must be fragile and weak based on how subdued the crater is we see today."

As the spacecraft made its closest approach it flew through cometary particles, and the effects of a dozen impacts that penetrated more than one layer of its protective shielding were recorded. "The data indicate Stardust went through something similar to a B-17 bomber flying through flak in World War II," said Don Brownlee, Stardust-NExT co-investigator. "Instead of having a little stream of uniform particles coming out, they apparently came out in chunks and crumbled."

The flyby revealed parts of the comet nucleus that weren't seen previously. In this image, three terraces of different elevations are visible, with dark, banded scarps, or slopes, separating them. The widest of the banded slopes is about 2 kilometres. The lowest terrace has two circular features that are about 150 metres in diameter. Image: NASA/JPL-Caltech/Cornell.

Stardust is a 12-year old spacecraft, and its first mission was to visit comet Wild-2, collect cometary particles and jettison them back to Earth, which it did successfully in 2006. It was then retargeted to rendezvous with Tempel-1 as Stardust-NExT (New Exploration of Comet Tempel-1). Meanwhile, the Deep Impact spacecraft was reassigned to fly by comet Hartley 2, which it achieved last November. The Stardust-NExT mission was carried out at less than 10 percent of the price a new discovery mission built from scratch would have cost, showing just how cost effective this new class of "recycled" mission can be at producing valuable scientific results.

Stardust-NExT will continue to make observations of Tempel-1 as it recedes from view over the coming 1-2 weeks, and we'll have a full, detailed report of the findings in the April issue of Astronomy Now.