A Tale of Two Comets - NASA's Stardust Samples Amaze Researchers, as Mothership is Eyed for Recon at Deep Impact Site

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This article is republished with permission and appears in the 20 March 2006 edition of Aviation Week & Space Technology.

In a dramatic bid to maximize the utilization of existing, low- cost planetary spacecraft, researchers want to divert the NASA/Lockheed Martin Stardust comet-sample-return mothership to intercept and image a second comet, blasted open last July 4 by the Deep Impact mission.

Stardust is about 20 million mi. behind the Earth in a solar orbit that earlier enabled it to collect samples from the comet Wild 2 (AW&ST Jan. 12, 2004, p. 29).

If the new mission receives final approval, the Stardust orbit would be adjusted later for a reconnaissance of the 70-ft.-deep, Rose-Bowl-sized crater that Deep Impact violently excavated on the comet Tempel 1, 83 million mi. from Earth.

The objective had been for the NASA/Ball Aerospace Deep Impact flyby spacecraft to image Tempel 1's subsurface geologic strata, a mission now being sought for Stardust.

Deep Impact was unable to image the crater because of the unexpectedly dense and opaque debris cloud from the blast caused by its 820-lb. impactor that struck the Tempel 1 at 30,000 fps. The flight’s total mission cost was $328 million.

The new Stardust plan is being kept largely secret by the space agency because it's an add-on to NASA's new Discovery mission selection process, just moving into its final stages at NASA headquarters, says Thomas Morgan, Stardust program scientist.

Although costing only a few million dollars and nowhere near what building an entirely new spacecraft would cost, the plan must undergo a competitive analysis within the tight NASA science budget. Final details on the new mission for Stardust must be submitted by Apr. 5, when the new proposals are due at NASA headquarters, says Lindley Johnson, a Discovery mission manager.

The new intercept, to take place about 2010, would be managed by the Jet Propulsion Laboratory in Pasadena, Calif. Developed for JPL by Lockheed Martin, Stardust and its 1999 launch on a Boeing Delta II cost only $210 million.

Comet and asteroid science missions are at the forefront of planetary exploration because they can be done cheaply and offer big science payoffs.

The new Stardust sample data are themselves colliding headlong with previous comet theories compiled without the benefit of samples. The analysis show the diverse minerals found in the Wild 2 Stardust samples (see photo above) had to have been formed as extremely hot materials near the core of a primordial planetary nebula around a star—either the Sun or some other distant star.

This is the opposite of existing models of comet formation that have them most likely made up of the coldest—not the hottest—debris from the frigid outer edges of a planetary nebula or the cur-rent Solar System. The distinction matters tremendously to planetary science, because it will affect broad theories on the formation of this and other solar systems.

The findings stunned the more than 1,500 international planetary scientists and managers at the 37th annual Lunar and Planetary Science Conference (LPSC) here near the Johnson Space Center (JSC).

Other major LPSC highlights include new evidence to bolster earlier controversial evidence for past microbial life on Mars and European and U.S. lunar mission actions (see pp. 29 and 34).

Another session was a contentious exchange between NASA headquarters managers and about 600 LPSC attendees. LPSC participants roundly applauded criticisms expressed by several researchers that NASA managers have ceased to communicate with the planetary science community and international mission partners.

Mary Cleave, the NASA Science Mission Directorate associate administrator, told the group she "gets the message" and is in new talks with the White House to help make NASA's Fiscal 2007 science budget request more responsive—within the budget realities forced by the space shuttle program recovery. The shuttle's next launch was itself delayed last week to no earlier than July 1 (see p. 56).

Although it failed to image its impact crater, the Deep Impact team presented key new details here about the bizarre 75%-porous, talcum-powder-like surface it found at Tempel 1.

The Deep Impact flyby stage itself will also likely be vectored to fly by another comet or asteroid eventually, although trajectory limitations would prevent it from returning to Tempel 1. New comet target options for it are also to be submitted by Apr. 5.

But the direct comet samples collected from jets shooting off Stardust's Wild 2 target have created a major new mystery in the study of Solar System formation, says Don Brownlee, Stardust principal investigator from the University of Washington. The mystery is: How do you get Stardust sample minerals that had to be formed in the hottest, innermost part of a solar nebula in a body roaming the coldest, most distant reaches of the Solar System?

Brownlee said the minerals found in the Stardust samples had to be formed in "red-hot or white-hot conditions" at about 2,000F. This compares with the team's pre-mission expectation to find samples formed more around absolute zero at, -459F.

"That is a very big surprise," said Michael Zolensky, the Stardust sample curator and co-investigator at JSC.

Brownlee said the samples are likely to prove more important for understanding Solar System formation as a whole, than have been the lunar surface samples collected by the Apollo astronauts and the three successful unmanned Soviet lunar sample-return flights.

The lunar samples have been a key to understanding the formation of the Earth/Moon system, but less revealing about the Solar System as a whole.

How the large international team answers the new questions posed by the Stardust samples "will place fundamental restraints on our understanding about how the Solar System was formed," Brownlee said.

Zolensky said the Stardust findings will factor into how zoning between the different planets and small bodies in our Solar System—and others—take place, and whether there was a major transport mechanism to fire material outbound 4-5 billion years ago as the solar nebula coalesced.

Brownlee said one possible theory involves gigantic jets of material seen in telescope images of other embryonic star systems firing material high above and below the ecliptic planes of these stars. Maybe that mechanism tossed early nebula material from near its core out beyond where Pluto now orbits, he mused.

Another theory is that the stuff Stardust tied into may come from a totally different planetary system than our own. Further research should be able to sort that out, Brownlee said.

The Stardust samples collected in aerogel within minutes of spraying off the comet, 242 million mi. from Earth are also far larger than expected.

Instead of the 0.25-micron-sized particles anticipated, Stardust collected many 10-micron-sized particles.

"We have got rocks," Brownlee says, at the end of the tracks in the aerogel that slowed and cushioned the particles as they hit Stardust at 6 km. per sec. "These are real rocks, real minerals." They include olivine, the primary component of green sand found on Hawaiian beaches—a surprising finding from a comet.

A major advancement in sample handling is also enabling each sample smaller than the width of a human hair to be sliced and sectioned for major analysis by researchers on six teams with members worldwide—including Antarctica, where one researcher is based. JSC is slicing 10-micron samples— 0.0004 in. across—into hundreds of individual sections.

The samples tracks in the aerogel are up to 11.7 mm. long and many have root-like sprays, as if the particles exploded on their way into the medium, says Peter Tsou, deputy principal investigator, who led development of the aerogel system at JPL.

The sample tracks in the aerogel are 100 times larger than the team simulated during the seven years between when Stardust was launched and the Jan. 17 return capsule's drop-off by the Stardust mothership that remains in space (AW&ST Jan. 23, p. 20).

Brownlee said that in the JSC Stardust sample facility, the mood is just like it was during Apollo, when technicians would hold the lunar samples and comment, "I just can't believe these came from the Moon." But this time around, they are saying, "I can't believe these came from a comet."


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