Extraterrestrial Capture

Interview with Don Brownlee

"This is an exciting time," says University of Washington astronomy professor, Don Brownlee, who is also the principal investigator for NASA’s Stardust mission. Stardust is scheduled to contact a comet on January 2nd, and return collected dust to Earth in January 2006. It is the first sample-return mission since Apollo. The spacecraft will fly a near-collision course within 300 kilometers (200 miles) of the wispy tail of a comet called Wild 2, while trapping tiny comet particles in a low-density silica material called aerogel.

donald_brownlee Donald Brownlee, principal investigator for Stardust. Brownlee is co-author of "Rare Earth," "The Life and Death of Planet Earth", and Professor of Astronomy at the University of Washington in Seattle.

To the delight of mission scientists on November 13, the cometary ball of dirty ice and rock–about as big as three Brooklyn Bridges laid end-to-end– was visually detected by the spacecraft’s optical navigation camera on the very first attempt. To make this detection, the spacecraft’s camera saw stars as dim as 11th visual magnitude, more than 1,500 times dimmer than a human can see on a clear night. Stardust’s main camera, built for NASA’s Voyager program, will transmit the closest-ever comet pictures back to Earth.

During the forthcoming January 2nd encounter, the comet particles will be traveling five times faster than a rifle bullet, but the collecting aerogel will stop this dust in a fraction of an inch. Because aerogel is as much as 99.9 percent empty space, the collision will not damage the grains or appreciably alter their characteristics.

The spacecraft is scheduled to return to Earth two years later when a capsule containing its treasure – less than an ounce of comet dust – will parachute to the Utah desert. While thousands of tons of microscopic comet particles blanket Earth each year, Brownlee said that, "unfortunately, they are difficult to find among the earthly materials. And even when extraterrestrial particles can be found, they are cosmic orphans – there is no way to determine their origin."

In addition to understanding the origin of comets, the mission may offer insights into how life originated on Earth. Comets are thought to have delivered a significant share of the Earth’s water, and "this [mission] gives us a real opportunity to find out if our long-held suspicions are right, that comets played a major role in the origin of life," Brownlee said. "No one really knows how life began, but we’re certain that carbon was key to the process. Comets are the most carbon-rich materials in the solar system, and we know they are full of organic compounds that fall on the Earth all the time."

Astrobiology Magazine had the opportunity to talk with Professor Brownlee as the Stardust spacecraft approached its dramatic encounter with a comet.

Astrobiology Magazine (AM): When did your mission work on Stardust begin?

Don Brownlee (DB): We actually began this work in 1980 as a proposal to do an "atomized sample return" from comet Halley. The mission (Halley Earth Return, or HER) did not happen but it began a chain of developments and proposals including sample return missions with ESA and Japan. The breakthroughs were the development of intact capture using aerogel and the NASA Discovery program.

Stardust was selected in 1995.

AM: What is the exact date predicted for the terrestrial return of samples in the Utah desert?

DB: January 15, 2006.

AM: What are the logistical preparations for this mission, as the first sample return since Apollo?

Dust grain or IDP, interstellar dust particle
Credit: UWSTL, NASA Hubble

DB: A cleanroom for curation and distribution of the samples is being built in Building 31 at the NASA Johnson Space Center in Houston. Special methods are needed to handle and extract samples from the silica aerogel that they are collected in, but after extraction the samples will be treated in ways that are similar to those used for cosmic dust particles in the Cosmic Dust lab also in Building 31.

Most samples will be 5 to 20 micron range– identical to the typical cosmic dust particles collected in the stratosphere. Stardust samples will usually be studied by consortia groups around the world using sequences of different techniques.

AM: What would be the typical organic compounds that one might expect to find arround a comet?

DB: The most common nonvolatile organics are likely to be similar to those found in carbonaceous chondites [carbon-rich meteorites]. This is likely to be a kerogen-like material [waxy, organic solid] but we will see.

AM: When the aerogel sample from Stardust is taken to Houston’s Johnson Space Center, what sorts of tests are run? Mass spectroscopy and electron micrographs?

DB: The initial inspection at JSC will be to measure track lengths and estimate particle mass and state of preservation. Other work will be usually done elsewhere by a wide range of techniques– essentially all methods that can be used to study nanogram and smaller samples.

Aerogel panel used to capture comet particles.
Credit: JPL/Tsou

AM: What is the tell-tale signature that the material is pristine and cometary in origin? A particular ratio of key chemical elements from the mass spectroscopy?

DB: If it collected by Stardust on the "comet side" of the aerogel it is nearly 100% certain to be cometary. If we do pick up a few interplanetary dust particles for even impact debris from the spacecraft, those particles will not produce tracks perpendicular to the front plane of the collector.

This is the beauty of Stardust– all particles on the comet side have a known origin and are collected at exactly the same speed (6.1 km/s).

AM: What type of temperature control is possible within the aerogel to preserve the state of the comet dust–for instance, what happens if a tiny amount of water is captured in frozen form?

DB: Particles are pulse heated during capture– a few microseconds. When they are returned to Earth they are heated to ambient conditions — perhaps as much as 35 C. No liquid water will survive except as surface films on the aerogel.

AM: There are many mission concepts for extending the Stardust collection method, in particular for projectiles fired into Europa to stir up enough particles that a flyby might collect surface samples without actually landing. What is your opinion of these concepts?

DB: Both a Europa (Ice Clipper) and a Mars sample mission ("Sample Collection for Investigation of Mars", or SCIM) have been proposed. They are very interesting. Stardust-style sample collection has the great attraction that it does not require landing.

AM: The actual encounter with the comet, Wild 2, will last a few minutes, correct?

DB: Stardust will fly 300 km from the comet at 6 km/s. In two minutes it will cover 90 degrees of phase angle. A few minutes is correct for most of the collection and imaging.

AM: After the January 2nd encounter, how will you know that collection has been nominal? Are there telemetry signals that give enough information to infer capture?

DB: We will have a carrier signal during the encounter telling us that the spacecraft is OK. The estimate of the success of the collection will be based on the number of particles detected by the dust collector (Dust Flux Monitor Instrument, or DFMI) and the mass spectrometer (Cometary and Interstellar Dust Analyzer, or CIDA).

What’s Next

first view of annefrank
First images of Annefrank asteroid from Stardust
Credit: NASA/JPL, U. Wash

Stardust, the fourth in NASA’s Discovery missions and the first mission designed to return samples from beyond Mars, was launched from Cape Canaveral, Fla., on Feb. 7, 1999. It is currently on its third giant loop around the sun, and will have traveled some 3.1 billion miles by the end of its voyage. In November 2002 during an earlier asteroid flyby of Asteroid 5535 Annefrank, the spacecraft successfully tested systems it will use in this week’s Wild 2 encounter. During its nearly five years in space, it has captured interstellar dust using the opposite side of the collector that will gather the grains from Wild 2.

In the next 5 or so years, there will be no fewer than four to five encounters of spacecraft with comets and asteroids. Of particular interest next year, NASA’s Deep Impact spacecraft is expected to make a stadium-sized crater in Comet Tempel 1 on July 4 of 2005, making a large enough impact to be visible through telescopes on Earth.

On Valentine’s Day in 2001, the Near-Shoemaker spacecraft successfully landed on the asteroid, 433 Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid about 176 million kilometers (109 million miles) from Earth– prompted Andrew Cheng, NEAR Project Scientist, to note: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."

All the following missions are fully funded, though not all have been launched:

2001 Sept. 22 Comet Borrelly Deep Space One (flyby)  
2004 Jan. 2 Comet Wild 2 Stardust (coma sample return)
2005 July 4 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. Asteroid 1998 SF36 Muses-C (sample return)
2014 Nov Comet Churyumov-Gerasimenko Rosetta (flyby/landing)

Stardust is a collaboration of the UW, NASA and its Jet Propulsion Laboratory at the California Institute of Technology, and Lockheed Martin Space Systems. Other key members are The Boeing Co., the Max-Planck Institute for Extraterrestrial Physics, NASA Ames Research Center and the University of Chicago. Stardust, a part of NASA’s Discovery Program of low-cost, highly focused science missions, was built by Lockheed Martin Astronautics and Operations, Denver, Colo., and is managed by JPL for NASA’s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena. The principal investigator is astronomy professor Donald E. Brownlee of the University of Washington in Seattle.

Related Web Pages

Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space