Stardust: The mission that forever changed our understanding of comets

DA: Can you tell us about your experience recovering the sample collector? 

Brownlee: It was the middle of the night in a very isolated Utah location [the capsule was recovered in Dugway, Utah, in the west desert near the Great Salt Lake]. The entry of the capsule was a wonder. It was [a] glowing red fireball with a luminous tail coming in from the west. It was coming down and getting closer but viewed from ground zero it oddly climbed up in the sky. People near [the town of] Wendover heard the sonic boom and a NASA aircraft got spectacular images as did a film crew from Japan on the ground. I was outside to see the fireball and then inside to watch images from tracking devices. It landed in the dark and it took several hours to find it. It was found by our helicopter crew and returned to a special cleanroom facility prepared for it where it was inspected and prepared for [a] flight the next day to the Johnson Space Center in Houston. 

DA: What did the analysis of the comet dust particles tell us? What were the main findings?

Brownlee: The major finding is that the rocky components of the comet, most of its total mass, formed at red hot temperatures. Comet formation included fire and ice. Some of the ice formed at near absolute zero temperature(s), but the rocky materials formed at white hot conditions. Many of the materials in the comet have also been found in meteorites. Comet Wild 2 is a broader mix of components suggesting that materials from a broad range of locations were transported out beyond Pluto where the comet formed. The rocky materials mostly formed at temperatures above 1832 degrees Fahrenheit (1000 degrees Celsius) and could not have had any ices or organics on them at their time of formation. The comet rocky silicate materials formed first, then assembled with ice and organics in a drastically colder place. This proved that the formation of comet dust and ice was clearly decoupled. The samples proved that the outer solar system was not isolated from the inner solar system, and that materials were clearly mixing over regions from near the Sun to regions beyond the orbit of Pluto.

DA: Looking back on the mission all these years later, what are your fondest memories of the experience?

Brownlee: Launch: a totally amazing experience when it is your own mission. I watched the launch from 1 mile (1.6 km) away, it was so clear that we could see the 4 solid rocket motors separate and fall with the naked eye.

The flyby: The tension was just incredible. No matter how careful you are, space exploration involves risk and unknowns, you are involved in a strange game of Russian roulette. Over the entire mission, [I] felt like a soldier on the beach during D-Day [in World War II]. Bullets are whizzing around you, and you hope that one doesn’t hit you. 

Atmospheric entry and recovery: The launch was so awe inspiring that many had tears in their eyes. This was nothing like seeing the fireball entry of the sample return capsule in the middle of the night, in the middle of the desert with a glowing luminescent tail behind it. 

Opening the collector: We opened the collector in a special cleanroom at the Johnson Space Center. Mike Zolensky and I were the only ones right next to it and when the aerogel array was finally exposed, we were looking at it from the back side. We couldn’t see impacts and the aerogel looked even better that it looked before launch. We wondered if the collector had actually opened. We were nervous but felt like we could see some capture tracks but [were] not sure because we were looking through the back side of about 1.6 inches (4 centimeters) of aerogel. When the collector was flipped up, we could instantly clearly see capture tracks. 

Probably my greatest thrill of the mission was presenting the first results of the sample analysis at the annual Lunar and Planetary Science Conference just 3 months after landing. When I showed results to 600 people packed into the room, you could hear gasps and see jaws drop. We had gone to a type of body that is famed for its ices, a body whose dust was believed to be dominated by solids formed around other stars. We had found that it contained the highest temperature material that could ever have existed in the solar system. To find such material in a comet was revolutionary. Our modest mission had returned samples to Earth that told us things about comets that could never [have] been known by remote sensing methods. 

Doug Adler is the co-host of The Right Stuff Companion podcast and the co-author of the book: From The Earth to the Moon: The Miniseries Companion