Rethinking Comets

The comet cloud defines the outer edge of the Solar System, ex­tending a thousand times further than the planets from the Sun, and a significant fraction of the distance to the nearest stars. It had always been presumed that these small bodies were frozen rel­ics from the formation epoch, essentially unchanged in 4.5 billion years. The expectation was that comets would be made of dust from a previous generation of stars, or pre-solar grains. That’s why the mission was named Stardust.

Instead, something quite unexpected was found: fire and ice.24 Comets contain ice that formed in the frigid zone beyond Nep­tune, but the bulk of a comet’s mass is rock, and that rocky ma­terial seems to have formed under conditions hot enough to va­porize bricks. The comet particles embedded in Stardust’s aerogel included two ingredients that are found in meteorites, debris from asteroids that formed between the orbits of Mars and Jupiter. One is chondrules, rounded droplets of rock found in many primitive meteorites that melted and quickly cooled as they orbited the Sun. The other is a much rarer mineral called a Calcium Aluminum Inclusion, irregular white particles of very unusual chemical com­position that can only form at very high temperature. The Stardust team puzzled over this discovery, but the implication seemed clear. Matter that formed readily in the inner Solar System was some­how transported to the edge of the young Solar System where the comets formed. Wild 2 does contain grains of pre-solar stardust material, but they’re very rare. Comets are not made of material left over from other stars; they’re made mostly from material that formed close to the Sun. As such, they provide insights into how planets and moons were built 4.5 billion years ago.

Stardust’s primary contribution to planetary science is verifi­cation of the idea that there was extensive radial mixing of ma­terial in the solar nebula just as the Solar System was forming. Also, comets are more physically diverse and variegated than any­one imagined. Each of the comets where we’ve got up close and personal—Halley, Borrelly, Tempel 1, and Wild 2—have different shapes, surface textures and features, and levels of activity. Com­ets may be modest in size compared to planets and moons, but their role in planetary systems is anything but modest. They deliver water and organic building blocks of life to terrestrial planets, and as we know from studies of our geological record, when they hit a planet they can decisively alter the history of ecosystems and even the entire biosphere. It’s appropriate that all human cultures have been fascinated by comets, and viewed them as harbingers of life and death.

To learn more requires an increase in our modest amount of di­rect information. So it was very exciting when Stardust was given a new lease on life after its sample return. In 2007, NASA approved the New Exploration of Tempel 1 (NExT) mission, a return to the site of the Deep Impact mission in 2005.25 Stardust had con­served just enough hydrazine fuel to make a second journey to a comet, and the goal was to observe the impact made by the ear­lier spacecraft and see any changes on Tempel 1 caused by its last close approach to the Sun (Deep Impact’s cameras were blinded by dust released from the impact). In early 2010, a controlled burn ensured Stardust would approach at the optimal speed. The flyby was challenging because the camera had to catch the correct side of the tumbling comet nucleus while coming up on it at 7 miles per second. Appropriately, our first second date with a comet took place on Valentine’s Day, 2011. Stardust showed that the 500-foot wide crater created in 2005 was indistinct and had a mound in the center, indicating that much of the ejected material had fallen straight back down in the weak comet gravity. The results indi­cated that the comet nucleus was fragile and only weakly held together.

All good things must come to an end. For Stardust the end came on March 24, 2011, when mission controllers deliberately ordered a “burn to completion.” They told the spacecraft, which was al­ready running on fumes, to fire its main engines until there was no fuel left. NASA used the final burn to refine fuel consumption models for these kinds of engines, ensuring that Stardust gave use­ful data right up to its last gasp. The next day, Project Manager Tim Larson put the spacecraft into a state called safe mode, turned off the transmitter, and walked away from the console. Mission accomplished.