• August 19, 2013

    Let's Swing By for a Closer Look: Sending Spacecraft to Comets

    by Bonnie Meinke

    If you ask almost any planetary scientist why she chose planetary science, she’ll likely say something along the lines of “I wanted to get my hands dirty!” The relative closeness of objects in our solar system makes them much easier to explore than our stellar neighbors, and technological advances have given planetary scientists the opportunity to truly touch other planets.


    We now live in a world where, given enough time and money, we can visit any body in our solar system using spacecraft. These time and money constraints, however, have often forced planetary scientists to rely on serendipity to study comets.


    Such was the first spacecraft encounter with a comet, when the International Cometary Explorer (ICE) flew through the tail of Comet Giacobini-Zinner in 1985. ICE wasn’t designed specifically for this purpose – in fact, it was sent into space to study the solar wind. ICE was actually the second incarnation of the International Sun/Earth Explorer 3 (ISEE-3) mission, which changed names when it changed course in order to intersect with the comet tail. Since then, many missions have capitalized on the opportunity to repurpose other probes to rendezvous with comets.


    Of course, comets are such interesting targets that there are also entire missions intentionally dedicated to studying them. In March of 1986, all eyes were trained on Comet Halley, including five missions dedicated to encounters with the comet. Two Russian Vega missions took advantage of a “two-for,” deploying a lander and balloon at Venus, then using a Venusian gravity assist to change course for encounters with Comet Halley. Japan sent the Susei and Sakigake probes. The European Space Agency mission Giotto came within 375 miles of the nucleus!


    Since the 1980s, so many international missions have encountered comets that there are too many to list here. (That’s what Wikipedia is for, right?) Many of these missions flew through comet tails and imaged the nuclei. Myriad others observed comets from space and the ground.. We began to understand comets and, with that, to understand the grand-scale structure of our solar system. If these little chunks of ice and dust, left over from the birth of our Sun and planets, can reveal the structure of our solar system, perhaps they can also reveal its origins. Unfortunately, observations can only tell us so much about such small, dark bodies.


    Get some planetary scientists in a room to discuss the next steps in probing comets, give them too much coffee, and soon a new idea will pop up: Let’s blow it up! In 2005, the aptly named Deep Impact mission deliberately smashed a projectile into Comet Tempel 1. The photographs the mission took of the high-speed impact helped scientists decipher the composition and structure of a comet nucleus.


    The last 30 years has been a time of great observing in the field of cometary science, indeed, but remember how I mentioned planetary scientists’ need to get their hands dirty? The next step was to bring comet material home to Earth for study. Enter STARDUST, which allowed scientists to literally touch a comet. STARDUST was able to grab a sample of Comet Wild 2 by flying through its coma and “catching” pieces of dust in “aerogel,” an amazing material 1,000 times less dense than glass. Aerogel can capture cometary dust without heating or physical alteration, thus preserving each speck of dust in its natural state. 


    Comet Wild 2’s nucleus, imaged by the STARDUST mission. Credit: NASA


    Once delivered back to Earth, a team of researchers were able to retrieve little bits of comet (each smaller than a grain of sand) out of the aerogel and study its composition. Comets are the remnants of solar system formation, composed of the oldest stuff in our solar system. Studying what a comet is made of is like looking back in time to an infant solar system.


    Piece of cometary dust in aerogel used in the STARDUST mission. The long streak is the result of the aerogel braking the high-speed particle. Credit: NASA


    After the sample collection and return, in the spirit of multiple encounters, the STARDUST spacecraft went into its extended mission, known as STARDUST-NExT, in which it flew by Comet Tempel-1 to observe the aftermath of the crater left during the Deep Impact mission!


    Over the last three decades, cometary scientists have made the most of each opportunity to study these icy visitors to the inner solar system, trying to get the biggest bang for their space-exploring buck by using innovative probing techniques and visiting several comets per mission. With such ingenuity, they have advanced the study of comets from the fuzzy blobs of our ancestors’ night sky to the concrete samples humans can literally reach out and touch. Comets may be infrequent visitors to our inner solar system, but we’ve been making the most of their occasional trips.