NASA's Cosmic Two-For-One Deal: A Return to Comet Tempel 1

Artist concept of Stardust-NExT approaching comet Tempel 1.
Credit: NASA/JPL-Caltech/LMSS

The human race is certainly leaving its marks on the Solar System. On Valentine's Day, NASA revisited the locale of one of those marks—comet Tempel 1—and far from being a simple "I was here" (like the three scratch marks left behind by Jules Verne's intrepid explorer in Journey to the Center of the Earth), this one was a crater 200 meters across created to see what makes the comet tick….

Cast your mind's eye back five years. Remember NASA's Deep Impact spacecraft, the one that lobbed a heavy metal projectile at Tempel 1 in hopes of seeing what came flying out of the blast, and more excitingly what the hole it made looked like afterward? The idea was to get a better grip on how the comet is put together (is it crunchy, powdery, ice-hard; is it light like Styrofoam, or weightier like block-ice, or concrete?).

Well, as it turned out back in 2006, Deep Impact successfully bullseyed the little 4.5-mile long ice potato—and the impact was so effective that the spectacular blast, as good as anything from the ILM special effects department, completely obscured the spacecraft’s camera-eye view, hiding the would-be crater from sight. The mission was a success, I should add; Deep Impact got plenty of good data and images of the comet and the blast plume—just not the man-made-object-made crater it made….

Fast forward to February 14, 2011. Exercising a good reduce-reuse-recycle ethic, NASA sent the "NExT" spacecraft past Tempel 1, visiting the same comet twice for the first time in history, and doing so with an existing spacecraft that had completed its initial mission years before.

Cast your mind's eye back to the Stardust mission, which flew through the tale of comet Wilde 2, collecting particles from the plume in a block of comet dust "fly paper" made of aerogel (basically glass spun up so light and fluffy as to barely register any weight or substance, a piece of which looks like a slab of solid smoke), and dropping the collector pod back on Earth for the first ever comet sample return mission. Stardust flew on, circling the Sun for several years, and was finally re-tasked as NExT—the "New Exploration of Tempel 1" mission.

It's as if NASA is playing a celestial shell game: Where's the spacecraft now? What is it named? Which comet is it going to this time?

NExT flew within 112 miles of the Tempel 1 nucleus and captured over 70 images. Yes, scientists hoped to capture an image of the crater left behind by Deep Impact—and they did; see if you can spot it too. But, there is plenty more to interest us than a hole in the snow. The fact that this comet had been visited five years prior means that we can compare images from then and now to see any changes that may have taken place. As it turns out, five years is not only the interval between the visits by Deep Impact and Stardust/NExT, it's also the orbital period of comet Tempel 1—so, the comet had made exactly one swing around the Sun between visits, providing the opportunity to study Sun-induced changes.

Tempel 1 is about 4.5 by 2.9 miles in size, and has an average density of about 0.62 grams per cubic centimeter—about five times denser than the densest Styrofoam, two-thirds as dense as ice, and 600 times more dense than aerogel…. Its elliptical orbit carries it between the orbits of Mars and Jupiter—and it is that range of exposure to solar radiation that has researchers looking for physical changes in the comet.

By the way, the Deep Impact spacecraft was also reused to make a second comet flyby. Five years after bombing Tempel 1, Deep Impact, renamed EPOXI, flew by the peanut-shaped comet Hartley 2, back in November. Between Stardust and Deep Impact, there were a lot of firsts: first sample return mission from a comet; first time a single spacecraft has visited two comets; first time two spacecraft have visited the same comet….

It's doubtful there is enough fuel left on either of these veterans for another opportunistic encounter, but talk about bang for the buck….

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