Second Comet Visit is a First
by Bob Silberg
On January 15, 2006, the Stardust spacecraft completed one history-making mission and began another. Returning from a rendezvous with comet Wild (pronounced “Vilt”) 2, the spacecraft approached Earth and jettisoned the capsule containing particles from the comet and interstellar dust. The capsule landed safely and on-target, southwest of Salt Lake City, Utah, completing the world’s first sample return from a comet. The spacecraft, responding to commands from the mission’s navigators, flew past Earth and began an orbit around the sun that would bring it near the planet every 3 years.
Now this spacecraft is on a new record-setting mission: a visit to Tempel 1, the comet that was the target of the Deep Impact mission. It’s the first-ever follow-up mission to a comet. And since NASA has already built, tested, and launched the spacecraft, the new mission (called “Stardust-NExT” for New Exploration of Tempel 1) will cost less than 10 percent of what a mission like this would normally cost. That’s a saving of hundreds of millions of dollars.
NASA plans to revisit Tempel 1 in 2011 to finish the investigation begun in 2005 when the Deep Impact mission blasted a crater into the comet. “The crater’s there,” said Joseph Veverka, Professor of Astronomy at Cornell University and Principal Investigator of Stardust-NExT, “but we’ve never seen it.” That’s because the cloud of material ejected from the crater obscured the Deep Impact spacecraft’s view. By the time the particles slowly settled back down to the comet’s surface, the spacecraft, traveling at about 10 km (about 6 miles) per second, was gone.
Looking into the crater with Stardust-NExT will provide mankind’s first view of a comet’s internal structure, information that is not only scientifically interesting, but vital to our future ability to keep a comet from hitting the Earth. Even the size of the crater will be revealing. “That will tell us the mechanical properties of the subsurface of the comet,” Veverka said. “In other words, how does the comet respond to impacts? And that’s one of the fundamental things that you’d need to know if you were trying to blow up a comet or push it out of the way.”
In order to examine the crater, the mission’s scientists and engineers will have to time the encounter with extraordinary precision, so that the crater side of the rotating comet will face the spacecraft as it zips by at around 11 km (nearly 7 miles) per second. “That’s by far the most challenging part of the mission,” Veverka said.
“The good news,” said Allan Cheuvront, the mission’s Spacecraft Engineer and Program Manager, “is we have enough fuel that, a year before our encounter date, we can change our time of arrival up to 20 hours. And that’s significant because the rotational period of Tempel 1 is a little over 40 hours. So we improve our chances tremendously of arriving with the crater on the right side, in the field of view.”
Why perform this maneuver a year before the encounter? Because that’s the most efficient time to use the relatively small amount of fuel left over from the original Stardust mission and a maneuver in January 2009 in which the spacecraft used Earth’s gravity to change trajectory and increase speed..
The spacecraft can increase its speed a little further by firing its thrusters behind it, or slow down a bit by firing the thrusters ahead of it. Performing this maneuver in February 2010 allows a year of traveling time for that small change in velocity to build up into a change in arrival time of up to 20 hours. (If this maneuver were performed 30 days before the encounter with the same amount of fuel, it would result in only a 15-minute change in arrival time.) The tricky part, of course, is that the scientists will have to decide a year before arrival exactly how much the spacecraft’s speed needs to be changed in order to see the crater in 2011. And if they make a small error in calculating the comet’s rotational rate, that error will also have a year to compound.
Stardust-NExT will encounter Tempel 1 at about the same point in the comet’s orbit where Deep Impact met up with it six years earlier—a couple of weeks after the comet’s perihelion, its closest approach to the Sun. This provides an opportunity for what Veverka says may be the most important part of the mission: “to do something that’s never been done before, and that is to see how much comets change each time they go by the Sun. We’ve never to this date seen a comet once and then again after it goes through perihelion to see how much things have changed,” he said. Thanks to Deep Impact and Stardust-NExT, scientists will have before-and-after pictures of Tempel 1.