Many From One

But the plucky spacecraftwhich completed the goals of its primary mission back in September 1999 and finally ceased operations December 18, 2001will be remembered not so much for the science it did during that comet flyby but for the broader impacts resulting from the technologies it tested (see sidebar). "In one day it produced enough [images and measurements] to revolutionize our understanding of comets," says Marc D. Rayman, project manager for the mission at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "But its primary mission will have consequences for so many future space missions. The long-term science return is going to be stupendous."

DS1 was launched in October 1998 as the first mission from NASA's New Millennium program, begun in late 1994. The mission goal was to test a dozen advanced technologies, including an ion-propulsion engine, an autonomous navigation system that replaces the standard total spacecraft control from the ground, advanced solar-concentrator arrays and miniaturized instruments. The mission cost a total of just under $160 million and went from start to launch in three yearsmaking it one of the least expensive and most rapidly developed interplanetary missions of the modern spaceflight era.

Of the 12 technologies that DS1 tested, perhaps ion, or solar electric, propulsion has received the most attention. And rightly so: although ion engines had been used in science-fiction books and movies for decades, before DS1 one had never been used for primary propulsion on a real spacecraft. The principle is simple: An electron gun ionizes a xenon propellant entering a chamber that is about the size of a coffee can. Two negatively charged grids of molybdenum at the other end draw the xenon out of the spacecraft to provide thrust; a cathode adds electrons to the departing xenon, so the spacecraft doesn't become negatively charged (see illustration). The xenon ions provide a tiny thrustroughly equivalent to the pressure exerted by a sheet of paper resting against your hand, and 10,000 times smaller than the force provided by typical chemical propulsion systems on planetary spacecraftthat slowly but efficiently builds spacecraft speed.

By the time DS1's ion engine was shut off, it had thrusted for more than 677 days and had used 73.5 kilograms of its original 81.5 kilograms of fuel. The minimum operational time required to achieve basic mission success was 200 hours. Because ion propulsion is 10 times more efficient than conventional chemical propulsion, DS1 was able to change its speed much more often and with much less propellant than would have been possible otherwiseimportant when launch costs tens of thousands of dollars per pound.

As a result of DS1's tests, ion propulsion is now part of the toolkit of acceptable technologies for mission designers to employ in their future flight plans. For instance, it is being considered for a mission that will rendezvous with asteroid Vesta, orbiting it for nine months and then continuing on to rendezvous with Ceres, studying it for nine months. "There's absolutely no way to do that with conventional chemical propulsion," Rayman says.

Another DS1-tested technology bound for space again is AutoNav, the autonomous navigation system that enabled the spacecraft to pilot itself for long stretches of time. A similar system will be used on Deep Impact, a mission launching in January 2004 that will drive a 350-kilogram impactor into a comet to reveal the subsurface for study.

With AutoNav, DS1 required much less of the precious, limited time of ground antennas to stay on course. And whereas all previous spacecraft missions were entirely operated from the ground by tens or hundreds of ground controllers, DS1 required fewer than a dozen. Rayman believes that more spacecraft will have to demonstrate this kind of independence for future missions that NASA is planning. "We can't build or operate the way we used to because we don't have the same conditionsand not the same science questions either," he says. "If we want to bring samples back to earth from more difficult locationscomets, asteroids, moons, other planets like Marswe need spacecraft that are smaller and smarter. Spacecraft are going to have to make decisions for themselves. We need to have them be more agile and adapt to changing situations in their environment. If we're going to be operating many [missions], we're going to have to have autonomy."

That's not to say that DS1 was agile enough to avoid all difficulties and disappointments during its voyage. For instance, in its first flyby encounter in July 1999, DS1 returned disappointing images of the asteroid Braille. The asteroid was fainter than the worst-case prediction and the camera did not respond to the faint light the way it was expected to, so it provided no useful signals to the tracking system. And then the spacecraft's star trackerthe critical device that enables spacecraft to stay its coursefailed in November 1999; the star tracker was not one of the test technologies, but a commercial device. Spacecraft engineers developed a creative mix of new software and new uses for the existing equipment that kept the aging spacecraft going to perform an extended mission. AutoNav and the science camera were pressed into additional service. "When the star tracker failed, we began using the science camera for a star tracker. Since early last year, it's taken one picture every 30 seconds [or one million pictures a year]," Rayman says. AutoNav used the pictures to help orient the spacecraft.

"By the time it got to the comet [Borrelly], the spacecraft was old and debilitated," Rayman says. "To use that to return such exciting sciencethis is so incredibly cool."

In the last couple of months after the Borrelly flyby, DS1 still continued to function for technology tests, which Rayman dubbed "the hyperextended mission." Mission engineers operated the ion-propulsion system in some nonstandard configurations and tested some of its failure modes to explore the effects of aging on the system. Indeed, all nine of the hardware technologies were exercised during the hyperextended mission.

Now that the engine has been turned off, DS1 will drift in its orbit around the sun, its job completed; its radio receiver will be left on, in case future generations want to contact the spacecraft. "It'll just become a piece of cosmic flotsam," Rayman notes, "but one for which we'll have a lot of fond memories."