From: Marshall Space Flight Center
Posted: Wednesday, September 19, 2001
NASA's Deep Space 1 probe is about to begin its greatest adventure yet -- a daring plunge into a comet.
Kirk: "I take it the odds are against us and the situation is grim."September 19, 2001: On October 24, 1998, a Delta rocket lifted off from Cape Canaveral. It was a routine launch bearing an ordinary-looking spacecraft. But NASA insiders knew better. This mission was something special -- even its name was out of the ordinary: Deep Space 1 (DS1).
Picard: "You could say that."
Kirk: "Sounds like fun!"
-- Captains James T. Kirk and Jean-Luc Picard, from Star Trek: Generations
Most space probes are put together using tried-and-true technology, minimizing the chances of failure. But DS1 left Earth bristling with the un-tried. Its mission was to test a dozen new exotic technologies. Among them: a long-lasting ion propulsion engine, a solar array that focused sunlight for extra power, and software that endowed the craft with artificial intelligence to conduct its own affairs.
Ground controllers must have nail-biting nightmares about such missions. Almost anything on Deep Space 1 might go wrong. And when it did the spacecraft itself would be in charge!
But as they say, fortune favors the bold. Deep Space 1 spent nearly a year trying out its uncertain technologies and, improbably, they all worked -- many beyond expectations. DS1's ion engine, for instance, is now the longest-operating propulsion system in the history of the space program.
"Deep Space 1 tested these advanced technologies so that other missions wouldn't have to bear the costs of being first," explains Marc Rayman, the Deep Space 1 Project Manager at the Jet Propulsion Laboratory. "And it was a big success."
Rayman leads a resourceful band of JPL scientists and engineers who share their spacecraft's trailblazing spirit. Always thinking of new things to try, they were plotting Deep Space 1's next adventure even before its primary mission of technology testing was done. "We wanted to fly by a comet," recalls Rayman. It's something few spacecraft had tried before. Indeed, comets are virgin territory compared to better-studied planets and asteroids. Flying into the unknown seemed a natural mission for Deep Space 1.
Rayman continued: "When we were firing the ion engine during tests in 1998 and 1999 we always steered to keep our options open for an encounter." Their favorite target was 19P/Borrelly, an intriguing comet that had veered toward the Sun during the 19th century when it passed too close to Jupiter. It's been swinging through the inner solar system every 6.9 years since. Borrelly would reach perihelion (its closest approach to the Sun at 1.36 AU) on Sept. 14, 2001 -- just right for a meeting with Deep Space 1.
But first NASA had to approve the scheme. There were good reasons to go, says Rayman, but approval was not a foregone conclusion. "NASA has many important objectives and, of course, limited funding. Still, we were hopeful because our primary mission had been so successful."
Only one spacecraft has ever captured close-up pictures of a comet before: the European Space Agency's Giotto probe, which flew a scant 596 km from the core of Comet Halley in 1986. Photographs revealed Halley's nucleus as one of the blackest objects in the solar system -- one of Giotto's most surprising finds! Unfortunately, flying comet dust destroyed Giotto's camera, so when the probe flew even closer to comet Grigg-Skjellerup in 1992, the craft was blind.
At least four more spacecraft -- CONTOUR, Stardust, Deep Impact, and the ESA's Rosetta -- will rendezvous with comets in the years ahead. But scientists aren't sure if Giotto's experience will be a reliable guide for those missions.
"Comets are as individual as people," explains Rayman. "You couldn't expect to learn about the whole human race by studying a single person. Likewise, Halley can't reveal all we need to know about comets."
By visiting Borrelly, Deep Space 1 could examine a second comet at close range, perhaps as near as 2000 km, and provide valuable intelligence to comet mission planners.
In late 1999 NASA headquarters agreed: Deep Space 1 could visit Borrelly. "We were overjoyed," says Rayman, ever game for a new adventure. There was little time to spare for celebration, though. When Rayman heard the good news Borrelly was already plunging toward perihelion. "We needed to leave soon," he recalls.
And, of course, that's when disaster struck.
Just as Deep Space 1 was setting sail for the comet in late 1999, its guidance system, called the "Star Tracker," suddenly failed. The Star Tracker was a computerized camera (ironically not one of the probe's experimental devices) that measured DS1's orientation with respect to the stars. Without it, the spacecraft was lost.
JPL engineers quickly came up with a solution. They would reconfigure "MICAS" (an experimental camera that DS1 had just finished testing) to serve as a makeshift Star Tracker. It was a brilliant plan, but there was one drawback: Deep Space 1 was 300 million km away. No one could lay their hands on the spacecraft! Their only option was to transmit radio commands across interplanetary space, telling the onboard computer how it could transform MICAS into a star tracking navigator.
It was a spine-tingling operation: Time was short and testing was hurried -- one false command to the navigation system might disorient the craft forever! But in the end, Deep Space 1 was saved. "It was a thrilling rescue," recalls Rayman, "and just in time to catch Borrelly." (You can read a more complete account here.)
On June 28, 2000, with MICAS at the helm, Deep Space 1 aimed for the comet and revved its ion engine -- the beginning of a 15-month journey. Twice since then solar storms have scrambled the makeshift navigation system, and once MICAS tried to fix on a star that was too dim. "In each case, the DS1 team managed to coax the spacecraft back to normal," says Rayman. "It was stressful work. And unforgettable. There's nothing quite like joining a half-dozen space experts (powered by Oreo cookies) in the middle of the night to joystick a spacecraft that's hundreds of millions of kilometers away."
One more such incident, notes Rayman, and the encounter with Borrelly will likely fail. Fortunately the long journey is almost over. On September 22nd, Deep Space 1 will finally reach the comet -- and begin its greatest adventure yet.
This month amateur astronomers can easily spot Comet Borrelly through a 10 inch or larger telescope. Glowing at 9th magnitude in the constellation Gemini, it looks like a fuzzy blob with a short, faint tail.
That blob is the comet's coma, a huge cloud of gas spanning 50,000 km which hides Borrelly's nucleus deep within. The nucleus is an asteroid-sized mixture of ice, dust and rock. Albeit small, it's the source of all we see. Ices in Borrelly's nucleus, warmed by sunlight, are vaporizing furiously -- spitting dust and blowing gaseous jets. It's a maelstrom that could be deadly to a ship like Deep Space 1.
"Unlike Giotto," says Rayman, "DS1 was not built to encounter a comet. It has no shielding to protect it from flying debris. A single particle of comet dust the width of a human hair can deliver as much energy as a bowling ball does when it crashes into the pins." Deep Space 1 could be rocked hard by such particles dozens of times as it transits the coma -- about an hour-long journey at 16.5 km/s (37,000 mph).
That's not much time to find Borrelly's nucleus. "Our goal is to capture a black and white photo of the sunlit side of the nucleus while the spacecraft is about 8000 km away," says Rayman. But, he adds, it won't be easy. As soon as Deep Space 1 plunges into the haze, the navigation camera will have to stop tracking stars and begin searching for the core. Onboard gyros will do their best to hold the spacecraft steady as MICAS snaps a rapid-fire sequence of photos and analyzes them for hints of the comet's core. The procedure is complicated by the fact that no one knows if the nucleus will be dark (like Halley's) or bright; or if the irregular 5 by 5 by 8 km core will appear end-on or edge-on. And, of course, the spacecraft itself might be listing, buffeted by comet dust.
If MICAS beats the odds and finds the nucleus, scientists will get more than just a black and white photo. In addition to its optical camera, MICAS is also equipped with an experimental infrared spectrometer that can sense minerals from afar. (Even the impressive Giotto probe didn't carry such a device.) Not only will scientists see what Borrelly's nucleus looks like, they'll also learn what it's made of!
While MICAS conducts its feverish search for the nucleus, other instruments on board DS1 will be calmly collecting valuable measurements of the surrounding coma. "We've reconfigured many of DS1's sensors to study Borrelly," Rayman says. For instance, ion engine diagnostic sensors will "listen" for plasma waves in the gaseous coma and attempt to detect the comet's magnetic field. Also, an experimental mass spectrometer named "PEPE" will identify atoms and molecules in gases blowing by the spacecraft. "This is a wonderful opportunity to learn what the coma is made of," says Rayman.
Whether DS1 will emerge from the coma with stunning new data -- or emerge intact at all -- remains to be seen. But no matter, the Borrelly encounter is a guaranteed success by virtue of its daring. If the spacecraft is destroyed, other missions will be more wary and perhaps survive thanks to DS1's sacrifice. If the spacecraft lives, it will beam back priceless scientific information.
"This journey has been filled with adventure -- much of it different from what we had planned," says Rayman. "But that's OK. You're not going to win the big prizes if you don't take the big risks."
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