Going Comet Wild

NASA's STARDUST spacecraft, bound for comet Wild-2, celebrated its first year in space this month.

February 28, 2000: In February 1999, NASA's STARDUST spacecraft blasted off from the Kennedy Space Center for a daring encounter with periodic comet Wild-2. Its ambitious goal is to intercept Wild-2 in 2004, to capture tiny bits of comet dust and debris, and then return them to Earth for analysis in 2006. On the way, STARDUST will also sample a stream of dust particles from outside the solar system.

STARDUST is the first comet rendezvous mission since the European Giotto spacecraft's fly-by of Comet Halley (1986) and Comet 26P/Grigg-Skjellerup (1992), and the first ever to attempt to return a comet sample to Earth. It's a long 7-year mission, but one most scientists feel is worth the wait.

Right: Comet P/Wild-2 photographed by K. Meech on Dec. 17, 1990 using an 88 inch reflector telescope on Mauna Kea, Hawaii.

"I'm Here! I'm OK!"

After one year in space STARDUST is doing well, say project officials. The spacecraft has executed several flawless course adjustments, and last week it deployed its aerogel collector for a first-ever sampling of interstellar dust particles. Still, STARDUST has given mission controllers their share of sleepless nights.

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"There have been storms to sail through," recounts STARDUST project manager Dr. Kenneth Atkins in a recent commemorative essay. "The first attempt to move from gyro-stabilized control to pure star-referencing found a software bug that caused the spacecraft [to go into safe mode]. When the ship invokes this routine, it shuts down all unnecessary activities, including communications with Earth, and turns to the Sun to ensure the lifeblood of solar energy floods its batteries and electronics with electricity. When it deems all is well, it sets up a plan to contact us on Earth, tell us what happened, and let us tell it what to do next. This routine, while carefully designed to protect the spacecraft, is still an 'anxiety event' for the crew back on Earth. It’s a bit like the feeling when your teenager is late coming home, and you get no phone call. The anxiety builds fear until the dutiful signal comes through. 'I’m here!' 'I'm O.K.!'"

STARDUST and its crew have successfully navigated three more safe mode events, all involving data handling by on-board software.

What's all the fuss about Wild-2?

Scientists are curious about comets because they are thought to be the oldest, most primitive bodies in the solar system. Comets are made up of the same stuff as the early Solar Nebula that collapsed to form the sun and planets. It is now known that comets contain significant amounts of water ice, dust, and carbon based compounds. They may have been an important source of water and organic molecules for Earth when many comets collided with our planet during a period of heavy bombardment over 4 billion years ago. Modern-day comets are like a time machine. They offer a window into the past when the Solar System was young and life on Earth was just beginning.

History is filled with famous comets. Halley's comet, Hale-Bopp, Hyakutake and others have dazzled observers with their brilliant nuclei and dramatic tails. Recent comets like Hale-Bopp have been viewed by hundreds of millions of people, and Halley's comet has had a real impact on history, as in 1066 when it was so bright that it terrified millions of Europeans and was widely credited with the Norman victory at the Battle of Hastings.

Unlike its famous cousins, comet Wild-2 is a relatively dim, new arrival to the inner solar system. Before September 1974, when it passed within 0.006 AU of Jupiter, Wild-2 circled the Sun in an orbit between Jupiter and Uranus. That encounter with the giant planet, at only 10 times the distance which fragmented P/Shoemaker-Levy 9 in 1994, altered Wild-2's orbit so that its closest approach to the Sun now lies just inside the orbit of Mars.

Above: Where is comet Wild-2 right now? This view, looking down on the sun shows the orbit of Wild-2 and its current location. This picture from JPL's Solar System Simulator is updated every 5 minutes. Hit reload for the latest image.

During its first passage by Earth (1.21 AU) on January 6, 1978, the comet was discovered by Paul Wild. Since then, the best apparition of Wild-2 was in March 1997 when it passed within 0.85 AU of our planet, brightening to an unimpressive 10th magnitude. That's too faint to be seen with the naked eye, but bright enough for modest amateur telescopes.

So, why visit an obscure, hard-to-see object like Wild-2, when there are so many more notorious comets to choose from? There are two important reasons:

#1 It's fresh. Before its near miss with Jupiter in 1974 comet Wild-2 was well-preserved in the frigid outer solar system. With its new orbit, Wild-2 now comes much closer to the Sun. When a comet passes close enough to the Sun, some of its material is boiled off into interplanetary space. After about a thousand trips past the Sun, it loses most of its volatile materials and no longer generates a coma or tail. Since Wild-2 has passed the Sun only a few times, it still has most of its dust and gases - it is "pristine." By the time STARDUST encounters the comet, Wild-2 will have made only five trips around the Sun. By contrast, Comet Halley has passed the Sun more than 100 times.

#2 It's in the right place at the right time. Wild-2 presents a unique opportunity -- it is in the right place at the right time. Scientists have found a flight path that allows the spacecraft to fly by the comet at a relatively low speed, only 13,600 mph. Because of this "low velocity" flyby, comet dust can be captured by collectors on the spacecraft, rather than blowing right through the collectors and out the back side! This comet dust can then be brought back to the Earth to be analyzed.

Above: If comet Wild-2 had passed much closer to Jupiter in 1974, it might have ended up like comet Shoemaker-Levy 9, pictured above. Comet SL-9 was often referred to as the "string of pearls" comet. It is famous for its suggestive appearance as well as its collision with the planet Jupiter! The comet's original single nucleus was torn to pieces by Jupiter's strong gravity during a close encounter with the solar system's largest planet in 1992. The pieces are seen in this composite of Hubble Space Telescope images to be "pearls" strung out along the comet's orbital path. [more information]

After the flyby is done, STARDUST will return to Earth. In 2006 the craft's aerogel sample collectors will descend by parachute toward the U.S. Air Force Test and Training range in Utah, about 100 miles southwest of Salt Lake City in the desert. By the time the mission is over, comet Wild-2 -- dim, obscure, and little-known -- will take its rightful place in the pantheon of historic comets.

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Production Editor: Dr. Tony Phillips Author: Dr. Tony Phillips