NASAs Stardust spacecraft -- en route to a rendezvous with comet Wild 2 -- zips by Earth on Monday. The flyby gives a speed boost to the probe, putting the solar-winged Stardust on a heading to meet and greet the comet in January 2004.

During the gravity-assist, the desk-sized craft makes its closest Earth approach at 6:20 a.m. Eastern Standard Time (11:20 GMT) January 15, flying just southeast of the southern tip of Africa. It will be scooting through space at about 22,400 miles (36,480 kilometers) per hour and 3,700 miles (5,953 kilometers) from Earth.

Stardust was hurled into space from Cape Canaveral, Florida on February 7, 1999. The craft was built for NASA by Lockheed Martin Astronautics in Denver, Colorado. The Jet Propulsion Laboratory (JPL) in Pasadena, California is managing the mission.

After the flyby, Stardust will be taking pictures of the Moon as it races outward from Earth, Brownlee told SPACE.com.

The picture-taking session is designed to test out Stardusts camera. Gasses eking from somewhere on the craft have mucked up the device. That gas condensed on colder parts of the camera, fogging up the images that Stardust can relay to Earth.

But the spacecraft was recently rolled in such a way as to receive an extra blast of warmth from the Sun. That maneuver, coupled to turning on the camera heaters, baked the hardware for an extended period.

"The heating made a major improvement. That makes us feel really good. Its better than it was," Brownlee said. "We still have quite a few years before we get to the comet," he said.

Radio scientists are poised to observe Stardust as it slingshots its way back into space. They are eager to gauge the probes velocity after the flyby. That data is important, not only to pinpoint Stardusts course following the flyby, but to help unravel a mystery.

For some reason, perhaps an unknown component of gravity, spacecraft have received an extra boost by a mysterious force. Scientists hope to confirm the purported effect as the Stardust spacecraft whips by Earth.

Analysis by radio scientists of the post-Earth flyby trajectories of three spacecraft have shown each craft to have picked up an unexpected increase in speed: The Galileo spacecraft in December 1990; the Near Earth Asteroid Rendezvous (NEAR) probe in January 1998; and the Saturn-bound Cassini spacecraft in August 1999.

Why this has been the case is a head-scratcher, admits John Anderson, a senior research scientist and member of the Stardust science team at the Jet Propulsion Laboratory in Pasadena, California.

"This problem has been with us for about 10 years, and we havent found a solution," Anderson said in an earlier SPACE.com interview.

In 2004, upon reaching comet Wild 2 just outside the orbit of Mars -- 242 million miles (389 million kilometers) from Earth -- Stardust will capture particles tossed off the comet, snagging the material in a paddle-like device loaded with aerogel.

Aerogel is a wispy material carried along to capture and contain both comet particles and interstellar dust particles.

After the encounter, Stardust is slated to return to Earth with its cosmic collectibles. In January 2006, a Stardust return capsule is to parachute into the Utah desert, carrying the treasure trove of particle specimens.

Once on Earth, the particles are to be distributed among laboratories around the globe for analysis. Data from the samples are expected to yield clues regarding the very beginnings of our solar system, and possibly the origin of life itself.