The solar flare as seen by the SOHO sun probe. Photo: NASA/ESA

Quick-thinking NASA engineers and scientists helped the Stardust spacecraft survive a close encounter with a storm of high-energy particles from the Sun after a recent solar flare.

Stardust, a NASA mission to return samples of a comet, was only 1.4 AU (130 million miles) from the Sun on the afternoon of Wed., Nov. 9. It was flying at about 20,000 kilometers per hour (over 12,000 miles per hour).

Engineers from the Stardust team were a little worried, since they had heard that the fourth largest solar flare since 1976 was heading toward Earth. This monster cloud of energized particles was 100,000 times more intense than usual, and it was heading toward Stardust.

The engineers' fears came to pass in the middle of the night, when the solar wind's stream of high-energy protons hit the spacecraft. Its two star cameras, which it uses to control the spacecraft's orientation, got a large dose of energy. Protons from the solar wind electrified pixels in the star cameras, producing dots that the camera interpreted as stars. The 12 brightest images, the ones the spacecraft relied on to point its way, were electrified pixels, which showed up as false stars. Hundreds of these star-like images inundated the star camera's field of view, which meant it could not recognize its attitude in space.

Top: Last image taken before entering safe mode. View shows hundreds of proton hits as solar flare occurred. Bottom: Taken days after the solar flare subsided, the image shows many bright objects, all identified as stars or Saturn. The camera had completely recovered from the proton hits at this time. Photo: NASA/JPL

The spacecraft did the safest thing it could -- it went into standby mode, turning its solar panels toward the Sun and waiting for communication from Earth. While it was waiting, the spacecraft tried again to determine its attitude by using two different sets of cameras. It repeatedly turned up hundreds of bogus star-like images. It also switched between electronics systems on either side of the spacecraft. So Stardust began to slowly rotate in place, pointing its solar panels at the Sun.

The flight team didn't hear from Stardust when they tried to communicate with it the next morning. They deduced that the solar flare had caused it to go into standby mode, and they knew that meant the spacecraft would send a signal within 24 hours.

Scientists confirmed their theory when they reviewed data from the spacecraft that verified that the problems had begun when the solar flare occurred. The influence of the proton stream would diminish over the next few days but still posed some danger, so the team left the spacecraft in standby mode until the threat passed.

On Saturday, November 11th, the flight team reset the first star camera and turned it back on. They used another method of orienting the spacecraft, called inertial measuring units, while they inspected the cameras. Engineers retrieved the last images the camera took before the spacecraft reset itself and saw hundreds of false star images. Although the camera normally uses a circular area in the middle to take pictures, the proton hits were so strong they even penetrated parts of the camera usually hidden from the light.

An artist's concept of the Stardust spacecraft during interstellar dust collection. Photo: NASA

On Monday, the Stardust flight team commanded the spacecraft to leave its safe mode. The star camera was back on the job, controlling the orientation of the spacecraft perfectly. The engineers retrieved more data from Stardust to ensure the entire spacecraft had not been affected by the solar flare.

An image taken days after the solar flare subsided shows that the camera had completely recovered from the proton hits. All the bright objects in the picture can be identified as stars, Jupiter or Saturn.

Stardust was launched onto a perfect flight path on Feb. 7, 1999 from Cape Canaveral, Fla. The spacecraft is headed for an encounter with Comet P/Wild 2 in 2004. Stardust's mission is to collect samples of dust flying off the comet nucleus, and to collect interstellar particles flowing through our solar system. Stardust will fly back toward Earth in 2006 to drop off the samples in a parachute-equipped return capsule. Stardust, part of NASA's Discovery Program of low-cost, highly focused science missions, is managed by JPL for NASA's Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology.