STARDUST Vision Nearly Restored

NASA's STARDUST Mission, sponsored by the National Aeronautics and Space Administration (NASA), was launched February 1999 from Cape Canaveral, Florida. STARDUST is the fourth Discovery mission and will be the first United States sample return mission beyond the Mars orbit. STARDUST's objective will be to fly through the coma of P/Wild 2 and collect cometary dust particles recently released from the outgassing comet. The STARDUST flight team will tackle a variety of challenging tasks including controlling the flyby distance on approach to P/Wild 2, ensuring the safety of the spacecraft, and collecting thousands of cometary dust particles. This is done by using a combination of Earth-based radiometric spacecraft tracking and on-board images taken from the Navigation Camera (NC) of P/Wild 2 against a star background. The flight team needs to maintain the spacecraft at a safe distance from the comet. If the spacecraft flies too close it risks being overwhelmed by more dust particle hits than it was designed to withstand. Too far could cause a loss of primary science by not collecting the desired amount of dust particles.

The STARDUST Navigation Camera (NC), built by the Jet Propulsion Laboratory (JPL) and flying on a spacecraft built by Lockheed Martin Astronautics (LMA), experienced contamination of its optical surfaces after launch. This contamination took energy that normally would be deposited on one picture element (pixel) and spreading it over tens of pixels, lowering the central brightness and blurring the image. The source of the contamination is not known, but may have occurred during the launch phase when any warm contaminant would come to rest on the coldest surface around, in this case the camera optics and detector.

There was no threat to the spacecraft or possible loss of dust capture when STARDUST encounters P/Wild 2. The camera's performance was still adequate for flyby navigation, though the accuracy was reduced somewhat. Resolution of scientific images of the comet nucleus also would be compromised.

Studies indicated that this contamination could probably be sublimated, i.e., boiled away, by heating the optics. Sources of such heat included electric heaters on the spacecraft, other subsystems that produce heat when powered on, and the Sun. The NC Charged Couple Device (CCD) heater was turned on for a week and small improvement was shown. Therefore, as the spacecraft approached nearer to the Sun, the NC's charge coupled device and mirror motor heaters were turned on for about one month. Also during a recent trajectory correction maneuver, the Sun fortuitously illuminated the NC radiator, which normally keeps the CCD cold, thus increasing the temperature even further. This combined heating effect brought the CCD, primary optics and mirror temperatures from -35 degrees C (-31 degrees F) to over +20 degrees C (68 degrees F). The image quality improved significantly. The following three images demonstrate this improvement.

Figure 2 shows an in-flight image with the calibration lamp on. All detail of the filament is lost behind a veil of scattered light, as the light passes through the contaminant. Star images taken during this time period were only detectable as low as 7th visual magnitude with a one second exposure. Under these conditions, many short exposures would have had to be added to navigate through the P/Wild 2 approach.

Figure 3 shows a calibration lamp image taken after the most recent NC heating cycle. The zigzag pattern of the filament appears again, and the scattered light has been reduced significantly. Star images taken at this same time easily detected 9th visual magnitude stars, an improvement of two magnitudes. Heating the navigation camera has recaptured almost all of its lost performance. Future heating of the NC will be reviewed after detailed analyses