According to Dr. David Young of SwRI, the principal investigator for the data analysis phase of the DS1 mission and spokesman for the PEPE team, the new instrument observed charged xenon particles returning to the spacecraft from the 1 ampere exhaust plume of the ion propulsion jet. At the same time, PEPE data have been used to place limits on electrical charging of the DS1 spacecraft. Taken together, these data suggest that the ion drive engine used to propel the spacecraft can indeed be used to power future scientific missions without interfering with the scientific payload. Before now, the use of such an engine in space had been unproved.

PEPE incorporates a half-dozen new technologies that will make future space plasma instruments smaller and less expensive. Among these is a novel time-of-flight mass spectrometer that measures the composition of charged atomic particles (ions) coming from asteroid 1992 KD, the target of the DS1 mission. Other technologies include high performance power supplies operating at up to 15,000 volts but weighing just over 100 grams.

PEPE also incorporates novel ion and electron optical systems that eliminate complex motors otherwise needed to steer a plasma instrument's line of sight. Young said that PEPE has achieved most of its technical goals: It weighs about 25 percent of the mass and uses 50 percent of the power of a comparably performing SwRI instrument on NASA's Cassini spacecraft, while costing about 75 percent less. During January, the PEPE team took advantage of favorable alignment of the DS1 and Cassini spacecraft to measure the solar wind at both locations while separated by a distance of nearly 40,000,000 miles.

Young indicated that these initial PEPE measurements of the solar wind confirm that the instrument will be able to detect extremely low densities of ions and electrons during the planned DS1 encounters with the asteroid. For ions, the lowest detectable density is about 1 ion per 100 cubic centimeters, and for electrons it is ten times less. This means that PEPE has a good chance of providing the first plasma sensor detection of an asteroid and should give the first indication of the composition of asteroid surface materials that are constantly knocked off the asteroid surface by the solar wind.

About SwRI
SwRI is an independent, nonprofit, applied research and development organization with more than 2,700 employees and an annual research volume of more than $300 million.