02/03/1998 08:00:00

ACE Satellite Now In Place Between Earth and Sun; Will Seek To Determine What Sun Is Made Of

PASADENA—Tanning aficionados, beach bums, surfers, and other solar enthusiasts may not realize it yet, but there is a new satellite making a huge looping halo around the sun. And it's a satellite that's going to be a benefit to weather forecasters in predicting solar flares as well as to astrophysicists in understanding the nature of the universe.

The satellite is called the Advanced Composition Explorer, or ACE for short. Launched August 25, the satellite has reached its destination about a million miles from Earth toward the sun at a position known as L1. That's the point at which the gravitational pull from Earth and sun, plus centrifugal effects, exactly balance each other.

"So, a spacecraft can orbit this invisible point, maintaining a fixed distance from Earth as Earth orbits the sun," says Ed Stone, Morrisroe Professor of Physics at Caltech and principal investigator of the ACE science mission.

Stone and Caltech physicist Dick Mewaldt are leading the satellite's science mission at the ACE Science Center at Caltech. There, they obtain spacecraft telemetry from the flight operations team at the Goddard Space Flight Center, and process the data for the astrophysics community.

The satellite is designed to collect a wide range of information on the matter it encounters. Its mission can broadly be classified in two phases:

® The satellite incorporates a real-time solar wind system that will provide around-the-clock coverage of interplanetary conditions that affect Earth. This is especially of benefit to those living at high northern and southern latitudes, because Earth's magnetic field is such that a coronal mass ejection can more easily disrupt power systems close to the poles.

While the ACE can do nothing to prevent this phenomenon from occurring, the satellite can at least provide an hour of warning that a coronal mass ejection may create a magnetic storm. The warning could help minimize and perhaps even eliminate some of the outages.

The National Oceanic and Atmospheric Administration (NOAA) will analyze the data and issue forecasts and warnings of solar storms. According to NOAA, it will be possible to issue geomagnetic storm alerts with virtually 100 percent accuracy.

® The ACE science mission is designed to measure and compare the composition of three samples of matter that can be found in interplanetary space. These are the solar material in the form of the solar wind and energetic particles accelerated by violent eruptions of the sun, the gas from the nearby space between the stars, and high-energy cosmic rays that come from more distant regions in the Milky Way.

Understanding the nature of this matter can help researchers provide answers to fundamental questions about the origin of matter. Additional information on the precise mix of elements in the solar wind, for example, will also serve as a benchmark for understanding the composition of other bodies in the solar system.

The ACE satellite is carrying nine scientific instruments that were developed by a team of scientists representing 10 institutions in the United States and Europe. These instruments are an array of mass spectrometers that measure the mass of individual ions. The satellite is already collecting data, and is expected to do so for at least five years.

"Our first look at the data tells us that the performance of the instruments is excellent," says Stone. "We should be learning what the sun is made of in the months ahead."

[Note to editors: See http://www.srl.caltech.edu/ACE/ for more on the ACE science mission. Also, NOAA on Jan. 23 issued a press release on the ACE satellite's space weather forecasting capabilities.]

Written by Robert Tindol