From: Johns Hopkins University
Posted: Wednesday, November 5, 2003
NASA Spacecraft Offers First Direct Look at Dynamic Region Before Interstellar Space
More than 25 years after leaving home, NASA's Voyager 1 spacecraft reached a key checkpoint on its historic journey toward interstellar space.
Analyzing six months of data from Voyager's Low-Energy Charged Particle instrument, a team led by Dr. Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md., determined that the spacecraft, while nearly 8 billion miles from Earth, passed through and later returned behind the turbulent zone known as the solar termination shock. At the termination shock, streams of electrically charged gas blown from the Sun - called the solar wind - slow down rapidly after colliding with gas and magnetic pressure from between the stars. The shock is also considered the last stop before the invisible boundary of the heliosphere, the bubble-like region of space under our Sun's energetic influence.
"Voyager 1 is giving us our first taste of interstellar space," says Krimigis, principal investigator for the Low-Energy Charged Particle (LECP) instrument, which was designed and built at APL. "This is our first direct look at the incredibly dynamic activity in the solar system's outer limits."
Voyager 1 is the farthest manmade object in space, and from about Aug. 1, 2002 to Feb. 5, 2003, scientists noticed unusual readings from several instruments on the spacecraft indicating it had entered part of the solar system unlike any encountered before. Science team members' views vary on what the data means; one instrument team maintains that Voyager approached, but didn't cross, the termination shock. (Each team presents its views in the Nov. 6 issue of the journal Nature.)
Krimigis says his team, however, found compelling evidence of a shock crossing in data from the LECP. The instrument, mounted on a motorized, rotating platform that allows it to scan the sky in all directions, determines the composition, charge and direction of certain energized particles as they zip through space.
First, the team noticed a hundred-fold increase in the intensity of these charged particles, and that they were streaming by the spacecraft mostly along the magnetic field perpendicular to Voyager's path. "This was remarkable," Krimigis says, "because for 25 years, particles from the Sun were flowing straight out. We knew something strange must have happened to the solar wind that helps push these particles out."
At a termination shock, the solar wind would brake abruptly from supersonic to subsonic speed. The instrument on Voyager 1 that could measure solar wind speed no longer operates; however, the LECP detector can measure it indirectly from the speed and direction of the ions riding with the solar wind. "The solar wind had slowed from 700,000 miles per hour to less than 100,000 miles per hour," says Dr. Edmond Roelof, an LECP science team co-investigator at APL who developed analysis tools for just this type of data.
"Flying a moving device on Voyager - in this case an electric motor - was considered a risk," says Dr. Robert Decker, an LECP science team co-investigator and the instrument project manager at the Applied Physics Laboratory. "But that rotating capability was key to collecting this data, and helping us figure out that the solar wind had virtually stopped."
The team also found a third crucial clue: by measuring the composition of particles in the area, the instrument detected signatures of interstellar materials - the atoms and other particles from explosions of dying stars. "That tells us materials originally from outside the solar system are becoming accelerated near the spacecraft - again, something you expect to happen at the termination shock," says Dr. Matthew Hill, a science team member from the University of Maryland, College Park.
Estimating the shock's exact location has been hard since no one knows the precise conditions of interstellar space, though scientists do believe the constantly changing speed and pressure of the solar wind causes the shock's boundary to expand and contract. In this case, LECP readings indicate Voyager 1 crossed the shock at about 85 times the Earth-Sun distance, before the shock moved past the spacecraft at 87 times this distance.
Such movement also makes it difficult to predict when the spacecraft will again encounter that boundary. Until then, LECP team is correlating its results with those from other instrument teams, hoping to get a clearer picture of the interplay between the solar wind and interstellar medium, and matching that information to long-held models of the outer solar system. Already, there are some differences.
"We saw the right mix of interstellar materials where we thought we would, but overall, things didn't behave the way we expected from models," Krimigis says. "It was strange, but just another indication that nature behaves the way it wants, not according to what our theories predict."
Voyager 1 launched on Sept. 5, 1977, and flew past Jupiter and Saturn before heading northward out of the planets' orbital plane. Voyager 2, which launched on Aug. 20, 1977, and explored Jupiter, Saturn, Uranus and Neptune, is also moving out but in a southward direction and hasn't traveled as far. An APL-built Low-Energy Charged Particle detector flies on each; the Laboratory later developed similar instruments for the Galileo spacecraft, which recently ended its mission at Jupiter, and the Cassini spacecraft, which will begin orbiting Saturn in July 2004.
LECP team members presenting their results in the Nature article are Krimigis, Decker and Roelof of APL; Dr. George Gloecker, Dr. Douglas Hamilton and Hill of the University of Maryland, College Park; Dr. Thomas Armstrong of the University of Kansas, Lawrence; and Dr. Louis Lanzerotti, Bell Laboratories, Murray Hill, N.J. and New Jersey Institute of Technology, Newark. For more information on the articles, visit www.nature.com/nature.
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