MIT instrument finds surprises at solar system's edge

MIT instrument finds surprises at solar system's edge
David Chandler, MIT News Office
December 10, 2007

http://web.mit.edu/newsoffice/2007/voyager-1210.html

The Voyager 2 spacecraft's Plasma Science instrument, developed at MIT
in the 1970s, has turned up surprising revelations about the boundary
zone that marks the edge of the sun's influence in space.

The unexpected findings emerged in the last few weeks as the spacecraft
traversed the termination shockwave formed when the flow of particles
constantly streaming out from the sun--the solar wind--slams into the
surrounding thin gas that fills the space between stars.

The first surprise is that there is an unexpectedly strong magnetic
field in that surrounding interstellar region, generated by currents in
that incredibly tenuous gas. This magnetic field is squashing the bubble
of outflowing gas from the sun, distorting it from the uniform spherical
shape space physicists had expected to find.

A second surprise also emerged from Voyager 2's passage through the
solar system's outer edge: Just outside that boundary the temperature,
although hotter than inside, was ten times cooler than expected.
Theorists had to scramble to come up with an explanation for the
unanticipated chilling effect.

"It's a different kind of shockwave than we've seen anywhere else," says
John Richardson, principal investigator for the Plasma Physics
instrument and a Principal Research Scientist at MIT's Kavli Institute
for Astrophysics and Space Science. The unexpected coolness, theorists
now think, is caused by energy going into particles that are hotter than
those that can be measured by the MIT plasma instrument.

Richardson will be taking part in a press conference reporting the new
findings on Monday, Dec. 10, at a meeting of the American Geophysical
Union in San Francisco.

The Voyager 1 and 2 spacecraft were designed primarily to study the
planets Jupiter and Saturn and their moons. After launch, Voyager 2's
path was adjusted to take it past Uranus and Neptune as well. Although
the craft were only built for a five-year mission, both are still
working well three decades later.

"We were incredibly lucky to have it last 30 years," says John Belcher,
professor of physics at MIT and former principal investigator for the
Voyager Plasma Science instrument. The craft is now expected to keep
working until about 2020, and still has important scientific objectives
ahead.

It is now passing through a boundary zone called the heliosheath, a
region where the solar wind interacts with the surrounding interstellar
medium. But sometime in the next decade, it will cross a final edge,
called the heliopause, where the sun's outflow of particles ends. At
that point, it will be able to measure characteristics of the
interstellar medium, for the first time, in a region unaffected by the
solar wind and the sun's magnetism.

Although Voyager 1 had already crossed the termination shockwave three
years ago, the MIT Plasma Science instrument on that spacecraft had
stopped working, so the spacecraft could only indirectly detect the end
of the sun's influence.

But with Voyager 2, the Plasma Science instrument not only detected the
boundary, making detailed measurements of the solar wind's temperature,
speed and density as the spacecraft crossed through it, but it actually
encountered the shockwave repeatedly. Because the outflow of the solar
wind varies with changes in the sun's activity level, building up during
large solar flares and quieting during lulls in sunspot activity, the
boundary itself pulsates in and out. These pulsations can wash across
the craft multiple times, just as a boat landing onshore may cross the
ocean's edge multiple times as waves crash in and then recede.

While Voyager 1 apparently made a single crossing, Voyager 2 apparently
crossed the boundary five times, producing a wealth of new data. It's
even possible that if there are large variations in that solar outflow,
the shock layer "could push past Voyager again," says Richardson. "That
would give us some idea of how elastic the shock is" -- that is, how far
out these pulsations may stretch. Until and unless such detections are
made, "we only have models" of how great such variations might be, he
says.

Voyager 2 is now 7.879 billion miles from Earth, traveling away at
almost 35,000 miles per hour. Voyager 1 is 9.797 billion miles away,
going more than 38,000 mph.

The Plasma Science instrument was developed by the late Professor
Herbert Bridge and Alan Lazarus, a senior research scientist in the
Department of Physics and MIT's Kavli Institute for Astrophysics and
Space Science. NASA has sponsored the work.