Postcards from the edge

Message from a craft at the solar system's final frontier holds surprises.

Science News

http://www.sciencenews.org/view/generic/id/33795/title/Postcards_from_the_edge

By Ron Cowen

July 2, 2008

There are no signs to announce the edge of the solar system, but when the
venerable Voyager 2 spacecraft approached this final frontier last Aug. 31 it
was in for quite a shock. So were the scientists who analyzed the data that the
craft radioed back to Earth, along with related observations by NASA’s twin
Earth-orbiting STEREO spacecraft.

The signals reveal that at a distance of 83.7 astronomical units (1 AU is the
average Earth-sun separation), Voyager 2 had at least five encounters with a
turbulent region known as the termination shock, the researchers report in the
July 3 Nature. That’s the place where the solar wind — the sun’s hot supersonic
wind of protons and other charged particles, which carves the heliosphere, a
bubble in space extending well beyond the orbit of Pluto — slams into cold
interstellar space and abruptly slows....

Researchers studying the flow of energy at the solar system’s edge found another
surprise. At the termination shock, the solar wind slows and dumps a large
amount of energy into space. This energy must then exist in some form, such as
heat. But John Richardson of the Massachusetts Institute of Technology and his
colleagues found that the temperature of protons — a main constituent of the
solar wind — in the slowed-down region is five to 10 times cooler than expected.

Using the STEREO spacecraft, ROBERT LIN and LINGHUA WANG OF THE UNIVERSITY OF
CALIFORNIA, BERKELEY, and their colleagues trace the missing energy to a large
group of “pickup protons”—particles that started out as neutral hydrogen atoms
from interstellar space and then infiltrated the solar system. The solar wind
ionized these atoms, turning them into protons that were then carried back out
again by the wind, to the termination shock. About 80 percent of the energy
released when the solar wind slows goes into accelerating the pickup protons,
the researchers report....

The Berkeley-led team concluded that these particles came from the interstellar
medium, providing the first map of particles from just beyond the solar system.
This new map is especially important because material at the solar system’s edge
is too tenuous and faint to be imaged by a visible-light telescope....