STEREO Creates First
Images of the
Solar System’s Invisible Frontier
Maryland (NASA/JPL) — NASA’s sun-focused Solar Terrestrial
Relations Observatory, or STEREO, twin spacecraft unexpectedly
detected particles from the edge of the solar system last year.
This helped scientists map the energized particles where the
hot solar wind slams into the cold interstellar medium.
The two STEREO spacecraft were launched in 2006 into Earth’s
orbit around the sun to obtain stereo pictures of the sun’s surface
and measure magnetic fields and ion fluxes associated with solar
explosions. From June to October 2007, sensors aboard both STEREO
spacecraft detected energetic neutral atoms originating from
the same spot in the sky, where the sun plunges through the interstellar
Mapping the region by means of neutral, or uncharged,
atoms instead of light "heralds a new kind of astronomy using
neutral atoms," said Dr. Robert Lin, professor of physics
at the University of California, Berkeley and lead for the suprathermal
electron sensor aboard the STEREO spacecraft. "You can’t
get a global picture of this region, one of the last unexplored
regions of the heliosphere, through normal telescopes," Lin
said. The heliosphere is a bubble in space produced by the solar
wind. It stretches from the sun to beyond the orbit of Pluto.
The solar wind streams off the Sun in all directions at great
speeds. Once beyond the orbit of Pluto, this supersonic wind
must slow down to meet the gases in the interstellar medium.
As the solar wind slows, it changes direction to form a comet-like
tail behind the sun. This subsonic flow region is called the
The results, reported in the July 3 issue of the journal Nature,
clear up a discrepancy in the amount of energy dumped into space
by the decelerating solar wind. The solar wind was detected when
Voyager 2 entered the heliosheath.
Researchers determined that the newly discovered population
of ions in the heliosheath contains about 70 percent of the dissipated
energy from the solar wind, exactly the amount unaccounted for
by Voyager 2’s instruments. The Voyager 2 results also are reported
in the July 3 issue of Nature. The Berkeley team concluded that
these energetic neutral atoms were originally ions heated up
in the termination shock area that lost their charge to cold
atoms in the interstellar medium and, no longer hindered by magnetic
fields, flowed back toward the sun and into the sensors aboard
"This is the first mapping of energetic neutral particles
from the edge of the heliosphere," Lin said. According to
Lin, the neutral atoms are probably hydrogen, which comprise
most of the particles in the local interstellar medium.
The charge exchange between hot ions and neutral atoms to generate
energetic neutral atoms is well known around the sun and planets,
including Earth and Jupiter. Spacecraft have used this as a means
of remotely measuring the energy in ion plasmas since neutral
atoms travel much farther than ions.
NASA plans to launch the Interstellar Boundary Explorer, or
IBEX, later this year to more thoroughly map the boundary of
the solar system.