[FPSPACE] Voyager 2: Sol system termination shock boundary is "squashed"

[LARRY KLAES]

VOYAGER 2 PROVES SOLAR SYSTEM IS SQUASHED

San Francisco, Calif. - NASA's Voyager 2 spacecraft has followed its twin 
Voyager 1 into the solar system's final frontier, a vast region at the edge 
of our solar system where the solar wind runs up against the thin gas 
between the stars.

However, Voyager 2 took a different path, entering this region, called the 
heliosheath, on August 30, 2007. Because Voyager 2 crossed the heliosheath 
boundary, called the solar wind termination shock, about 10 billion miles 
away from Voyager 1 and almost a billion miles closer to the sun, it 
confirmed that our solar system is “squashed” or “dented”– that the bubble 
carved into interstellar space by the solar wind is not perfectly round. 
Where Voyager 2 made its crossing, the bubble is pushed in closer to the sun 
by the local interstellar magnetic field.

“Voyager 2 continues its journey of discovery, crossing the termination 
shock multiple times as it entered the outermost layer of the giant 
heliospheric bubble surrounding the Sun and joined Voyager 1 in the last leg 
of the race to interstellar space.” said Voyager Project Scientist Dr. 
Edward Stone of the California Institute of Technology, Pasadena, Calif.

The solar wind is a thin gas of electrically charged particles (plasma) 
blown into space by the sun. The solar wind blows in all directions, carving 
a bubble into interstellar space that extends past the orbit of Pluto. This 
bubble is called the heliosphere, and Voyager 1 was the first spacecraft to 
explore its outer layer, when it crossed into the heliosheath in December 
2004. As Voyager 1 made this historic passage, it encountered the shock wave 
that surrounds our solar system called the solar wind termination shock, 
where the solar wind is abruptly slowed by pressure from the gas and 
magnetic field in interstellar space.

Even though Voyager 2 is the second spacecraft to cross the shock, it is 
scientifically exciting for a couple of reasons. The Voyager 2 spacecraft 
has a working Plasma Science instrument that can directly measure the 
velocity, density and temperature of the solar wind. This instrument is no 
longer working on Voyager 1 and estimates of the solar wind speed had to be 
made indirectly. Secondly, Voyager 1 may have had only a single shock 
crossing and it happened during a data gap. But Voyager 2 had at least five 
shock crossings over a couple of days (the shock “sloshes” back and forth 
like surf on a beach, allowing multiple crossings) and three of them are 
clearly in the data. They show us an unusual shock.

In a normal shock wave, fast-moving material slows down and forms a denser, 
hotter region as it encounters an obstacle. However, Voyager 2 found a much 
lower temperature beyond the shock than was predicted. This probably 
indicates that the energy is being transferred to cosmic ray particles that 
were accelerated to high speeds at the shock.

"The important new data describing the termination shock are still being 
pondered, but it is clear that Voyager has once again surprised us," said 
Dr. Eric Christian, Voyager Program Scientist at NASA Headquarters, 
Washington.

The two Voyager spacecraft will be the only source of local observations of 
this distant but highly interesting region for years to come. But in the 
summer of 2008, NASA will be launching a mission specifically designed to 
globally image the termination shock and heliosheath remotely from Earth 
orbit. The Interstellar Boundary Explorer (IBEX), led by Dr. David McComas 
of the Southwest Research Institute in San Antonio, Texas, will use 
energetic neutral atoms (ENAs) to create all-sky maps at various energies of 
the interaction of the heliosphere with interstellar space. ENAs are formed 
when energetic electrically-charged particles “steal” an electron from 
another particle. Once neutral, they travel straight, unaffected by the 
solar magnetic field. IBEX will detect some of the particles that happen to 
be headed towards the Earth, and the number and energy of the particles 
coming from all different directions will tell us much more about the 
overall structure of the interaction between the heliosphere and 
interstellar space.

Results on the Voyager 2 shock crossing from the entire Voyager science team 
are being presented at the Fall 2007 meeting of the American Geophysical 
Union in San Francisco. The Voyagers were built by NASA’s Jet Propulsion 
Laboratory (JPL) in Pasadena, Calif., which continues to operate both 
spacecraft.

For images, refer to:

http://www.nasa.gov/mission_pages/voyager/termination_shock.html