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A Mission to Probe the Interstellar Boundary

February 15, 2005 —
After 25 years of blood, sweat and tears, USC astronautics professor Mike Gruntman finally has his wish: a chance to probe the very edges of our solar system with a spacecraft that can measure from afar the interactions of the solar wind with interstellar dust and gas. 

Gruntman, who is chairman of the Viterbi School’s Astronautics and Space Technology Division, is a co-investigator on the “Interstellar Boundary Explorer” (IBEX), a mission recently selected by NASA for development and launch in 2008.

IBEX will be the first mission to remotely sample the edge of the solar system, an uncharted region of space called the “heliopause,” which is thought to lie about 10 billion to 14 billion miles from the Sun.  This is the zone in which the Sun’s
Gruntman's Interstellar Boundary Explorer (IBEX) will search for the heliosphere, a three-dimensional sphere (depicted in blue) enveloping the solar system and shielding it from interstellar space. 
powerful influence all but disappears and the solar wind slows from   1 million miles per hour to about 250,000 miles per hour. 


Scientists believe an invisible “shock front” – called the termination shock – girdles the outer edge of the solar system at this distance.  This plasma shock front is created in much the same way that an air shock is formed in front of a supersonic aircraft as it flies through the air.  The wave of plasma deflects ionized interstellar material and shields the solar system from harmful cosmic radiation streaming between the stars.  Only two spacecraft in history have operated long enough in space to near that region — Voyager 1 and Voyager 2  — but neither carried the proper instruments to measure in-situ the properties of complicated flow patterns created when the solar wind collides with interstellar matter.
25 Years In The Making
Gruntman has been working on an interstellar mission to explore this tenuous boundary since 1983, when he published his first paper on the concept. His idea was to remotely probe the solar boundary by measuring fluxes of heliospheric energetic neutral atoms.
Along the way, advances in imaging technology brought his ideas into the realm of practicality and a team of scientists from several institutions came up with a new proposal. They suggested building a simple spinning spacecraft, endowed with a pair of large, ultra-sensitive cameras, to detect energetic neutral atoms produced at the solar system boundary.
The timing was just right.  Many of the IBEX mission instrument requirements were ripe for development and successfully demonstrated on missions in the intervening years, including IMAGE, a satellite designed to image Earth’s magnetosphere.  IBEX builds on some of the technology flown on IMAGE.  Once it begins to collect data, IBEX may also help scientists decipher any long-awaited data they could receive from Voyager 1, the likelier of the two Voyagers to reach the termination shock, if it ever does. 
The mission's science goals are central to NASA's Sun-Earth Connection program: to help scientists understand the connection between the Earth and the Sun, and more fundamentally, how the Sun and solar wind interact with the galaxy.   
“IBEX will make the first global map of the boundary between the solar system and interstellar space, which is about 100 times farther away from the Sun than the Earth,” Gruntman says. “Every four to five days, the spacecraft will travel outside of Earth’s magnetosphere at the farthest point in its orbit and be able to study these neutral atoms streaming from the edge of the solar system.” 
In the future, humanity will send spacecraft hurtling far past this planetary edge and into the galactic medium, which is dominated by stellar processes similar to those occurring at the edge of the solar system.  Gruntman says if spacecraft are to survive their journeys into the galaxy, scientists must know what to expect.
Mike Gruntman


Outbound To Other Stars
This is the first step, he contends, to a precursor mission for future space-faring vehicles that will be heading to very far-out places, like Alpha Centauri, our nearest star neighbor.   
“Alpha Centauri is just 4.3 light years away,” he says with matter-of-fact resolve.  “It’s not that far-fetched of an idea and it’s not that far away.  One day our starships will be going there.”  
Sound incredible?
Ask the IBEX science, hardware and management teams.  For the next three years, they will be designing the cameras -- two energetic neutral atom imagers – under the direction of mission principal investigator David McComas, a scientist at the Southwest Research Institute in San Antonio, Texas.

Orbital Sciences Corp. in Dulles, Virginia, will build the spacecraft, a lightweight design based on the company’s MicroStar spacecraft design.  Dozens of these spacecraft are already in orbit, performing communications and remote sensing missions.  Launch from a Pegasus rocket will put the spacecraft into a highly elliptical orbit of about 150,000 miles above Earth — or about two-thirds of the distance from the Earth to the Moon.

The mission, which costs $134 million, is part of NASA’s Small Explorer (SMEX) program of rapid, small and highly focused science exploration missions, which are designed to further scientific discoveries in astronomy and space physics.

                                                                --Diane Ainsworth