Launched on Oct. 19, 2008, the Interstellar Boundary Explorer, or IBEX, spacecraft, is unique to NASA's heliophysics fleet: it images the outer boundary of the heliosphere, a boundary at the furthest edges of the solar system, far past the planets, some 8 million miles away. There, the constant stream of solar particles flowing off the sun, the solar wind, pushes up against the interstellar material flowing in from the local galactic neighborhood.
IBEX is also different because it creates images from particles instead of light. IBEX, scientists create maps from the observed neutral atoms. Some are of non-solar origin, others were created by collisions of solar wind particles with other neutral atoms far from the sun. Observing where these energetic neutral atoms, or ENAs, come from describes what's going on in these distant regions. Over the course of six months and many orbits around Earth, IBEX can paint a picture of the entire sky in ENAs.
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During its first five years, IBEX has made some astounding discoveries.
Mapping the Boundaries
In its first year, IBEX scientists created the first-ever all-sky map of the heliosphere's boundary, where the influence of the solar wind diminishes and interacts with the interstellar medium. The most startling finding is that the map was not uniform or symmetrical, but shows a bright ribbon of energetic neutral atoms snaking through it.
During its second and third years, IBEX showed that the heliosphere's boundaries changed more rapidly than expected, with variations as short as six months. Additional sets of all-sky maps showed the evolution of the interstellar boundary region: the mysterious ribbon feature at the nose, of the heliosphere – in the front as it moves through space – evolved. Also, a knot-like feature spread and diminished. This variation over time is challenging scientists to try to understand how the heliosphere can change so rapidly.
ENAs Near Earth
Because IBEX is orbiting Earth, it also can look back toward Earth's neutral-atom environment and so has provided the first ENA images of the magnetosphere from the outside.
Nearby, IBEX has scanned the moon, as well. The moon has no atmosphere or magnetosphere, so the solar wind slams unimpeded into its surface. IBEX observations showed that the moon creates a backscattered, neutral solar wind: about 10 percent of the impinging solar-wind protons bounce off the lunar surface, becoming ENAs as they do.
The Heliosphere: Looking Ahead and Looking Behind
Measurements by IBEX announced in 2012 show the influence of the heliosphere on the local interstellar medium is different than expected. Previous models showed a boundary ahead of the heliosphere, outside the influence of the sun: a shock formed by the entire heliosphere pushing through the interstellar material around it. IBEX data suggests that there is no bow shock preceding the heliosphere's movement through space.
IBEX also offered up the first observations of the heliotail. If our eyes could see particles and we looked straight down the tail we would see an unexpected shape a little like a four-leaf clover. The two side leaves are filled with slow moving particles and the upper and lower leaves with fast ones.
Into the Galaxy
Much further away, IBEX also provided information about the local galactic environment. It made the first direct measurements of neutral hydrogen, oxygen, and neon coming into the heliosphere from the interstellar medium. The measurements show that the composition of the current galactic neighborhood is different than that of the sun and the solar system. This puzzle may mean that the sun has moved out of the region where it formed, or that some of the oxygen has been captured by dust in interstellar space.
IBEX also found that the speed of the galactic wind registered around 52 thousand miles per hour. By comparing this wind to previous results from other missions over the last 40 years, scientists believe that the direction of the wind has changed by about 7 degrees in the last four decades. While the cause of this shift is unknown, it may be telling us something about changing conditions as we move through our region of the Milky Way.
IBEX is a NASA Heliophysics Small Explorer mission. The Southwest Research Institute in San Antonio, Texas, leads IBEX with teams of national and international partners. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Explorers Program for the agency's Science Mission Directorate in Washington.