What questions will IBEX help to answer?

IBEX’s sole, focused science objective is to discover the global interaction between plasma from the solar wind and the interstellar medium at the boundary region of our Solar System. IBEX achieves this objective by detecting energetic neutral atoms (ENAs) coming from the boundary region and interstellar neutral atoms coming inward from outside the boundary. From these detections, IBEX scientists hope to answer four fundamental science questions:

• What is the global strength and structure of the termination shock?
• How are energetic protons accelerated at the termination shock?
• What are the global properties of the solar wind flow beyond the termination shock and in the heliotail?
• How does the interstellar flow interact with the heliosphere beyond the heliopause?

The heliosphere helps define one type of boundary of our Solar System. The solar wind from our Sun blows outward against the material between the stars, called the "interstellar medium", and clears out a bubble–like region. This bubble that surrounds the Sun and the Solar System is called the "heliosphere." It is a definable, measureable region in space. The use of the word "global" above means "as seen around the entire sky." The "termination shock" is the region where the solar wind slows down and begins to interact with the interstellar medium. The "heliotail" is the region of our heliosphere opposite to the direction of travel of our Solar System through the Milky Way Galaxy. The part of our heliosphere in the direction of travel is often called the "nose." The "heliopause" is the outermost boundary, where the solar wind does not travel outward any farther.

All of the planets in our Solar System are within the heliosphere. In the late 1970s and 1980s, the Voyager spacecraft expanded our knowledge of the outer Solar System. Voyager 1 launched September 5, 1977, and Voyager 2 launched August 20, 1977. Voyagers 1 and 2 both explored the planets Jupiter and Saturn, and Voyager 2 explored Uranus and Neptune. After their planetary observations, both spacecraft continued outward in different directions. The Voyagers were only supposed to last a few years, but they have continued to operate for over 30 years, well past their designed lifetimes.

Voyager 1 reached the termination shock on December 16, 2004 at a distance of 8.4 billion miles (14.1 billion kilometers) from the Sun. Voyager 2 reached the termination shock on August 30, 2007 at a distance of 7.8 billion miles (12.6 billion kilometers) from the Sun. The discrepancy in distances and dates can be explained by the fact that Voyager 1 is traveling faster than Voyager 2, and that the termination shock is not at a uniform distance from the Sun. This distance can vary with the activity level of the Sun.

Today, the Voyager spacecraft are at our Solar System’s boundary region, but they can only sample the conditions at those two specific points. IBEX images the entire sky, giving us an all–sky view of the boundary. The data from the Voyagers is combined with IBEX’s data, allowing scientists to work to create a more complete model of the boundary of our Solar System.