Feature

Battle at the Interstellar Boundary
11.04.08
A battle of galactic proportions is taking place at the boundary of the solar system and has been for millions of years.

An artist's drawing of the heliosphere

This diagram shows the solar system at the center of the heliosphere. The enlarged detail shows the IBEX spacecraft in orbit around Earth. Image Credit: NASA

On one side is the solar wind, a stream of charged particles that blows away from the sun out past the planets. On the other side is the interstellar medium, which contains streaming gas and dust found between stars in the galaxy.

The solar wind is constantly playing defense as it tries to keep the interstellar medium from muscling its way into the solar system.

What's so bad about the interstellar medium? Among its gas and dust, traveling at nearly the speed of light, are cosmic rays. These rays are high-energy particles that could be harmful to humans.

Fortunately, the solar wind is strong enough to form a bubble-shaped region known as the heliosphere around the sun and solar system. The heliosphere keeps the interstellar medium and most of its dangerous cosmic rays at bay.

Still, some cosmic rays are able to penetrate the outer edge of the heliosphere
-- the heliopause -- and head toward Earth. But most of those are shielded away from the planet by the magnetic force of the magnetosphere, or weakened by collisions with molecules in the atmosphere.

The Termination Shock, From Up Close and Far Away

The area near the edge of the solar system, far beyond the orbit of Pluto, where the force of the interstellar medium becomes strong enough to slow the momentum of the solar wind is called the termination shock. NASA's Voyager 1 and Voyager 2, both launched in 1977, became the first spacecraft to reach the termination shock in 2004 and 2007, respectively.

Unlike Voyagers 1 and 2, NASA's Interstellar Boundary Explorer, or IBEX, won't have to travel for decades before studying the boundary of the solar system. IBEX launched on Oct. 19, 2008, and orbits Earth at a distance a little closer than the moon.

How is IBEX learning anything about an area billions of miles away?

Eating up ENAs

IBEX studies the boundary of the solar system by collecting a kind of particle that is created in the region where the solar wind and interstellar medium mingle. These particles are called energetic neutral atoms, or ENAs.

An ENA is born when a fast-moving, positively charged atom in the solar wind steals electrons from a slow-moving, neutral atom in the interstellar medium. This "charge exchange" makes the solar wind atom neutral itself. Because it is no longer positive or negative, it does not react to magnetic fields and thus travels in a straight line from the point of the charge exchange.

An artist's drawing of the IBEX spacecraft in orbit around Earth

The IBEX spacecraft orbits Earth at a distance a little closer than the moon. Image Credit: NASA

In the region where the solar wind and interstellar medium interact, the solar wind is no longer heading straight out from the sun. In fact, some of the solar wind atoms are pointed back toward Earth. When these become ENAs, they travel directly toward IBEX, which has two ENA-collecting sensors. The spacecraft keeps track of the direction the particles come from, the time they enter the sensor, the mass of the particles and the amount of energy each particle has. Using this information, scientists build a map showing the number of particles at each energy level that come from each direction.

Building on the Past, Preparing for the Future

IBEX's full-sky map complements and builds on the single-point measurements made by the two Voyager spacecraft. By analyzing the map, scientists hope to learn more about how the solar wind and interstellar medium interact with each other.

A better understanding of how the solar wind fights off cosmic rays, for example, will help engineers design suits and equipment to better protect astronauts and spacecraft. The farther from Earth astronauts travel -- NASA is planning to eventually send astronauts to Mars -- the more cosmic rays they encounter.


Key Definitions

Cosmic rays: High-energy particles that travel close to the speed of light. Most of these particles come from beyond the solar system. But some, lower-energy cosmic rays, originate from the sun. Contrary to what the name implies, cosmic rays are not streams of particles. Rather, they are individual particles.

Heliopause: The outer edge of the heliosphere where the interstellar medium stops the forward progress of the solar wind.

Heliosphere: The bubble-shaped region around the sun and solar system where the solar wind pushes the interstellar medium away from the solar system.

Interstellar medium: All the gas and dust found between stars. Cosmic rays are contained within the interstellar medium.

Magnetosphere: The region of space around Earth that contains Earth's magnetic field, which deflects cosmic rays and other dangerous particles away from the planet.

Solar wind: A stream of charged particles, mostly protons and electrons, that escapes into the sun's outer atmosphere at high speeds and blows outward past the planets.

Termination shock: The area near the edge of the solar system where the force of the interstellar medium is strong enough to slow the momentum of the solar wind.


Related Resources
Windows to the Universe: Cosmic Rays   →
IBEX Museums & Planetaria   →
What Is the Heliopause? Video   →

Dan Stillman, Institute for Global Environmental Strategies