IBEX
Published on Oct 3, 2008 at 3:02 pm.
2 Comments.
Filed under solar system, space exploration.
In biology, an ibex is a type of wild goat. But, that is not what I am writing about here. Rather, I am writing about NASA’s Interstellar Boundary Explorer (IBEX) Mission, set to launch in just over two weeks, on October 19 by a Pegasus rocket. IBEX is the latest of the long list of Explorer missions that started with the United States’ first satellite, Explorer 1. The missions are no longer simply numbered, but now are generally given individual
names. Interestingly, the IBEX mission team decided to use a stylized drawing of an ibex goat as part of the mission logo, as seen here.
This particular mission is aimed at studying the farthest reaches of the Solar System, where the Sun’s influence wanes and interplanetary space gives way to interstellar space. But, isn’t that just the space past the farthest planets? That is how we used to think of the difference between interstellar space and interplanetary space a century ago, but it is not how we think of it today. At one time, the space between the planets was thought to be empty. Eventually, it became apparent that there was still something, though not much, between the planets. There are meteors, interplanetary dust grains, solar wind particles, etc. between the planets. All of this is called the interplanetary medium. But, that was in the Solar System. The space between the stars was still thought to be largely empty. But, we now know that even the space between the stars, though amazingly empty by earthly standards, and thinner than the best vacuum pump could possibly pump on Earth, is still filled with things. There are interstellar dust grains, interstellar gas atoms, and magnetic fields associated with the galaxy. Also, interstellar space is populated with some extremely high energy particles flying at near the speed of light. Some of these ultra high energy particles are quite large by particle standards, as large as the nucleus of an iron atom. All of these things make up the interstellar medium between the stars. One of the ways that you can define the “edge” of the Solar System is to say that it is where the boundary exists between where space is dominated by solar wind particles and other components of the interplanetary medium and where space is dominated by interstellar particles, cosmic rays, and other components of the interstellar medium. But, since the interstellar medium and the interplanetary medium are so vaporous anyway, you would expect that the boundary is not so sharp as to say that a particular distance marks the boundary between being in the Solar System and being in interstellar space. Rather, there is a sort of transition region, inside of which the Sun dominates and outside of which the galaxy dominates. The region of space where the Sun dominates is the heliosphere. Beyond that can be thought of as interstellar space. The boundary zone between the two is the heliosheath. The outer edge of the heliosheath, beyond which is interstellar space, is the heliopause. That is usually regarded as the “edge” of the Solar System. Now, it is important to realize that the Sun’s light and gravity continue outward beyond the heliopause. You could even have bodies orbiting the Sun beyond the heliopause, but they would be moving through an environment that would be characterized as that of being like that of the interstellar medium rather than being like the interplanetary medium.
The heliopause, marks where interplanetary space gives way to interstellar space, but as I said, it is not a sharp edge. The solar wind particles moving away from the Sun eventually slow down and start to bunch up. The point where they slow to subsonic speeds is called the termination shock. IBEX seeks to investigate this zone. Four spacecraft are currently in the vicinity of the heliosheath: Pioneer 10 and 11 and Voyager 1 and 2. The Pioneers are dead, but the Voyagers are still operational (though at far reduced capacity than they were when launched). Both have passed the termination shock, and Voyager 1 is closing on the heliopause. One other spacecraft, New Horizons, is on its way out of the Solar System, but it will be quite a few more years until it reaches the termination shock.
Unlike the spacecraft leaving the Solar System, IBEX will study the edge of the Solar System from right in our backyard: in Earth orbit. So, how can it do that? The answer lies in the interstellar cosmic rays that I mentioned. Not all cosmic rays stay in interstellar space. Many of these are very energetic and they make in into the Solar System. Granted, probably less than 25% make it past the heliosheath, but some do make it. These cosmic rays can then strike Earth’s atmosphere, where they can cause showers of particles raining down on us from above. These particles mean that we are constantly exposed to radiation from space. The higher you are, on mountains or in aircraft, the more radiation you receive. And, changes in the heliosheath can change the amount of cosmic radiation that Earth receives. IBEX is going to study the particles that are showering Earth from that distant edge of the Solar System. By steadily measuring the particles, then gradually a picture will emerge of the heliopause, heliosheath, and termination shock (at least that is the hope!). And, that is what IBEX hopes to accomplish over its two year mission.
-Astroprof
Images courtesy NASA, IBEX
Darnell Clayton on October 5, 2008 at 7:57 pm: 1
Wow, that is great to hear! I wonder whether or not the changes in the heliosheath would be significantly noticeable to us fragile humans? (as the radiation level would probably still be at dangerous levels either way).
Sara on October 8, 2008 at 2:17 pm: 2
I have a question about comets and eccentricity…didn’t know how to contact since the e-mail link was taken down due to spam. I was wondering if eccentricity is related to whether a comet is short or long-period? Sorry to post this where it doesn’t really belong, but I’d appreciate an answer if anyone knows.