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Last Post 2/27/2007 9:36 AM by Kris Sigsbee

Van Allen Belts

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2/27/2007 8:14 AM
I was wondering how close the International Space Station and Space Shuttle missions are able to fly to the Van Allen Belts? Has one ever flown to close to them? Harry

Kris Sigsbee

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2/27/2007 9:36 AM
Hi Harry! That is a great question! There are actually two Van Allen radiation belts, which we call the inner and the outer belt. The inner belt is relatively stable and consists mostly of protons. The inner belt extends from just a little bit above the top of the Earth's atmosphere out to a distance of about 1 RE (1 RE = radius of the Earth = 6371 km = 4000 miles). The outer belt is less stable and consists of relativistic electrons. The outer belt extends from 3 RE out to beyond geosynchronous orbit (where communications satellites are located) at 6.6 RE. The International Space Station and the Space Shuttle fly at altitudes less than 400 km, which is usually just below the inner radiation belt. However, a region called the South Atlantic Anomaly (SAA), where the inner radiation belt makes its closest approach to the Earth's surface, can cause problems for spacecraft in a low-Earth orbit. Inside the SAA, the radiation intensity is higher than other places at the same altitude above the Earth. The SAA is caused by a "dip" in the strength of the Earth's magnetic field at that location because the center of the Earth's magnetic field is offset from the Earth's geographic center by about 450 kilometers (280 miles). The SAA can cause problems for satellites that have orbits around the Earth that are tilted (inclined) between 35° and 60° relative to the Earth’s equator and have altitudes of a few hundred miles. A satellite or other spacecraft with this kind of orbit will travel through the SAA periodically, exposing it to several minutes of strong radiation each time. The International Space Station has an orbit with an inclination of 51.6° and required extra shielding to protect it from the SAA. Some satellites, like the Hubble Space Telescope do not take observations while passing through the SAA. Scientists from the University of Iowa, home of James Van Allen, are involved with a new mission to study the radiation belts called the Radiation Belt Storm Probes Mission (http://geospace.jhuapl.edu/index.php). You can see where the International Space Station is located right now on this web site: http://spaceflight.nasa.gov/realdata/tracking/ Kris