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A Space Oddity
Gabriel Spera
Earth can be viewed as a gigantic bar magnet spinning in space. Its toroidal magnetic field encases the planet like a huge inner tube. This field shields Earth from the solar wind—a continuous stream of charged particles cast off by the sun. It also traps charged particles, which tend to congregate in distinct bands based on their charge, energy, and origin. Two primary bands of trapped particles exist: the one closer to Earth is predominantly made up of protons, while the one farther away is mostly electrons. Evidence of these bands was first made public by James Van Allen, and so they are often referred to as the Van Allen radiation belts. This radiation can cause all sorts of malfunctions in spacecraft electronics. In fact, the Geiger counter used to measure cosmic rays on Explorer 1 stopped functioning because it was overloaded by radiation!
Anyone who has used a compass knows that magnetic north and geographic north do not exactly line up. That's because Earth's magnetic dipole is tilted by about 11.5 degrees from its rotational axis and shifted slightly off-center. At the north magnetic pole, the field is stronger, effectively keeping the inner proton belt farther away; at the south magnetic pole, the field is weaker, allowing the proton belt to come closer to the planet's surface. Most of the proton belt is about 1200–1300 kilometers high, but it dips down as low as 200–300 kilometers off the lower coast of Brazil, creating a phenomenon known as the South Atlantic Anomaly. At certain altitudes, the South Atlantic Anomaly is bigger than Brazil itself.
Count rate of protons and electrons greater than 0.5 MeV in low Earth orbit measured by the NASA/SAMPEX satellite. |
A satellite in a typical low Earth orbit remains safely below the proton belt—except at the South Atlantic Anomaly. Spacecraft passing through this region are bombarded by protons with energies exceeding 10 million electron volts at a typical flux of 3000 particles per square centimeter per second. These particles can be a hazard for space systems and astronauts.
NASA launched the Terra Earth Observing System spacecraft in 1999 as part of a broad mission to study global climate change. Just one day after launch, the satellite's high-gain antenna spontaneously went into "safe" mode, interrupting communications with the Tracking and Data Relay System satellites. A series of diagnostic tests indicated that an anomalously high current had passed through the motor drive assembly. In fact, there was no high current—only a glitch in a semiconductor component that made it look as though a high current had occurred. This electronic glitch was the result of a single-event upset, an error caused by the action of ionized particles. Most flight components had been tested beforehand, but a few (including the one that experienced problems) had been overlooked. The flight software had to be revised to correct for these events.
Similarly, the Hubble Space Telescope experienced bit errors in communications between subsystems when traveling through the Anomaly. Error detection and correction schemes prevented data loss, but the problem was still annoying to ground controllers. As a result, several high-voltage instruments are powered down before the Hubble enters the South Atlantic Anomaly, an event that happens several times a day.
Numerous other missions have been affected as well. ROSAT, the Roentgen Satellite, was an X-ray observatory that flew for much of the 1990s. The unit's position-sensitive proportional counters had to be turned off during passage through the South Atlantic Anomaly to prevent severe damage. ROSAT's high-resolution imager could be left on, but could collect no useful data while in the region. The Topex satellite, which flies at an altitude of about 1000 kilometers, is still prone to random upsets in its altimeter as it passes through the Anomaly, preventing proper data collection.
Examination of nearly 1300 single-event upsets from one computer on the TAOS mission shows that nearly 50 percent occured in the South Atlantic Anomaly, whereas only 5 percent of orbital time was spent there. |
Perhaps the most serious case was NASA's Modis satellite, which was rendered inoperative in 2001 as it passed through the South Atlantic Anomaly. The failure seemed to be caused by an overvoltage shutdown, probably started when a high-energy ion struck a vulnerable metal-oxide semiconductor field-effect transistor (MOSFET), causing it to fail. It took 16 days to get the satellite back on line.
Random glitches affect humans as well. Since the days of Apollo 11, astronauts in space have reported seeing random flashes of light—with their eyes closed. These flashes are believed to be caused by energetic particles striking sensitive areas of the retina. In a recent experiment, astronauts aboard the Mir wore detector helmets to help researchers correlate the number of reported flashes with the measured particle flux. If the flashes increased when Mir entered the South Atlantic Anomaly, then protons would be revealed as the likely cause; if not, then heavy ions (which appear in equal amounts inside and outside the proton belt) would be indicated. The frequency of the flashes increased in the Anomaly, but only slightly, suggesting that protons alone are not responsible, but neither are heavy ions.
So it seems that the South Atlantic Anomaly may well have a few more surprises in store.
