Added 1 new A* page:Yesterday I was talking about the PAMELA satellite finding 1000 times the galactic average amount of antimatter in the Van Allen radiation belt (I should have specified, in the *inner* Van Allen belt). PAMELA was able to measure the radiation in the belt from its orbit around Earth because of a peculiarity caused by the way the Earth's magnetic pole is offset from its rotational axis: the magnetic pole is tilted 11 degrees, and shifted (although I couldn't get Wikipedia to tell me why) about 500 kilometers (300 miles) to the north. The combination of this tilt and shift in the axis of the Earth's magnetic field means that the inner Van Allen belt, which forms sort of a torus centered on the Earth's magnetic axis, comes closer to the Earth's surface at an area--whose location varies over time--in the southern hemisphere. This area is called the South Atlantic Anomaly.|
Here's a visualization of the Anomaly at an altitude of about 560 km, as seen in X-rays by the German ROSAT satellite in the '90's ("ROSAT" is short for "Röntgensatellit"; "in German X-rays are called Röntgenstrahlen, in honour of Wilhelm Röntgen," who discovered them--and, incidentally, in English articles soon after their discovery they were called "Röntgen rays"; hm and in a bit of current news, an analysis in February of this year found that ROSAT, containing a lot of heat-resistant ceramics and glass, probably won't burn up entirely in re-entry--so chunks up to 400 kg could come crashing violently to Earth when ROSAT re-enters the atmosphere between October and December of this year :o):
image by NASA / Steve Snowden (source)
PAMELA took its measurements for antimatter while its orbit was taking it through the South Atlantic Anomaly. The Anomaly moves and varies in size over time; it drifts to the west at a speed of about 0.3 degrees per year, which may be related to "the rotation differential between the Earth's core and its surface, estimated to be between 0.3 and 0.5 degrees per year" (that would be the Earth's solid, *inner* metal core, which is surrounded by a layer of molten metal, allowing the center to turn independently of the rest of the Earth; it's the convection of the liquid metals in the outer core, though, that is thought to create Earth's magnetic field, which, in case you're wondering, or even if you weren't, is about 50 times stronger at the Earth's core). The Earth's magnetic field fluctuates over time, though, and it is currently weakening, which is thought to be responsible for an observed gradual widening of the Anomaly: "Predictions estimate that by 2240 the SAA may cover approximately half of the southern hemisphere."
PAMELA was made to survive the intense cosmic radiation of the inner Van Allen belt, but a lot of spacecraft just have to avoid the Anomaly altogether, or switch off while passing through it. The International Space Station has extra shielding to allow it to pass through safely, and the Hubble Space Telescope doesn't take observations while passing through.
NASA has a pretty thorough article about the Space Shuttle's five general purpose computers (GPCs), including a section comparing their performance under space radiation--including passing through the Anomaly--compared with more contemporary laptop computers:
|Although the GPCs run the spacecraft during a mission, astronauts take a number of relatively modern computers with them into orbit in the form of laptops. Crews carry modified IBM ThinkPad A31p computers into space with them and use them for rendezvous assistance, entry and landing simulations and e-mailing Earth.|
The laptops also are much faster than the GPCs and connect with devices not available to the GPCs. The Thinkpads use one of these connections to relay photos of the external tank falling away after launch to mission control at NASA's Johnson Space Center in Houston.
But that modernity has a trade-off: the laptops are not nearly as reliable as the GPCs due to radiation effects and use of less critical commercial off-the-shelf software, Klausman said. [...]
"For critical operations, I can't come anywhere close to that reliability with the laptops," Klausman said. "They are wonderful items, but they are susceptible to radiation particles, they are susceptible to badly written software. I could put five laptops on board and all five would suffer radiation upsets within the first day."
With a ThinkPad 760XD laptop, two to three memory changes due to radiation occur during a shuttle flight to the Station, Klausman said. That number balloons up to 30 for a mission to NASA's Hubble Space Telescope. The reason is that Hubble orbits about 150 miles higher than the station, where the radiation protection from Earth's magnetic field is not as strong.
Designers also found out that laptops would crash when the shuttle passes through the "South Atlantic Anomaly," which is an area where the magnetic field draws in to Earth, again offering less radiation filtering for spacecraft flying through it.
The GPCs don't crash for radiation concerns because the GPC hardware includes a memory scrubber that prevents the system from reading radiation-changed memory.
That "scrubber" sounds kind of neat. According to Wikipedia's radiation hardening page, "a 'scrubber' circuit must continuously sweep the RAM; reading out the data, checking the parity for data errors, then writing back any corrections to the RAM."