Alpha Magnetic Spectrometer In Limbo
Now that Atlantis has safely returned to Earth, she only has two scheduled missions left. Her last flight, STS-125, is scheduled for September 10, 2008. That will be the servicing mission for the Hubble Space Telescope. As part of the mission a docking adapter will be installed, making future servicing missions by Orion possible. Refurbishing HST was nearly cancelled, but was saved by support from both scientists and the public.
The Alpha Magnetic Spectrometer is designed to be attached to ISS. It incorporates superconducting magnets to detect cosmic rays striking Earth. It may find the signatures of antimatter in the early Universe. A Space Station is an unprecedented platform for such observations. AMS is a collaboration of 16 countries who have spent 1.5 billion US on the project. It is the most important physics experiment planned for ISS.
Though they strike Earth constantly, relatively little is known about cosmic rays. These particles reach energies far higher than any human accelerator can achieve. The highest energy cosmic rays, nicknamed "Oh My God" particles, have energies far greater than physics can explain. These particles may have originated at a time near the Big Bang. Their immense energies are one more indicator that the speed of light has slowed.
Nearly everyone has experienced the power of a thunderstorm. We are taught in school that lightning originates from static discharges within storm clouds. What triggers those discharges is unknown. The tracks of cosmic rays, striking and scattering particles in the atmosphere, are very similiar to lightning. Some researchers have suggested that cosmic rays are the cause of lightning! Since cosmic rays fall nearly steadily across Earth's surface, that is a hypothesis that needs to be tested. If cosmic rays cause lightning, that is one more example of how our lives are intimately entwined with Space.
The AMS is nearly completed and sitting in a clean room. Unfortunately, with the shuttle program scheduled to end in 2010, there is no longer a flight scheduled to take AMS into Space. Alternatives have been studied, but adapting the experiment to another spacecraft would cost hundreds of millions. Grounding the experiment would be a major disappointment for scientists and international partners. Physicists have not given up; the experiment is still scheduled to be received by NASA in 2008 and prepared for flight.
There are one, possibly two hopes left. NASA has allowed two "contingency" flights to ISS in case something goes wrong with a scheduled mission. If these flights are not used up, one of them could be used to orbit AMS. More ISS missions are a good thing, for they allow more supplies and crew changes. This may mean extending the shuttle program past 2010, but with all the delays that may happen anyway.
Adding a flight will also narrow the "gap" between shuttle retirement and the first Orion flights. If shuttle is retired in 2010 and Orion does not fly until 2013 (2014? 2016?) it will be the longest gap in US human spaceflight since 1975-81. During that period ISS would be completely dependent on Russian spacecraft. We can only hope that private spacecraft can help fill the gap.
Just as scientists and the public successfully lobbied to repair Hubble, now is the time to start lobbying for AMS. As before, NASA may examine alternatives before concluding that a shuttle mission is needed. This is a very important experiment that international partners have already paid for. Leaving AMS on the ground would remove one scientific justification for ISS. The Alpha Magnetic Spectrometer deserves to fly.
Labels: Cosmic rays, ISS, space shuttle, Space Station
posted by L. Riofrio at 8:39 PM
2 Comments:
perhaps we could bury the AMS in the flooded SSC tunnel in Texas and spend more starting civil wars!
"Nearly everyone has experienced the power of a thunderstorm. We are taught in school that lightning originates from static discharges within storm clouds. What triggers those discharges is unknown. The tracks of cosmic rays, striking and scattering particles in the atmosphere, are very similiar to lightning. Some researchers have suggested that cosmic rays are the cause of lightning! Since cosmic rays fall nearly steadily across Earth's surface, that is a hypothesis that needs to be tested. If cosmic rays cause lightning, that is one more example of how our lives are intimately entwined with Space." - Louise
Yes! It's simply the Geiger counter effect: there is a massive (400 kV) electric potential between the ionosphere at high altitude and the earth's surface, and the ionospheric gas is at low pressure, just like the inside of a Geiger counter tube with high voltage and low pressure gas!
A sufficiently ionizing, high-energy cosmic ray, can set off a lightning bolt, just as a "count" (electron avalanche) is set off by a beta particle entering a Geiger counter tube.
If you actually have a Geiger counter with a glass window, turn out the lights and bring some Sr-90 near it. You can see miniature lightning flashes! The gas (argon) sparks when particles of radiation set off electron avalanches, even at the normal operating voltage (you get trouble if you turn the potential up too high, because the gas no longer quenches so the first particle that ionises it causes it to light up like a neon tube and remain glowing until the counter is switched off; that's damaging to the lifespan of the Geiger tube, and of course the scalar or ratemeter attached to it which is providing the HV can't detect any pulses if that happens).
So the atmosphere is in a sense a giant Geiger counter, and lightning bolts are likely just electron avalanches in triggered at altitude in low pressure air by cosmic rays. The direction of the lightning bolt marks the direction of the electric field lines in the atmosphere. Usually they're vertical, corresponding to the natural electric field gradient of the atmosphere, 120 v/m vertically near sea level.
Feynman has a great lecture about this in his Lectures on Physics. The base of the electrically conductive ionosphere is 50 km above sea level, and it forms one electrode with the sea (conductive salt water) or wet ground as the other one. The Earth has a net charge of 1,000,000 Coulombs. The vertical electric potential between them is V = 400,000 volts, and since the capacitance of the Earth (treating the oceans and the ionosphere as two concentric capacitor plates with the air between them as the dielectric) is C = 0.091 F, the atmosphere normally stores (1/2)CV^2 = 7.3 GJ of energy!
Because the air is normally fairly non-conductive, the vertical current flowing as a result of that natural vertical electric field is normally small, just 3.5 pA/m^2, but this means that 1,800 Amps is flowing vertically at any one time over the entire Earth.
This vertical current is compensated-for by an average of 40 lightning discharges per second (in 2,000 thunderstorms concentrated mainly in warm oceanic tropical locations which suffer the most frequent electrical storms; South America, central Africa, Southeast Asia, and Northern Australia) which maintains the balance of charge between Earth's surface and the ionosphere. (This figure of 40 lightning strikes per second is based on 1995 data from the Optical Transient Detector satellite and this figure is only about half of the old obsolete estimate used in Feynman's lectures.)
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