Feb. 15, 2008 -- This week, as the military unveiled its unprecedented plan to shoot down an ailing spy satellite, an ammonia tank once used aboard the International Space Station plummeted to Earth. So did the second stage of a Delta rocket that put an Italian radar imager into orbit in December. By week's end, part of a Russian Molniya rocket that left Russia's Plesetsk Cosmodrome in 1996 should dive into the atmosphere as well.
The Aerospace Corp.'s Center for Orbital and Re-entry Debris Studies estimates there are some 70,000 objects about two centimeters (0.8 inches) in diameter orbiting between 525 and 625 miles above Earth.
Now the U.S. military wants to do away with one nagging piece of debris: a 2.5-ton satellite that failed shortly after reaching orbit in December 2006.
It won't be easy -- especially the timing -- but this piece of space debris poses a particular risk.
While the chances of being hit by falling space junk are minute -- the Aerospace Corp puts the risk at less than one in one trillion, compared with, for example, a one in 1.4 million chance of being hit by lightning in the United States -- the military says the dead satellite contains about 1,000 pounds of toxic propellant, making it potentially more dangerous should it fall on a populated area.
The plan, reviewed and approved by President George W. Bush, is to modify a missile designed to intercept short- and medium-range ballistic missiles, aim it at the wayward satellite just as it reaches the edge of Earth’s atmosphere, and shoot the fuel tank so it spills its toxic load before reaching the ground.
Without intervention, the tank and some of its frozen load of hydrazine, would survive the re-entry into the atmosphere, experts say, pointing to the fuel tanks recovered from the space shuttle Columbia wreckage.
"The analysis that we've done is as certain as any analysis of this type can be," said NASA administrator Michael Griffin. "The hydrazine tank will survive intact … the hydrazine in it is frozen solid. Not all of it will melt. So you will land on the ground with a tank full of slush hydrazine that would then later evaporate. The tank will have been breached -- not probably, but the tank will have been breached -- because the fuel lines will have been ripped out of the main spacecraft, and so that hydrazine will vent."
Straight shot
The missile would be launched from a Naval cruiser, likely from the Pacific Ocean, sometime between Wednesday, when the space shuttle Atlantis is due to land, and next Friday, a time period when the satellite is expected to be close to re-entering the atmosphere.
The challenging part of the operation is not the shot, which military officials say is very similar to what the missile is designed to do, namely intercept and destroy an incoming missile.
"If they can't hit this, they can't hit anything," said John Pike, a Washington, DC-based military policy analyst with GlobalSecurity.org.Earth's atmosphere expands depending primarily on how much extreme ultraviolet radiation is streaming from the sun. Traveling at the speed of light, 186,000 miles per second, photons from the sun reach Earth in about eight minutes.
The energy typically is absorbed, but if there is enough energy, an electron from an atmospheric particle can break off, contributing to the ionosphere. Otherwise, the extra energy becomes kinetic energy, which causes atoms and molecules in the atmosphere to rotate faster, causing a gradual rise in temperature.
Heating up
The warmer the atmosphere, the more friction is placed on objects in orbit. Tugged by Earth's gravity, they lose altitude as they slow, until they no longer have the momentum to remain pinned in orbit and plunge to a fiery destruction in the atmosphere.
The physics of atmospheric heating are well understood, Kent Tobiska, with Space Environment Technologies in Pacific Palisades, Calif., told Discovery News. But only in the last decade have physicists had the tools and computer models to correlate solar flux with changes in Earth's atmosphere.
Several satellites constantly monitor the sun and their data is used to create simulations to predict atmospheric heating.
"Since the start of the decade, we've made some real advances in operationally specifying what the upper atmosphere is doing in real time and predicting it out two or three days into the future," Tobiska said.
Ironically, the best indicator of how the atmospheric predictions are doing come from tracking orbital debris. The North American Aerospace Defense Command, or NORAD keeps tabs on more than 13,000 Earth-orbiting objects, continually refining their paths based on radar and other observational data.
"There's a predicted solar energy that's coming in and those are fed into atmospheric density models, producing a global atmospheric map for a given time and going out into the future," said Tobiska. "You use the junk satellites to recalibrate what the atmosphere is doing every few hours."
The hit on the dead spy satellite is being timed so that hopefully its debris will re-enter the atmosphere as quickly as possible. Still, there are uncertainties.
Greenhouse gases in the lower atmosphere, which are on the rise, trap heat, triggering cooler temperatures in the upper atmosphere, which serves to keep space junk in orbit longer.
Five studies conducted since 2000 have all reached the same conclusion: temperatures in the upper atmosphere are showing a 10 percent decrease overall, about a difference of 70 or 80 degrees, Tobiska said.
"We need to start taking into account atmospheric cooling and that's what we as a community as starting to do," he said.
Even with a perfectly timed hit, the military's troubles are not over. Atmospheric physics can carry debris far from an intended fall zone. Wreckage from the space shuttle Columbia, for example, was spread over 2,000 square miles.
"Pieces of old Russian and U.S. rockets have been found washed up on the shores of remote islands in the Pacific," said Scott Hubbard, a member of the board that investigated the 2003 Columbia accident.
The military plans to target its wayward satellite when it is about 160 miles above Earth.
"Once it hits the atmosphere, it tumbles, it breaks apart, it is very unpredictable and next to impossible to engage. So what we're trying to do here is catch it just prior to the last minute, so it's as absolutely low as possible, outside the atmosphere, so that the debris comes down as quickly as possible," said Gen. James Cartwright, vice chairman of the Joint Chiefs of Staff.
"The satellite … will deorbit more quickly, and we can predict more accurately where it will de-orbit, so we can potentially put it in a position in the ocean," he said.
"The worst case is we miss and then we have a known situation which is where we are today," he added.
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