Photograph courtesy NASA
Published August 8, 2011
Earlier this month Russia's space agency Roscosmos caused an inadvertent media frenzy when deputy head Vitaly Davydov stated in a video that the International Space Station (ISS) would sink into the Pacific Ocean in 2020.
But NASA officials say the statement about the roughly $150-billion, 15-nation partnership was premature.
Officially NASA, Roscosmos, and partners in Japan, Europe, and Canada have agreed to keep the station operational until at least 2020, but "that's only half the story," said NASA spokesperson Joshua Buck.
"The international partners have been discussing extending the mission through 2028. At this point, there's no reason we wouldn't do that."
Still, at some point the mission will end, and the orbiting laboratory will be directed to plunge toward Earth. The station can't simply be left in orbit, or it will eventually fall from the skies on its own, raining debris over a wide swath of the planet and possibly endangering people on the ground.
So what exactly will happen when we no longer need the biggest artificial space object in history?
Space Station to Have Controlled Fall
The U.S. government tracks about 8,000 baseball-size or bigger objects in low-Earth orbit, a zone within a few hundred miles above the planet's surface. Most are pieces of used rockets and dead satellites, but more than 500 are functional spacecraft.
To stay in orbit, an object must be traveling at a constant speed over Earth's surface. For the ISS, which orbits at a height of about 200 miles (322 kilometers), that's roughly 17,500 miles (28,163 kilometers) an hour.
But low-Earth orbit isn't a perfect vacuum. A whisper-thin veil of molecules at the atmosphere's outer reaches bumps into and slows down orbiting objects.
"You can't just leave something you want up there or you'll lose it. It's going to require boosting," said aerospace engineer and reentry specialist William Ailor of the Aerospace Corporation, a federally funded research and development center based in El Segundo, California.
The ISS weighs roughly 920,000 pounds (417,305 kilograms) and spreads out over an area the size of an NFL football field. To counteract its orbital drag, mission controllers must use propellant in attached spacecraft to periodically loft the station to a higher orbit.
Without those propellant burns, the station would eventually drop from orbit. But it's not practical to let the propellant simply run out: The ISS won't be completely destroyed by atmospheric reentry, and unless it's brought down intentionally, no one would know when or where chunks of debris would reach Earth.
Aiming for Spacecraft Cemetery
To keep people on the ground safe, it's best to deorbit a spacecraft with a strong burn of propellant, to slow it down at the right time. That's what Roscosmos did for Mir, which, prior to the ISS, was the biggest human-made object in space.
The 145-ton Russian space station zoomed around Earth until March 2001, when Roscosmos slowed Mir down so that it broke up over the South Pacific Ocean at a location called Spacecraft Cemetery, a spot known for its remoteness from civilization.
Watch video of Mir's debris trail taken from a South Pacific island.
"Larger things like this come down with big footprints, usually 400 miles [644 kilometers] long and as wide as 30 miles [48 kilometers]. Basically a big, long ellipse," Ailor said.
Because deorbit footprints are so enormous, Ailor said no one is likely to recover any hunks of Mir or the next crashed space station for museum displays.
"You've got debris spread out over a long footprint. Trying to find something like that, even though targeted in the same area, is like looking for needles in a very big haystack," he said. "It would be an expensive proposition."
Space Station to Have Mir-rored Demise
Despite its larger size, the ISS should experience a textbook reentry resembling Mir's.
According to a NASA presentation released in October 2010, space station controllers will dock an unmanned vehicle to the forward end of the laboratory. At the right time, they'll burn its propellant at full blast and sink the entire complex deep into the atmosphere.
From there, things start heating up.
As the ISS plows through the thin outer atmosphere at hypersonic speeds, cone-like shock fronts will surround parts out in front. Air molecules will compress, collide, emit infrared radiation, and heat pieces of the space station upward of 3,000 degrees Fahrenheit (1,649 degrees Celsius).
"You're converting all of that kinetic energy from the spacecraft's orbit into heat to slow it down," Ailor said, adding that "the things that [will disintegrate] first are the flimsiest components, like solar panels."
Bigger modules will heat up and begin to soften or melt as the plunge continues. Aluminum has the lowest melting point and gives way first, followed by steel and titanium.
The space station's metallic joints should break apart next. And when the weakened structure sinks into thicker parts of the atmosphere, deceleration up to eight times stronger than Earth's gravity will rip the components to shreds.
"A spacecraft is essentially all broken apart by 120,000 feet [36,576 meters] or so. Surviving pieces are now cooling and continue to slow down," Ailor said. "Most fragments go subsonic at 60,000 to 80,000 feet [18,288 to 24,384 meters], then fall straight down—pianos falling out of a window is a good analogy."
Between 10 and 40 percent of the space station's mass should drop back to Earth, based on the intentional destruction of other large objects such as the Johannes Kepler spacecraft.
Pieces that do survive will either be those dense enough to not fully melt or break apart, or those shielded by components that bore the brunt of atmospheric reentry.
"When debris were recovered from space shuttle Columbia, things survived reentry that were never designed to," Ailor said. "They even found living organisms in a biological experiment that" the shuttle was bringing back to Earth.
Is the End of the ISS a Done Deal?
The plan to safely take down the ISS was initially developed to deal with unexpected emergencies, such as if the station is hit by space debris and is no longer safe for astronauts. Barring that, the laboratory was only recently fully assembled and still has plenty of useful life.
"There's an international crew of six up there doing science and active experiments. It's doing what it was designed for in a full capacity," Buck said.
NASA also has tentative plans to use the space station as a testbed for technologies that would be used on future missions to an asteroid, the moon, or perhaps even Mars. Buck added, however, that there are currently no plans to boost it out of low-Earth orbit and on to some other destination.
For now, it's too soon to say when Roscosmos, NASA, and its partners will send the orbiting laboratory to its doom.
Although plans are in place to eventually deorbit the space station, he said, "who knows what will happen in the next 17 years."
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