American spacecraft had to dodge space debris four times in 2008, NASA revealed Tuesday, a fact that highlights both the extent of the space junk problem and the primary mitigation option open to NASA.
By tracking pieces of debris larger than around four inches, space engineers can identify some dangerous space junk and meteoroids. If a satellite or spacecraft is in danger of getting hit, they simply move it out of harm’s way. The International Space Station, for example, had to make an evasive maneuver back in August 2008, to avoid a piece of an old Russian craft.
“On average, Shuttle and ISS have conducted several collision avoidance maneuvers over the past ten years on the order of about one per year,” said Nicholas Johnson, chief scientist at NASA’s Orbital Debris Program Office, in a teleconference with reporters ahead of a House subcommittee on space and aeronautics meeting about space debris.
Space debris is an increasing problem. Johnson noted that from the 1960s until the past year, orbital debris had increased linearly, despite advances in decreasing the amount of debris left behind per trip to space. But recently, a Chinese missile test on a satellite and the collision of two satellites in orbit, sent the amount of space debris up considerably. The satellite collision alone increased the risk to the upcoming May shuttle mission to the Hubble Space Telescope by 8 percent.
That complicated an already unusual mission. The Hubble Space Telescope, orbiting at 340 miles above Earth’s surface, has a far more densely crowded space debris environment than the ISS. The orbits near 560 miles are the most crowded with junk.
“The orbit of the telescope is up in a higher risk region than the Space Station is,” said Bryan O’Connor, chief of safety and mission assurance at NASA.
When the initial risk calculations came in for the mission to the Hubble at 1 in 185, they exceeded the general NASA guideline for risk, which states the risk of a catastrophic space debris hit should be less than 1 in 200. That sent the engineers back to the table to figure out ways to reduce that risk. They did so by tilting the orientation of the shuttle to take possible debris hits in less vulnerable parts of the craft and dipping to a lower orbit towards the end of the mission.
NASA also reduces the risk that a space-debris-related event will be catastrophic by inspecting the craft before re-entry. It is important to note that this doesn’t decrease the risk of a piece of space junk hitting the shuttle, but rather increases the chance that any problems resulting from such a strike could be fixed on orbit, or the crew rescued by another space craft. Their latest risk estimate is down to 1 in 221.
We already know small impacts occur regularly during shuttle flights. Wired Science obtained the Hypervelocity Impact Database last month, which revealed that in the 54 missions from STS-50 through STS-114, space junk and meteoroids hit shuttle windows 1,634 times necessitating 92 window replacements. In addition, the shuttle’s radiator was hit 317 times, actually causing holes in the radiator’s facesheet 53 times.
NASA officials downplayed the space debris problem, however, in their teleconference with reporters.
“We don’t think this is a page one story,” Johnson said. “There is no serious new issue that we have not been aware of and [that] needs to be addressed. This is just part of what we’ve been working with for many, many years.”
That contrasted with the scene during a meeting of the House subcommittee on space and aeronautics in which Dana Rohrbacher, a Republican from California, suggested that the United States needed an active strategy for getting rid of space debris.
“If we can chart it, if you’re already charting the course, all we have to do is get something up there that will knock it down,” Rohrbacher said. “That doesn’t have to be something very sophisticated. Just a big bulldozer in the sky, you might say. And perhaps something like that might not be as expensive as you think, especially if we were doing it internationally.”
Lieutenant General Larry D. James, commander of the Joint Functional Component Command for Space for U.S. Strategic Command, described U.S. monitoring capabilities as generally giving an accurate four-day forecast for space junk around our satellites, generally providing ample time for taking evasive action.
But he also said that some smaller, more erratic pieces of debris could pop into view less than half a day before they were scheduled to come across a space asset. While that wouldn’t matter to the more mobile shuttle, the International Space Station needs about 30 hours notice to maneuver.
Indeed that scenario nearly played out in early March. A previously unknown small piece of debris appeared with too little time for the ISS to move out of its way and its crew members were forced to take refuge in a Soyuz space craft that would have acted as an escape pod in the event of a catastrophic hit.
For that reason, James recommended that Congress fund the next generation of orbital sensors, colloquially known as the “Space Fence.”
“With the capability to perform 750,000 observations per day and track over 100,000 objects, the Space Fence will significantly reduce coverage gaps and significantly improve our low Earth and medium Earth orbit [space situational awareness],” he wrote in his prepared testimony.