Walking with her friends in the park on a crisp night in early 1997, Lottie Williams of Tulsa, Oklahoma, saw a stream of light flash across the sky. Suddenly a shard of metal the size of a fist smashed into her shoulder. Fortunately, the piece was metal wasn’t heavy and left Williams unharmed, but startled. It also left her the only person known to have been struck by debris falling from space, with the metal coming from a Delta 2 booster rocket falling back to Earth. It could have been worse; a 260 Kg fuel tank from the same rocket slammed to the ground in Texas around the same time, just missing an occupied farmhouse, NASA reports.
It’s a little known fact that approximately once a week, a large object like a defunct spacecraft or a rocket body falls out of space and plunges back to Earth, likely landing in the ocean, or a vast area like Siberia or the Canadian outback. And smaller objects are falling from space back to Earth daily in a fiery descent. The Antares rocket for example, which launched on April 21, crashed back to Earth on Saturday. “About once a year, we’ll find piece of spacecraft or a rocket body that survived re-entry,” said Nicholas Johnson, NASA’s chief scientist for orbital debris.
While this cascade of raining space junk sounds menacing, the objects actually pose little risk to humans. Smaller objects usually burn up during re-entry and no one object, Johnson said, poses a larger than 1 in 10,000 risk to humans. The Department of Defence has a space surveillance network that tracks tens of thousands of man-made objects in orbit, including the biggest pieces of space junk out there. These are the 23,000 objects bigger than a baseball, or 10 centimetres or larger in size. And scientists also monitor the objects falling back to Earth.
“We track, on average, two to three objects re-entering the Earth’s atmosphere each day,” said Defence department spokesperson Lt. Col. Monica Matoush. Flying through space at about 7 miles per second, these objects can wreak havoc on a spacecraft or satellite. And they’re potentially harmful to an astronaut doing a spacewalk. The Defence department sends out warnings of large objects in the trajectory of a spacecraft or satellite about three days in advance.
Then there are the smaller ones – the half million 1 centimetre objects and the 100 million 1 millimetre objects. “Small objects can do a lot of damage to a spacecraft if it’s not adequately protected,” Johnson said. “But the shielding is such that we can defend against objects up to 1 centimetre.” Critical components of the International Space Station, for example, are shielded to withstand debris up to 1 centimetre. Once or twice a year, Johnson said, NASA will manoeuvre the entire 400-metric ton space station to avoid massive space junk barrelling toward it. They also perform so-called “collision avoidance manoeuvres” with the 1,100 operational robotic spacecraft in low-Earth orbit.
With these shields protecting against smaller objects and the Defence department monitoring the larger ones, it’s the in-between pieces — the 5 centimetre objects — which pose the greatest threat, Johnson said. “If a 5 centimetre piece of debris were to hit any spacecraft, it could cause very severe damage and loss of the mission,” Johnson said.
On Feb. 10, 2009, the Iridium 33 satellite collided with the defunct Kosmos 2251 satellite at a speed of 26,000 miles per hour, creating 2,000 additional pieces of space debris. And in 1996, a piece of space debris collided with the Cerise satellite, tearing off a portion of the satellite’s gravity-gradient stabilization boom, damaging the spacecraft.
During an international meeting, scientists discussed possibilities to remove some of the more high-risk materials from low-Earth orbit. One possibility would involve launching new spacecraft into orbit, latching onto the defunct spacecraft or debris and boosting it into the ocean. And future space missions must include safe disposal. No definitive plan has been launched yet, although global experts say there is an “urgent need” to remove space debris. But it’s challenging, Johnson said: “It took a lot of effort and money to put those things into orbit, and it will take a lot of effort and money to bring these things back down,” he said.
Credit to PBS Newshour for this piece, from: