The governments of Britain, Japan and Australia are voicing concern over China's apparent test of an anti-satellite missile. The United States says China shot down one of its own aging weather satellites last week, in a kind of target practice in low Earth orbit.
Not much information about the event has been released. But scientists say hitting a satellite from the ground takes fairly sophisticated technology.
The satellite was 500 miles above the Earth's surface. The explosion created a cloud of debris in space, adding to the amount of "space junk" circling the Earth.
Hans Kristensen, a weapons expert at the Federation of American Scientists, says that while it has been assumed that China was working to develop such capabilities, the satellite strike still surprised him.
"I was surprised that they were able to do it," Kristensen says.
U.S. officials say the Chinese hit the satellite with the help of a medium-range ballistic missile — most likely the DF-21.
A satellite is a fairly small thing to hit with a missile. Kristensen says the DF-21 can probably hit a spot on the ground with an accuracy of several hundred feet. But the satellite was probably close to the size of a refrigerator.
So the assumption is that the Chinese device had some sort of advanced guidance system — maybe a kind of telescope to pick out the satellite. Then, it used thrusters to steer it toward the target. The force of the collision would have destroyed the satellite.
Kristensen says that when the United States and Russia were developing anti-satellite missiles in the 1980s, hitting the target was the hard part.
The United States successfully shot down one of its own satellites in 1985. So could a Chinese missile now take out a U.S. satellite? A rule of thumb is that a missile can go to a height about half of its horizontal range. So this missile might be able to reach an altitude of 600 miles.
The Global Positioning System satellites, now in common use, are much higher, tens of thousands of miles out.
But Kristensen says there are plenty of targets lower in the atmosphere. Spy satellites tend to be very low," he says, "because you need to get close."
David Wright, a physicist with the Union of Concerned Scientists, has been using computer models created by NASA to estimate debris from collisions. He says the latest smash-up would have been the equivalent of a half ton of high explosives going off.
This satellite was maybe 3/4 of a ton in mass," Wright says. "And it would have thrown off about 2 million pieces that were bigger than a millimeter in size. Sounds pretty small but at speed going a millimeter object could be deadly."
Deadly, that is, for other satellites. The odds of the debris hitting something look small he says. But this single event probably doubled the number of pieces of space junk at its altitude range.