Have you ever wondered what happens to all of the old objects launched into space, such as rockets and satellites? It's called space junk, and much of it is still out there now. According to NASA, the definition of space junk is "any man-made object in orbit (轨道) around Earth that no longer serves a useful function".
One large piece of space junk, a disused rocket, hit the dark side of the moon on March 4. The rocket's origin is not clear, but scientists know that it was massive — weighing about 3 tons. It must have left a large crater (坑), estimated 10 to 20 meters wide, on the moon's surface. It won't be visible right away, but scientists are trying to locate it. "We will find the crater, eventually," Mark Robinson, lead investigator for NASA Lunar Reconnaissance Orbiter Camera, told USA TODAY. "Depending on its location,itcould take as long as 28 days."
What's significant about this space junk crash is that it draws attention to some of the potential dangers of space junk. First, the accumulation of space junk in Earth's orbit means that satellites and the International Space Station (ISS) are at risk. In 2016, tiny space debris (残骸), smaller than a millimeter, caused a tiny chip in the ISS' window. Imagine the damage that a larger piece of debris could cause. The European Space Agency website states that objects "up to 1 cm in size could disable an instrument or a critical flight system on a satellite" while objects over 10 cm "could shatter a satellite or spacecraft into pieces". If this sounds terrifying, that's because it is. Any collision with larger pieces of debris could be especially dangerous for manned spacecraft.
In addition to potential damage to spacecraft and satellites, space junk also poses a threat to the Earth itself. Crashes can send debris into the atmosphere where it can remain for many decades. Some powerful crashes can even send large debris hurtling (猛冲) toward Earth's surface where it can affect the local environment. Leftover rocket debris can contain toxic (有毒的) materials that are harmful to plant and animal life. Some people in Siberia have even reported serious health problems after rocket debris landed in their regions in 2012, a local doctor told the BBC.
