The US is livid at Russia for conducting “recklessly" blowing up one of its own satellites in a missile test, saying that the “long-lived debris created by this dangerous and irresponsible test" will “threaten satellites and other space objects… for decades to come". Following the blowing up of the satellite, astronauts and cosmonauts aboard the International Space Station (ISS) had to shelter in place amid fears that some of the debris was headed their way. So, how many pieces of space debris are floating up there and what’s the danger that they can pose?
Why Does Space Debris Not Fall Back To Earth?
Blame it on Newton’s First Law of Motion, which says that an object will continue moving at a constant speed and in a straight line unless acted on by an unbalanced force. Up there in space, where the pull of gravity is low, an object tends to carry on in the orbit that it is placed in.
Objects in low Earth orbit (LEO), the region in space where the International Space Station and a host of satellites move around the Earth, avoid falling back to Earth by maintaining enough velocity to neutralise its gravitational pull. So, they tend to continue travelling in their demarcated routes for long periods.
But to be sure, even though the gravitational force is weak and the atmosphere thin, these objects do eventually start getting drawn back to Earth, although that may take years, or even decades. As US space agency Nasa points out, debris in orbits below 600km usually fall back to Earth within a few years but objects at heights of over 1,000km can stay in orbit for more than a century.
How Dangerous Can Small Pieces Of Space Junk Be?
Even the tiniest pieces of space debris can pose a danger because of the speed at which they are moving. As Nasa puts it, most space junk is moving “super fast", reaching blinding speeds of up to 8km per second.
“The average impact speed of a piece of orbital debris running into another object is 22,370 miles per hour (36,000kmph)," it says, which means that even a tiny piece of orbital debris can cause a lot of damage. For comparison, a bullet moves at speeds of under 3,000kmph. Thus, an approximately 4kg piece of space debris could have the same impact as a car moving at close to 100kmph, the US space agency adds.
How Many Pieces Of Space Debris Are Flying Around?
Virtually everything that ever went up to space and didn’t carry on beyond the planet’s gravitational field and is yet to fall back to Earth is still revolving in some orbit out there. This includes legitimate pieces like satellites and the ISS, and also hundreds and thousands of space junk.
Thousands of objects have been launched ito the great void since the erstwhile Soviet Union kicked off the space age with the launch of the Sputnik 1 satellite in 1957. Many such objects have been retired from service, conked out, or disintegrated after being turned into a target for the kind of missile test carried out by Russia, but which even the likes of US, China and India, too, have conducted.
Nasa says that orbital debris can range in size from tiny flecks of paint that have come off spacecraft to large satellites that are no longer working. It notes that the most common source of orbital debris larger than 1cm “is the explosion of objects orbiting the Earth", which most often are the upper stages of rockets that can contain fuel or high-pressure fluids.
According to Nasa, there are about 23,000 pieces of space debris larger than the size of a cricket ball orbiting the Earth along with half a million pieces of debris the size of a marble or larger. That’s not all, there are also approximately 100 million pieces of debris about 1mm or larger in size along with pieces of even smaller micrometre-sized debris.
The Russian missile test is said to have broken up the satellite into thousands of pieces of which the US is reported to have identified more than 1,500 trackable pieces.
How Does The International Space Station Avoid Being Hit By Space Debris?
Given the threat that even the smallest piece of fast-moving space debris can pose to satellites and spacecraft, Nasa actually tries to track them as they orbit around the Earth. Nasa says that more than 27,000 pieces of orbital debris are tracked by the US Department of Defence global Space Surveillance Network (SSN) sensors. While much of the debris may be too small to be tracked, they are “large enough to threaten human spaceflight and robotic missions".
Nasa says it has “a long-standing set of guidelines on how to deal with each potential collision threat" to ISS, laying down the conditions under which “evasive action or other precautions to ensure the safety of the crew are needed".
It says that collision risks are categorised depending on the size of the space debris involved. Where it known well in advance that space debris is headed towards the ISS, it is moved slightly to get out of harm’s way, something known as a “debris avoidance manoeuvre". The ISS has conducted 29 debris avoidance maneuvers since 1999, including three in 2020.
But where the data is not precise enough or the forecast of a collision course comes too late, “the best course of action is to move the crew into the Russian Soyuz or US commercial crew spacecraft that are used to transport humans to and from the station". That is what the ISS crew had to do after Earth-based mission control centres found out about the debris headed their way from the Russian satellite explosion.
“The crew would be able to leave the station if the collision caused a loss of pressure in the life-supporting module or damaged critical components. The spacecraft act as lifeboats for crew members in the event of an emergency," it says.