Space debris a danger to satellites
The plot for the 2013 film Gravity centres on astronauts dealing with space debris generated by a Russian missile strike on a defunct satellite.
The film convincingly portrays the lethal risk to spacecraft posed by dust and metal shards orbiting at hyper-velocity. The fireball spotted over the South Island on July 19 is believed to have been space junk.
Space debris is the name given to a wide range of human-made detritus in space, ranging in size from dust particles to old satellites. Most of the dust comes from satellite disintegration fragments, collision damage and rocket-fuel remnants.
The danger arises from the huge orbital speed of this material. A head-on collision with a metal shard in Low Earth Orbit (LOE), 160km to 2000km up, has a speed of 58,000kmh, enough to destroy a satellite and produce substantial quantities more of space debris.
NASA estimates there are 600,000 pieces of space debris between 1 and 10cm in size and that a piece hits the Earth daily. Most are harmless, but if it weren’t for luck some would have been deadly.
In 2007 the pilot of a LAN Chile commercial flight out of Auckland reported seeing debris from a Russian spy satellite falling into the Pacific Ocean. The closest approach of the debris to the aircraft was about 8km but its rate of descent caused a sonic boom (there was subsequently a debate about whether it was space debris or a meteorite).
In LOE the atmosphere is extremely thin. However, there is still sufficient atmosphere to cause air drag which, after some time, decelerates larger pieces of debris to below orbital speed, causing the debris to fall towards the Earth and hopefully burning up en-route.
But there is no air drag at the altitude of a geostationary satellite orbit (36,000km) and debris can remain there for millennia.
A geostationary orbit is one in which the satellite appears to be in the same place above the Earth because the time it takes to orbit is the same as the time the Earth takes to spin once on its axis.
A near collision is when two satellites approach within 50m of each other. NASA estimates this occurs about once per year.
There are two nightmare events – the Kessler Syndrome and missiles destroying satellites.
The Kessler Syndrome, proposed by NASA scientist Donald Kessler in 1978, refers to a scenario in which there are so many satellites in LEO that a collision produces enough debris to cause a cascade in which another satellite is hit soon after, followed by another then another – a runaway collision gradually piling more and more debris into the orbit.
The air drag on dust and small particles isn’t sufficient to decelerate these pieces and pull them out of orbit for many decades. After a Kessler event it would be years before it was possible to send satellites to or through LEO without major damage.
Given the huge dependency on satellites for strategic and commercial uses, winning a future war could necessitate destroying the enemy’s satellites using missiles. An all-out assault could have the same effect as the Kessler Syndrome.
As if to underline this threat, the precedent for a missile strike on a satellite was set by the Chinese who in 2007 targeted one of their weather satellites to test their ability to engage in satellite warfare if the need arose.
The test was a spectacular success and caused alarm in the West, not least because of the amount of space debris the collision generated.
Most satellites and spacecraft are protected to some extent from minor debris by a Whipple shield, a kind of metal bumper attached to the spacecraft wall. It presents an impact target and prevents the fabric of the spacecraft being damaged. The International Space Station has one, but, solar cells cannot be obstructed and so power generation on spacecraft remains vulnerable.
There is a tacit agreement that defunct satellites be sent to a graveyard orbit, one remote enough for old satellites to disintegrate in safety.
However, the agreement is not binding. As a result, space is still a modern-day Wild West.