With increased news coverage of recent months of large satellites/space craft re-entering the Earths atmosphere and returning to Earth, I thought it worthwhile to look at just what is up there!
Last September, the decommissioned NASA Upper Atmosphere Research Satellite (UARS) fell back to Earth in an uncontrolled re-entry at 0400 UT Saturday, September 24 last year somewhere in the Southern Pacific Ocean between Australia and South America, well away form land. UARS weighed 5,900 kg and NASA announced in early September that at least 26 pieces of debris were expected to survive re-entry and strike the Earth’s surface, the largest of which had an estimated mass of 158.30 kg possibly reaching the surface at a velocity of 44 metres per second or 160 km/h. Smaller pieces were expected to strike the surface at up to 107 metres per second or 390 km/hr.
Then between 1:45 UT and 2:15 UT October 23rd, the 2,426kg German X-ray satellite ROSAT bid farewell to space via a fiery plunge into the atmosphere in the early hours of Sunday. Interest was high in ROSAT due to its 400kg primary mirror, which held the potential of surviving the break up of the spacecraft in the atmosphere during entry. The space telescope entered the Earth’s atmosphere somewhere over the Bay of Bengal and again there were no reports of debris hitting the ground having been reported.
Then in January this year the 13,200 kg Russian Phobos-Grunt Mars Probe returned to Earth somewhere in the Pacific Ocean west of Chile after a failed attempt to fly to Mars. The spacecraft was launched on November 9th and after weeks of failed attempts to regain control it made an uncontrolled re-entry at 1800 UT on January 15th this year. A Chinese Mars Orbiter, Yinghou-1 was also on board. Roughly 7.51 metric tonnes of highly toxic hydrazine and nitrogen tetroxide were on board, according to the head of Roscosmos. This was of most concern. Again no eye witness accounts of the actual landing and re-entry were recorded and no reports of any pieces actually making land fall.
In a recent report in NASA’s Orbital Debris Quarterly newsletter, it appears that the Earth’s atmosphere has been puffing up in response to increasing levels of Ultra Violet (UV) radiation from sunspots. UV levels increase as sunspot activity on the Sun increases. This is good news for satellite operators, because a puffed up atmosphere helps clean up low-Earth orbit. One of the greatest risks to satellites is being hit by debris or another satellite. Fortunately, according to the report, the number of catalogued debris in Earth orbit actually decreased during 2011.
Table fo Debris Re-Entry from Chinese Weather Satellite Fengyun-1C destroyed by their own Anti-Satelite Test in 2007. From Spaceweather.com
Orbital debris, or “space junk,” is any man-made object in orbit around the Earth that no longer serves a useful purpose. Space junk can be bad news for an orbiting satellite. On February 11, 2009, a U.S. communications satellite owned by a private company called Iridium collided with a non-functioning Russian satellite. The collision destroyed both satellites and created a field of debris that endangers other orbiting satellites.
Since the launch of Sputnik on 4 October 1957, more than 4,600 launches have placed some 6000 satellites into orbit. Currently about 800 satellites are used operationally for science and other applications. Space debris comprise the ever-increasing amount of inactive space hardware in orbit around the Earth as well as fragments of spacecraft that have broken up, exploded or otherwise become abandoned.
An Artists Impression of the Amount of Space Debris in Low Earth Orbit - From ESA
The debris field comprises burnt-out launch vehicle upper stages, dead or inactive spacecraft and other objects ranging in size from as big as an automobile to microscopic dust.
To minimize the risk of collision between spacecraft and space junk, the U.S. Space Surveillance Network tracks all debris larger than 10 cm. These images represent all man-made objects, both functioning and useful objects and debris, currently being tracked. The images were made from models used to track debris in Earth orbit. Of the approximately 19,000 manmade objects larger than 10 cm in Earth orbit as of July 2009, most orbit close to the Earth.
In January 11, 2007, China intentionally destroyed one of its aging weather satellites – Fengyun-1C by anti-satellite (ASAT) device. The successful test led to what has been the largest single space debris incident in history in terms of new objects. It is estimated that it created more than 3,200 pieces of trackable debris, that is debris that is around 10cm in size or larger and over 35,000 pieces between 1cm and 10cm and more than or larger, and 1 million pieces between 1 mm and 1cm. Even though only 6% of the total 3218 catalogued debris from the ill-advised engagement had re-entered by the end of 2011, half of these debris pieces fell out of orbit in the past 12 months.
At 16:56 UT on February 10, 2009, the first major space debris collision occurred between the deactivated 950 kilograms Russian satellite Kosmos 2251 and an operational 560 kilograms communications satellite, Iridium 33 some 800 km over northern Siberia. The relative speed of impact was about 11.7 kilometres per second or approximately 42,120 kilometres per hour. Both satellites were destroyed and the collision scattered considerable debris, which poses an elevated risk to spacecraft. The collision created a debris cloud, although accurate estimates of the number of pieces of debris are not yet available. Pieces of this debris are now accelerating their departure from Earth orbit. In the absence of a new major satellite breakup, the overall orbital debris population should continue to decrease during 2012 and 2013.
Since it is estimated that there are millions of pieces of man made debris out there floating around, and all of it combined weighs about 5,500 tons, does all this space junk create any problems for the International Space Station (ISS) — or even people on the ground, is a question that I get asked a fair bit.
Firstly, in relation to the ISS, space debris objects are tracked remotely from the ground, and the station crew can be notified. This allows for, what is known as, a Debris Avoidance Manoeuvre (DAM) to be carried out. This is where the thrusters on the Russian Orbital Segment are used to alter the station’s orbital altitude, avoiding the debris. DAMs are not uncommon; taking place if computational models show the debris will approach within a certain threat distance. Eight DAMs had been performed prior to March 2009, the first seven between October 1999 and May 2003. Usually the orbit is raised by one or two kilometres by means of an increase in orbital velocity of the order of 1 m/s. Unusually there was a lowering of 1.7 km on 27 August 2008, the first such lowering for 8 years. There were two DAMs in 2009, on 22 March and 17 July.
If a threat from orbital debris is identified too late for a DAM to be safely conducted, the station crew close all the hatches aboard the station and retreat into their Soyuz spacecraft, so that they would be able to evacuate in the event the debris damaged the ISS. This partial station evacuation occurred on 13 March 2009 and 28 June 2011.
Ballistic panels, also called micrometeorite shielding, are incorporated into the station to protect pressurised sections and critical systems. The type and thickness of these panels varies depending upon their predicted exposure to damage.
Now what about those of us here on Earth, is there a possibility space junk could fall back to the ground? Everything in orbit will eventually be pulled back down by Earth’s gravity. When that happens depends on how high the object is and how fast it’s going. The higher the altitude, the longer the object will take to fall, and it’ll take even longer the faster it’s speeding around the Earth. Some of these objects could stay in orbit for thousands of years.
And to the more important question – what are the risks of getting hit on the head? Fortunately, most debris burns up during re-entry, and no one has ever been killed by space junk There have been only 2 recorded instances I can find of people being hit by space debris and both lived t tell the tale; a 6 year old boy in China in November 2002 and Lottie Williams in Oklahoma in the US in 1997. UK bookmakers note the chances of space junk landing on a person are at least 20 billion to one.
