Space Situational Awareness
from Gerhard Holtkamp, 15. October 2010, 23:06
Space is anything but empty and the satellites which we depend on more and more are facing a number of threats...
Half a century into the Space Age satellites have become an integral part of our lives. Navigation, communication, global surveillance and other activities would be unthinkable in their present form without the reliable operation of satellites.
But those satellites have to work in a hostile environment. They are subject to the strong radiation from the Sun in what has become known as Space Weather. Atomic Oxygen in the upper atmosphere can corrode their surfaces and they might get hit by micro-meteoroids at any moment.
In addition to these natural causes man has created another problem: Space debris. Of the roughly 15000 objects currently tracked individually active satellites account for only about 900. The rest are inactive satellites, upper stages used to launch these payloads and debris from breakups and collisions of rocket stages and satellites.
Yet this is just the tip of the iceberg. Only objects larger than 10 cm can be tracked regularly (and even this is only for low Earth orbits - further out into the geostationary belt it's something like 1 meter). There are vastly more objects smaller than that.
The kinetic energy of an object of 1 gram hitting a satellite with a speed of 12 km/sec (a typical relative speed between two objects in near Earth space) would be similar to throwing a washing machine off the roof of a 100 m tall building (make sure nobody is standing downstairs if you try this experiment!).
To ensure the safety of their satellites now and in the future space agencies around the world are engaged in programs of what has become known as Space Situational Awareness (SSA). These activities range from setting up a space surveillance system to keep track on what's up there to issuing guidelines of how to prevent space debris and also theoretic modelling to get a better understanding of the situation.
Space Surveillance.
Objects in low Earth orbit up to a height of 2000 km can be easily tracked by radar. To get as complete a picture as possible so called radar fences are set up. They consist of radar arrays which scan a particular part of the sky. Satellites orbiting the Earth will fly through these fences every now and then and can thus be tracked. For higher altitudes however radar signals become too weak and optical means (that is telescopes) have to be employed to look for such objects. (We are talking about space debris here. Active spacecraft have transponders and can be tracked all across the solar system if necessary.)
Up to now this space surveillance has been done primarily by the military of the two former Cold War superpowers. Orbit data on space objects are publicly availabe on the Internet thanks to the U.S. Space Command and these data are widely used for checking whether objects might get close to active satellites. Should there be a possible conjunction event a small evasion maneuver might be possible to get out of harms way. Satellites in particularly crowded orbits might have to perform such a maneuver about once a year.
But there are two major problems with the publicly available orbit data. Secret U.S.-, Japanese- and French military satellites are not included (not even the upper stages of the rockets that launched them nor any space debris resulting from such payloads) and there is also no public information on the accuracy of these orbit data. So your satellite might end up being hit by such an object that you didn't even know was there or you might do an unnessary maneuver in case of a known offending object because you were not sure just how accurate the orbit data were.
Realizing that this is not a healthy situation the European Space Agency ESA is currently in the process of setting up their own civilian space surveillance system. Like their military counterparts it will comprise radar fences and optical telescopes. Special surveillance satellites might be added at some point in the future and the monitoring of space weather would be a part of it.
Modelling.
To better assess the impact of space debris now and in the future various computer models and data bases have been created. In addition to the tracked larger objects a representative distribution of smaller micro-meteoroids and space debris needs to be included. Special measurement campaigns are being conducted to get statistical samples of these untracked objects.
In order to project the space debris population into the future breakups and collisions of objects must be simulated. These models are checked and tuned by special laboratory experiments in which particles are shot with high speed at typical satellite surfaces or even at complete satellite models. Also actual collisions that happened in space are carefully analysed and compared to the simulations.
On the whole these computer models seem to represent the space environment well giving their predictions into the future credibility. And this future doesn't look too well - at least if nothing is done about it. If we continue with our current practices of how to launch, operate and dispose of satellites then we can expect certain orbits to show a snowball effect of colliding space debris which could make those orbits almost unusable.
Space Debris Mitigation.
It would be best if only active satellites stayed in orbit and inactive ones as well as upper stages etc. were removed immediately when no longer needed. However this requires extra fuel which increases the launch mass. Therefore in the beginning only payloads which needed to return to Earth (like manned spacecraft or military intelligence film cassettes) were actively de-orbited. The rest were left to decay naturally. Another problem was that upper stages and retired satellites still had some unused fuel on board which could explode at a later time.
Over the years a number of instantaneous breakup events let to an ever increasing number of space debris. To prevent this measures are now being taken at the end of an active satellite's life or after a rocket stage has done its job. Any fuel left will be vented, batteries are deactivated etc. At the beginning of a satellite's life debris is often generated by expelling lens covers and other such items only needed during the launch and early orbit phase.
A rule adopted by NASA, ESA and other space agencies now requires satellites with altitudes up to about 1000 km to lower their orbit at the end of their active life enough so that they will reenter within 25 years. Although better than no rule it still means that a dead satellite of a few thousand kg will be around for a quarter of a century during which it can collide with some other debris. It isn't even sure whether the time span might not exceed 25 years because the reentry depends largely on how the highly variable solar activity increases the density of the upper atmosphere.
At best this 25 year rule is a stop-gap measure. Simulations show that only the total removal of selected satellites will really improve the situation. In the future there may be a requirement to carry enough fuel for a complete end-of-life deorbit. There are also various plans for something like a space garbage truck. A Japanese proposal for example sees a small satellite which can attach itself to an "non-cooperating" satellite, then unreel a tether which can conduct electricity and use the Earth's magnetic field to help deorbit the satellite within a few months.
Satellites in higher orbits are retired into so called graveyard orbits. For geostationary satellites this means lifting them by about 300 km above the geostationary belt. GPS satellites are lifted by 800 km. Numerical simulations show that satellites in these new orbits stay out of the way of the operational orbits for at least 200 years. But should there be a collision between any such retired satellites some of the resulting debris could find its way back to the operational orbits.
In a few decades we hopefully posses more efficient methods to launch new satellites and to completely retire old ones.
Protecting Satellites against Space Debris.
While active satellites can maneuver to avoid larger objects they have to be prepared to be hit by small untracked objects. Surrounding satellites with thick metal plates like a battleship might protect them but the resulting mass at launch would be prohibitive.
A more efficient method was proposed by astronomer Fred Whipple a decade before the first satellite was actually launched. In its most basic form the so called Whipple shield consists of two thin plates seperated by a certain distance. A projectile hitting the first plate will penetrate it but starts to disintegrate into fragments which are spread out when reaching the second plate. This second plate can then stop the now smaller and weaker fragments.
Modern Whipple shields might use more than two plates, special light-weight high strength materials (like Keflar which is used in bullet proof wests), metallic meshes and foam etc. Consideration is also given to which parts of the satellite need more protection than others.
For obvious reasons the most fortified human object currently in space is the Internation Space Station ISS. At assembly complete ISS space debris shielding will exceed 23 metric tons or more than 5% of the total mass of the ISS. This shielding is not uniform but it already takes into account that certain parts of the station are partly shielded by other sections and that space debris doesn't arrive uniformly from all directions.
Reentry Risk Assessment.
Manned spacecraft and a few other payloads which have to be recoverd perform pinpoint landings at a specific place and time. The current reentry rate of the remaining objects amounts to about 1.1 tons per day. While smaller objects completely burn up meteor-like in the atmosphere between 10% and 40% or the mass of larger objects can survive. And this can be anywhere along the groundtrack posing a risk to people and property.
In 1997 Lottie Williams from Tulsa, Oklahoma saw a bright flash of light like a meteor and was hit shortly afterwards by a 15 cm long metallic object. Although the object was never properly identified the time and place is consistent with the reentry of the second stage of a Delta rocket. Luckily Lottie had received just a glancing blow and remained uninjured.
This is the only instance to date that a person was (probably) hit by a piece of space debris. Somebody figured out that the risk of being hit by space debris is about 50 to 100 thousand times lower than being hit by lightning. Reassuring as this may sound it underestimates the problem. Space debris falling for instance on a busy road can easily result in casualties even if it doesn't hit a person directly on the head.
Esperance is a small town on the South coast of Western Australia. If you are ever in the region don't miss visiting their Municipal Museum. Surrounded by many items from the pioneer days of local settlement you can find pieces of the early American space station Skylab which broke up and reentered over that part of the world in July 1979.
The 75 ton Skylab did not have a propulsion system for deorbit. Friction with the tenuous upper atmosphere would cause it to sink ever lower. By changing the attitude of the station a few days before reentry was imminent mission controllers tried to change the drag in such a way that the reentry would occur over an empty ocean rather than a densly populated region. It almost worked. Much of the surviving Skylab debris is assumed to have fallen into the Indian Ocean before reaching the coast of Australia.
Luckily the region that Skylab parts did come down is only sparsely populated. Nobody got hurt and no damage was reported. The event even provided the local residents with a few days worth of entertainment driving around the countryside trying to locate Skylab pieces. Visiting the museum I was particularly impressed by the size of the oxygen tank and some other parts which survived almost unscathed (see pictures).
Another spectacular reentry once occured when a Russian satellite with a nuclear reactor on board came down over Northern Canada. This required a special cleanup effort due to radioactive contamination. Once again we have been lucky that reentry occured over a sparsely populated area.
But luck has a habit to eventually run out. The reentry part of a satellite's life needs a lot more thought starting with the very design of satellites. Small satellites can probably be built in such a way that they completely burn up during reentry. Larger satellites will typically have to be designed to be deorbited in a controlled way over some empty stretch of ocean. This is already the practice for the unmanned Progress, ATV and HTV supply craft to the ISS. And otherwise we are back to the space garbage truck.
Whether designing, launching, operating or disposing a satellite - Space Situational Awareness is bound to play an ever increasing role in the future.
Half a century into the Space Age satellites have become an integral part of our lives. Navigation, communication, global surveillance and other activities would be unthinkable in their present form without the reliable operation of satellites.
But those satellites have to work in a hostile environment. They are subject to the strong radiation from the Sun in what has become known as Space Weather. Atomic Oxygen in the upper atmosphere can corrode their surfaces and they might get hit by micro-meteoroids at any moment.
In addition to these natural causes man has created another problem: Space debris. Of the roughly 15000 objects currently tracked individually active satellites account for only about 900. The rest are inactive satellites, upper stages used to launch these payloads and debris from breakups and collisions of rocket stages and satellites.
Yet this is just the tip of the iceberg. Only objects larger than 10 cm can be tracked regularly (and even this is only for low Earth orbits - further out into the geostationary belt it's something like 1 meter). There are vastly more objects smaller than that.
The kinetic energy of an object of 1 gram hitting a satellite with a speed of 12 km/sec (a typical relative speed between two objects in near Earth space) would be similar to throwing a washing machine off the roof of a 100 m tall building (make sure nobody is standing downstairs if you try this experiment!).
To ensure the safety of their satellites now and in the future space agencies around the world are engaged in programs of what has become known as Space Situational Awareness (SSA). These activities range from setting up a space surveillance system to keep track on what's up there to issuing guidelines of how to prevent space debris and also theoretic modelling to get a better understanding of the situation.
Space Surveillance.
Objects in low Earth orbit up to a height of 2000 km can be easily tracked by radar. To get as complete a picture as possible so called radar fences are set up. They consist of radar arrays which scan a particular part of the sky. Satellites orbiting the Earth will fly through these fences every now and then and can thus be tracked. For higher altitudes however radar signals become too weak and optical means (that is telescopes) have to be employed to look for such objects. (We are talking about space debris here. Active spacecraft have transponders and can be tracked all across the solar system if necessary.)
Up to now this space surveillance has been done primarily by the military of the two former Cold War superpowers. Orbit data on space objects are publicly availabe on the Internet thanks to the U.S. Space Command and these data are widely used for checking whether objects might get close to active satellites. Should there be a possible conjunction event a small evasion maneuver might be possible to get out of harms way. Satellites in particularly crowded orbits might have to perform such a maneuver about once a year.
But there are two major problems with the publicly available orbit data. Secret U.S.-, Japanese- and French military satellites are not included (not even the upper stages of the rockets that launched them nor any space debris resulting from such payloads) and there is also no public information on the accuracy of these orbit data. So your satellite might end up being hit by such an object that you didn't even know was there or you might do an unnessary maneuver in case of a known offending object because you were not sure just how accurate the orbit data were.
Realizing that this is not a healthy situation the European Space Agency ESA is currently in the process of setting up their own civilian space surveillance system. Like their military counterparts it will comprise radar fences and optical telescopes. Special surveillance satellites might be added at some point in the future and the monitoring of space weather would be a part of it.
Modelling.
To better assess the impact of space debris now and in the future various computer models and data bases have been created. In addition to the tracked larger objects a representative distribution of smaller micro-meteoroids and space debris needs to be included. Special measurement campaigns are being conducted to get statistical samples of these untracked objects.
In order to project the space debris population into the future breakups and collisions of objects must be simulated. These models are checked and tuned by special laboratory experiments in which particles are shot with high speed at typical satellite surfaces or even at complete satellite models. Also actual collisions that happened in space are carefully analysed and compared to the simulations.
On the whole these computer models seem to represent the space environment well giving their predictions into the future credibility. And this future doesn't look too well - at least if nothing is done about it. If we continue with our current practices of how to launch, operate and dispose of satellites then we can expect certain orbits to show a snowball effect of colliding space debris which could make those orbits almost unusable.
Space Debris Mitigation.
It would be best if only active satellites stayed in orbit and inactive ones as well as upper stages etc. were removed immediately when no longer needed. However this requires extra fuel which increases the launch mass. Therefore in the beginning only payloads which needed to return to Earth (like manned spacecraft or military intelligence film cassettes) were actively de-orbited. The rest were left to decay naturally. Another problem was that upper stages and retired satellites still had some unused fuel on board which could explode at a later time.
Over the years a number of instantaneous breakup events let to an ever increasing number of space debris. To prevent this measures are now being taken at the end of an active satellite's life or after a rocket stage has done its job. Any fuel left will be vented, batteries are deactivated etc. At the beginning of a satellite's life debris is often generated by expelling lens covers and other such items only needed during the launch and early orbit phase.
A rule adopted by NASA, ESA and other space agencies now requires satellites with altitudes up to about 1000 km to lower their orbit at the end of their active life enough so that they will reenter within 25 years. Although better than no rule it still means that a dead satellite of a few thousand kg will be around for a quarter of a century during which it can collide with some other debris. It isn't even sure whether the time span might not exceed 25 years because the reentry depends largely on how the highly variable solar activity increases the density of the upper atmosphere.
At best this 25 year rule is a stop-gap measure. Simulations show that only the total removal of selected satellites will really improve the situation. In the future there may be a requirement to carry enough fuel for a complete end-of-life deorbit. There are also various plans for something like a space garbage truck. A Japanese proposal for example sees a small satellite which can attach itself to an "non-cooperating" satellite, then unreel a tether which can conduct electricity and use the Earth's magnetic field to help deorbit the satellite within a few months.
Satellites in higher orbits are retired into so called graveyard orbits. For geostationary satellites this means lifting them by about 300 km above the geostationary belt. GPS satellites are lifted by 800 km. Numerical simulations show that satellites in these new orbits stay out of the way of the operational orbits for at least 200 years. But should there be a collision between any such retired satellites some of the resulting debris could find its way back to the operational orbits.
In a few decades we hopefully posses more efficient methods to launch new satellites and to completely retire old ones.
Protecting Satellites against Space Debris.
While active satellites can maneuver to avoid larger objects they have to be prepared to be hit by small untracked objects. Surrounding satellites with thick metal plates like a battleship might protect them but the resulting mass at launch would be prohibitive.
A more efficient method was proposed by astronomer Fred Whipple a decade before the first satellite was actually launched. In its most basic form the so called Whipple shield consists of two thin plates seperated by a certain distance. A projectile hitting the first plate will penetrate it but starts to disintegrate into fragments which are spread out when reaching the second plate. This second plate can then stop the now smaller and weaker fragments.
Modern Whipple shields might use more than two plates, special light-weight high strength materials (like Keflar which is used in bullet proof wests), metallic meshes and foam etc. Consideration is also given to which parts of the satellite need more protection than others.
For obvious reasons the most fortified human object currently in space is the Internation Space Station ISS. At assembly complete ISS space debris shielding will exceed 23 metric tons or more than 5% of the total mass of the ISS. This shielding is not uniform but it already takes into account that certain parts of the station are partly shielded by other sections and that space debris doesn't arrive uniformly from all directions.
Reentry Risk Assessment.
Manned spacecraft and a few other payloads which have to be recoverd perform pinpoint landings at a specific place and time. The current reentry rate of the remaining objects amounts to about 1.1 tons per day. While smaller objects completely burn up meteor-like in the atmosphere between 10% and 40% or the mass of larger objects can survive. And this can be anywhere along the groundtrack posing a risk to people and property.
In 1997 Lottie Williams from Tulsa, Oklahoma saw a bright flash of light like a meteor and was hit shortly afterwards by a 15 cm long metallic object. Although the object was never properly identified the time and place is consistent with the reentry of the second stage of a Delta rocket. Luckily Lottie had received just a glancing blow and remained uninjured.
This is the only instance to date that a person was (probably) hit by a piece of space debris. Somebody figured out that the risk of being hit by space debris is about 50 to 100 thousand times lower than being hit by lightning. Reassuring as this may sound it underestimates the problem. Space debris falling for instance on a busy road can easily result in casualties even if it doesn't hit a person directly on the head.
Esperance is a small town on the South coast of Western Australia. If you are ever in the region don't miss visiting their Municipal Museum. Surrounded by many items from the pioneer days of local settlement you can find pieces of the early American space station Skylab which broke up and reentered over that part of the world in July 1979.
The 75 ton Skylab did not have a propulsion system for deorbit. Friction with the tenuous upper atmosphere would cause it to sink ever lower. By changing the attitude of the station a few days before reentry was imminent mission controllers tried to change the drag in such a way that the reentry would occur over an empty ocean rather than a densly populated region. It almost worked. Much of the surviving Skylab debris is assumed to have fallen into the Indian Ocean before reaching the coast of Australia.
Luckily the region that Skylab parts did come down is only sparsely populated. Nobody got hurt and no damage was reported. The event even provided the local residents with a few days worth of entertainment driving around the countryside trying to locate Skylab pieces. Visiting the museum I was particularly impressed by the size of the oxygen tank and some other parts which survived almost unscathed (see pictures).
Another spectacular reentry once occured when a Russian satellite with a nuclear reactor on board came down over Northern Canada. This required a special cleanup effort due to radioactive contamination. Once again we have been lucky that reentry occured over a sparsely populated area.
But luck has a habit to eventually run out. The reentry part of a satellite's life needs a lot more thought starting with the very design of satellites. Small satellites can probably be built in such a way that they completely burn up during reentry. Larger satellites will typically have to be designed to be deorbited in a controlled way over some empty stretch of ocean. This is already the practice for the unmanned Progress, ATV and HTV supply craft to the ISS. And otherwise we are back to the space garbage truck.
Whether designing, launching, operating or disposing a satellite - Space Situational Awareness is bound to play an ever increasing role in the future.
Gerhard,
Thanks for summing up this depressing problem and the measures that are undertaken (or should be undertaken) to control it.
In April 12009, there was a conference at the Europan Space Operations Centre ESOC in Darmstadt on this very subject.
Here is the press release with the key findings:
http://www.esa.int/esaCP/SEMKO5EH1TF_index_0.html
What impressed (and depressed) me most about this conference were the scenarios that were outlined.
1.) "Business as usual" - continue the messy and careless approach that humanity has taken up to now: In a few hundreds of years, we'd see a snowballing number of large debris objects in orbits. Spaceflight would come grinding to a halt.
2.) "Undertake mitigation measures", especially those you outlined: deorbiting upper stages, ensuring that no objects remains in near Earth orbit for more than 25 years. Consequence: We'd still see things get worse before they get better, but compared to the unbridled scenario, the increase would be much more moderate and with luck, we'd avoid the avalanche scenario. But as you rightly say, luck has a habit of running out.
3.) "Stop all spaceflight": One would think that at least then the problem would not get worse. But, depressingly, it does, because of all the stuff that is already up there. So even if we renounced all the benefits of spaceflight, the problem would still get worse before it gets better.
Well, we can't stop spaceflight. It is too important for civilization. Technological societies rely on space-based data gathering for a myriad of vital applications. We don't have the choice. We just have to do everything we can to avoid more debris and we should start - now! Not some day - to develop means of deorbiting defunct upper stages and satellites before they eventually blow up.
But if anyone reading this in involved in a planned satellite project for low Earth orbit, please undertake everything you can to avoid your spacecraft from becoming a problem. Have it launched into as low an orbit as acceptable and add devices that will reduce its orbital lifetime. Just a few appendages made out of thin sheet metal to increase the aerodynamic cross section should do the trick.