Avoiding satellite collisions
Space junk could be the cause of satellite failures.
When a European satellite nearly collided with part of a spent Chinese rocket in January it highlighted the issue of our increasingly crowded orbital environment. Had the European Space Agency not been alerted, its eight-tonne Envisat craft would have passed the four-tonne rocket body by about 50m - a hair's breadth in orbital terms. As it was, the ESA was able to signal the Envisat to fire its manoeuvring thrusters to push it to a safer distance.
In February 2009, however, things did not turn out so well. A collision between an Iridium satellite and a deactivated Russian Cosmos craft destroyed them both, creating two large clouds of high-speed debris. The scale of the crash was unprecedented and, worryingly, not only had it not been predicted but the Iridium craft had actually been due to pass closer to another object.
These are signs of things to come, experts warn. As more and more satellites are launched, orbits are becoming congested, raising the probability of further collisions. These will in turn produce further debris and may trigger 'collisional cascading' - now dubbed the Kessler Syndrome after its co-originator Donald Kessler - where secondary collisions would act like a chain reaction to create a dense belt of debris around the Earth.
At the moment, concern is focused principally on the low-Earth orbit (LEO) domain (see box on p38), where the Iridium-Cosmos collision and Envisat near-miss occurred. It's already so congested that some experts are predicting that even if launches were halted immediately, the Kessler Syndrome could still be triggered within a few decades. And, as more satellites are launched, to all orbital altitudes, the medium-Earth orbit (MEO) and geostationary-Earth orbit (GEO) could also be affected.
So how much of a role does control technology play in enabling satellite operators to avoid debris and other satellites? 'Not much, because the sky is simply so full,' asserts Balaji Srimoolanathan, research programme manager, aerospace and defence, at consultancy Frost & Sullivan. 'Once a satellite is in orbit the main issue is its orbital station or 'slot', and once it's in that slot then control is more a case of traffic management than actual control system technology.'
Satellites have to keep to these slots rather like a string of cars would along a busy road, and operators file for them from UN agency the International Telecommunication Union (ITU) in anticipation of their future launch programmes.
Demand is therefore keen, as Des Curran, spacecraft management authority at the UK MoD's Skynet 5 operator Paradigm Secure Communications explains, 'Obtaining an orbital slot is more a case of frequency allocation by the ITU than anything else - although in our case we go through Ofcom - and you'll find that satellite operators file for slots way in advance, a bit like a property developer who'll buy a plot of land before building on it.
'But once you have a slot filed you have to use it within seven years of the initial application, or it goes back to the ITU for reallocation. And once you have a slot you never let it go - you maintain it for as long as you can. Prime slots will always be sought-after and they are likely to become busier,' he says.
The main job of the technology is to control the satellite's orientation (attitude) during orbit, the path of the orbit itself, antennas or other payload equipment and guidance and navigation, mostly using thrusters - and maintain those functions in the face of, say, drag from the outermost layers of the atmosphere, interaction between the Earth's magnetic field and the satellite, and even mechanical oscillations induced by liquid splashing around in thruster propellant tanks or radiation falling on solar panels.
These, however, are onboard and increasingly autonomous systems used for minor adjustments, hence Srimoolanathan's distinction between controlling individual satellites and controlling all of them. He adds: 'It's less about the [onboard] control systems than the technology for radio comms between the satellite and the ground.'
So tracking is the key here - keeping what Curran calls 'a constant light hand on the tiller, and sometimes more than that,' adding, 'It's a very hostile environment up there, which can give rise to myriad anomalies - not necessarily all of them recurring.'
But while it's one thing to track and control a functioning satellite, doing so with defunct craft and collision or other debris in the way is quite another. 'There is a definite need to improve the accuracy of the knowledge of the position of orbiting debris objects in order to be confident that they can be avoided even with such manoeuvres,' says Dean Hope, manager of flight dynamics at telecoms satellite operator Inmarsat.
And at the moment there's no such thing as space traffic-control, nor is there likely to be for some time. As business development executive at commercial operator Surrey Satellite Technology Ltd (SSTL), Dr Stuart Eves, explains, 'Current tracking systems have a number of issues that make space traffic control impractical at present.
'They are not sufficiently numerous or sufficiently spread around the globe to provide continuous accurate tracking coverage; they cannot provide sufficient positional accuracy to allow evasion manoeuvres to be performed with confidence, and they can't track objects smaller than 5-10 cm in size. Even a 1cm sugar-cube sized piece of debris, travelling at 7.5km/s, could easily render a satellite inoperative if a collision occurred.
'So there is a considerable gap between the amount of information we have and the amount we need. Even if the Space Fairy were to wave her magic wand and tell us where all the debris was, our satellites are not currently equipped with the propellant budget that might be required to increase our 'margin of safety' enough to be sure of avoiding collisions.
'Now you might argue that a very clever Space Fairy - for example one that could predict the variations in orbital drag caused by 'weather-like' variations in the Earth's upper atmosphere - would be able to provide such accurate orbital predictions that near-misses could be predicted very accurately, and only very rarely would objects that were truly on a collision course need to manoeuvre. The bottom line though is that we don't believe in fairies,' he says.
In the meantime, calls are growing for operators to share the orbit data they do have. Here again though, there are problems. 'Military operators are usually unwilling to advertise their satellite orbits,' says Dr Eves. 'While two cooperative, civilian satellites which happen to find themselves on a collision course might 'share' the task of avoiding a catastrophe by manoeuvring in opposite directions, this is less straightforward if one of them is a military mission that does not wish to advertise its presence.' But the degree of collaboration is improving. For example, in the wake of the Iridium-Cosmos crash, the US Strategic Command's Joint Operations Center now assesses the probability of close approaches between all operational satellites in orbit, regardless of who owns them, while a new body, the Space Data Association (SDA), has been set up to help standardise the regular exchange of orbital information between operators.
Inmarsat is a founding member of the SDA, and as Hope explains, 'The SDA can accept tracking data from sources such as US Strategic Command for both active and inactive objects, as well as tracking data from its member operators, then calculate the probability of collisions between these objects. An SDA member can therefore be warned of close approaches and perform an avoidance manoeuvre.'
Of the 18,000 or so objects in orbit, however, only about 5 per cent are operational satellites - the rest are defunct craft, rocket bodies and debris. So tackling this problem comes down to minimising the production of further debris and perhaps even removing the most troublesome objects - remediation - although it's acknowledged that this presents technical and economic obstacles.
One way to cut down on further debris, says Curran, is to stop producing it, for example by using reusable launch craft. But, as he concedes, 'Any launch vehicle can have components that come off of it and become debris. So the challenge is to design the vehicle so that all of it re-enters the Earth's atmosphere and becomes harmless to spacecraft in the LEO belt. Ultimately though, people simply need to start taking responsibility for their own debris - as we do on the ground with our trash.'
Another move, already adopted by some operators, is to push GEO craft out into 'graveyard' orbits at the end of their life. 'Indeed,' says Hope, 'all responsible GEO operators are now budgeting a reserve of [thruster] fuel sufficient to allow their satellites to be disposed of safely a few hundred kilometres above the GEO arc, as recommended by standards developed by the ISO - we placed our 2F3 satellite into this graveyard region in 2006, for example.'
That isn't practical for LEO craft, so SSTL for example is looking at a possible solution that involves 'de-orbiting', where a satellite is sent into re-entry. 'In the past, the cost of both satellites and launches demanded that operators achieve the longest possible lifetime from their satellite hardware.' Dr Eves explains. 'But as an increasing proportion of the hardware in orbit comes to consist of smaller, cheaper satellites, it will become more financially acceptable to allow them to re-enter and burn up in the Earth's atmosphere after a few years.
'So one technology we are working on is an inflatable system designed to increase the cross-sectional area of the satellite at the end of its mission, thereby increasing the drag and shortening its lifetime.'
On the face of it, better protection against impacts could be an option but, as Curran explains, 'Most modern satellites are designed so that impacts on them by space debris are minimised anyway, for example by separating out the redundant systems around the satellite and keeping the components as deep inside the body as possible.
'But even redundancy can't help in some situations, such as loss of power, so the system is monitored for signs of danger, warning us if the telemetry starts to fall outside set parameters so that appropriate action can be taken to recover the situation. You have to ask yourself whether the extra weight of protection is worth the extra costs given the probability of being hit,' he says.
It's generally agreed though that a space traffic control system is the way forward, although no one sees this happening within the next decade. So if in the meantime you lose your cellphone signal or TV picture, the reason may be right above you.
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