The same drivers that led to the global air traffic control system on Earth could also result in the creation of an analogous space traffic control (STC) system, as the quantity of space debris continues to increase.
discussed in our previous space blog ‘Fishing space debris from orbit’
, space is cluttered with a vast assortment of objects. Each new satellite launch increases the possibility of yet more space shrapnel and the risk of a future collision. Satellites in low earth orbit (LEO), where many of SSTL’s spacecraft are orbiting, have an increased likelihood of collision, as this is the region of space where the density of man-made space objects is greatest. Orbital collisions have become a particularly worrying thought in the light of plans for Space Tourism
, where a collision with space debris would be a potential risk to human life.
In 1978 NASA scientist Donald J. Kessler, predicted a scenario in which the density of objects in LEO would become high enough that collisions between objects could cause a kind of chain reaction, with each collision generating further space debris and increasing the likelihood of further collisions. In the air domain, it is sometimes appropriate to designate regions as “no-fly zones”. Without appropriate STC measures it may become necessary to designate certain altitudes as “no-orbit zones” to avoid the Kessler scenario.
So, is the time right for Space Traffic Control?
Stuart Eves, Lead Mission Concepts Engineer at SSTL (and now on secondment at Astrium), recently presented the concept of STC at a lecture for the Royal Aeronautical Society, and has suggested that the key aims of an effective system should be to:
· Track smaller debris objects than is currently achieved.
· Reduce the tracking errors around space objects to improve collision avoidance operations.
· Increase the frequency of space object tracking.
· Improve space weather monitoring.
Achieving these aims requires new, global collaborations, data sharing, and a robust regulatory system, much like the one operated to ensure the safety of air traffic on Earth. Some of the technical considerations for such a system are discussed more fully below.
Solar Activity and Space Weather
The Sun’s 11-year activity cycle has a direct effect on the density of the Earth’s atmosphere, and hence the drag experienced by satellites in low Earth orbit. The more solar activity there is, the more extended the upper atmosphere becomes. An improved understanding of solar activity would allow us to more accurately predict the time-variant drag that our atmosphere is creating, and hence make more astute future predictions of satellite and debris orbits. This would also improve our models of when space debris will be pulled down to burn up in the earth’s atmosphere.
Forecasting and modelling of space weather is also of vital importance to the space-faring community, especially the frequency and severity of extreme events that may impact on the operational functionality of spacecraft, and more importantly, the health of any astronauts inside.
More accurately tracking the location of space debris is another way to reduce the risk of collisions and reducing the size of the object’s “error ellipsoid” is required to achieve this. An error ellipsoid is a three dimensional representation of the uncertainty in the position of an object in space. A range of factors can affect the shape of these ellipsoids, but in LEO it is dominated by the drag that the earth’s atmosphere is creating on that object.
An error ellipsoid can be made smaller through the sharing of orbital data between satellite operators. As with Air Traffic Control, where aircraft are required to report their positions when they are in uncontrolled airspace, satellite operators could report their current orbital positions and planned manoeuvres to each other periodically. Work like this is currently voluntarily carried out by a small group of satellite operators that make up the Space Data Association [http://www.space-data.org/sda/], but in future it could be made a mandatory regulation.
At present there is no global requirement to provide up-to-date information or tracking of any of the various objects entering space, including the pieces of the launch vehicles themselves that do not return to Earth. A new regulation could see the mandatory provision of orbital information on launch vehicles and payloads – the number of separate objects that will be injected into orbit, their positioning, size and weight. In time there could also be regulations governing the design of the launchers, limiting the number of debris objects generated by each launch.
Currently (and perhaps surprisingly) there is no requirement for spacecraft to carry any form of tracking device, or to share orbit and positioning data. Under a new space traffic control system, satellites could be required to periodically broadcast their positions to a network of receivers, based either on Earth or to a secondary monitoring network based in orbit, relying on data relay satellites to collect and transmit multiple space object position and velocity data for analysis. Many satellites in orbit today do carry beacons or GPS receiver systems. However these have the drawback of increasing the mass and cost of the mission for the satellite operator and they also draw some of the satellite’s power in order to operate, so some manufacturers are reluctant to include them in their designs. Also, once the satellite’s life cycle is over a powered tracking system is longer operable.
One tracking method that negotiates this problem is laser retro-reflectors. These are special mirrors, angled so that any light shone onto them is reflected back to the source. Laser retro-reflectors do not require any power, so even once the satellite has reached the end of its operational lifetime this system could still be used to track it very accurately.
Another approach to improve the knowledge of the orbital population would be to perform more tracking from LEO via space situational awareness spacecraft such as Sapphire
. These missions offer near-real-time continuous reporting, without any interference from the various factors that limit tracking from Earth, such as the day-night cycle and adverse weather.
Telemetry data and station-keeping manoeuvres
Another possible aid to collision avoidance would be the reporting of satellite telemetry data to a central regulatory body. Telemetry data documents the satellite’s state of health, orbit and attitude through the use of on-board sensors and thus a regulatory body could provide early warnings of when satellites are experiencing problems which might impact on other nearby satellites, and so give operators more time to take precautionary measures, such as performing avoidance manoeuvres. It would also be an obvious benefit to share information on planned manoeuvres, de-orbit burns, and deployment of de-orbit sails with the global space community.
The rise of the cubesats – a new threat?
Although many of these suggested regulations would be difficult for very small satellites, such as cubesats, to adhere to, it could be argued that a less restrictive set of regulations could be applied to these microsats if they were placed into a very low orbit, at less than 500km above earth, thereby ensuring that they would re-enter the Earth’s atmosphere and burn up within a year or two. And if it is judged that their sheer numbers, increasing year-on-year, is the bigger problem, then a cap could be agreed, using some kind of global licensing.
There has been much talk lately about putting mandatory requirements on satellite owners/operators to provide space debris removal systems on future spacecraft and for space agencies to fund active debris removal missions. This would be a useful part of any STC system, but the “value” of removing any given piece of debris will depend on parameters such as:
· Expected natural lifetime
· Current orbit
· Number of other objects in a possible collision path/zone
In the final analysis, international space traffic control standards to regulate satellite operators are undoubtedly in the best interests of everyone who relies on satellites in their daily lives – and these days that is just about everyone on the planet. After all, with space traffic control, the space industry stays in business!