Space debris manifestly represents a problem; one that urgently needs to be addressed if humankind is to continue to benefit from the capabilities that satellite systems provide.
To date, the discussion of space debris has inevitably focussed on the risks that it poses to the safe operation of both manned and unmanned space missions, (and occasionally to the hazards that it may pose to people and infrastructure on the Earth below).
But is there a different perspective? Given the starting condition of a large, (but still relatively poorly characterised), debris population, can observations of the existing debris population help us to resolve the current uncertainties about exactly how much debris there is in Earth orbit, and where it is located.
And if we can improve our characterisation of the debris population, does this improve the prospects for conducting orbital clean-up operations in the future. And possibly for recycling debris objects as propellant for satellites?
In these respects, although debris still constitutes a challenge, it can also be viewed as part of the solution too.
As is widely acknowledged, the population of man-made objects in Earth orbit is now significant. The majority of the radar-reflective objects down to about 10 cm in size are actively tracked, and constitute a catalogue of over 20,000 objects. So, we know about most of the big stuff, but the population of debris objects between 1 cm and 10 cm is still very uncertain, since it is too small to be tracked reliably from the Earth’s surface. Our models of this population make assumptions about the sizes of fragments generated in hypervelocity collisions and during individual satellite fragmentation events, but they do not agree. NASA’s estimate of the population in this size range is 500,000, whereas ESA’s figure is 900,000, almost a factor of two greater.
And the sub 1cm population is largely a mystery. A limited number of “in-situ” measurements have been possible by analysing hardware that was exposed to the space environment and was then returned to the Earth, (such as solar panels from the Hubble Space Telescope, and also the LDEF mission). These objects were in relatively low orbits, and even examination of their surfaces using high-powered microscopes often finds it hard to differentiate between the damage caused by very small man-made debris and the impacts caused by natural micro-meteorites, (which are even smaller, but travel at even higher velocities).
Fragmentation models of this population suggest that there are millions of man-made debris particles less than 1cm in size in Earth orbit, but there is also dynamical modelling which suggests that the lifetime of these particles is very short, (with durations of “weeks” to “months”), due to the influence of solar radiation pressure on their orbits. These two apparently contradictory conclusions can potentially be reconciled if the fragment generation rate is very high. Small objects may re-enter the atmosphere routinely, but if collisions between debris objects also happen frequently, the sub-cm population may be regularly replenished.
In order to address this question, ESA is planning an in-situ measurement mission which will probably consist of a “sail-like” deployed surface which will sweep out a statistically significant volume of space over the course of the satellite’s lifetime. Provided that the man-made and natural particles which hit the sail can be identified, we may get a reasonably reliable measurement of the debris population at the orbital altitude of the mission. (This assumes that the mission occupies a circular orbit – it might instead use an elliptical orbit in order to sample more orbital altitudes, but then there is a question of whether it would spend a statistically significant time at any given altitude, and whether the results could then reliably be extrapolated to estimate the population as a whole). Arguably, what the space community really needs is not just one mission, but a large number of significantly-sized “witness plates” at a variety of orbital altitudes in order to measure the population reliably.
This is the first way in which the debris population can be used against itself, because we already have a whole series of suitable witness plates in orbit, and some of them have been in orbit conducting this experiment for many years. We more usually refer to these “in-orbit witness plates” as “rocket bodies”.
Commercial precision SSA assets, (e.g. laser rangefinders and radar fences), are being developed that could reliably monitor the orbits and the rotation rates of these rocket bodies. If the debris generation rate is as high as has been suggested, then there is the potential to make observations of these objects and to deduce when they have been hit, (by noting when there are simultaneous changes to their rotation rates and their orbits).
In addition to these “baseline” measurements, it is conceivable that advanced sensors might additionally be able to measure temporary vibrations in the rocket body structure, (using micro-doppler techniques), excess heat from the energy dissipated in the collision, and possibly fragments generated in the collision, although most would be too small to see from the ground.
Previous studies (by Darren McKnight) have demonstrated the large rocket bodies represent the most significant threat of a major collision, so arguably there should be a regular collection campaign underway for these objects anyway. The difference would be that, rather than simply confirming an object’s orbit and rotation with observation lasting a few seconds, (which is what happens currently), sensors would be dedicated to track them across the sky. By increasing the “time on target”, the chances of detecting an impact event would be significantly enhanced.
Rocket bodies have relatively simple shapes, and we know their material composition. We should, therefore, be able to estimate their expected radar signatures, and then compare these estimates with inverse synthetic aperture radar (ISAR) images of the hardware in orbit. Small impacts are unlikely to be detectable, due to the limited resolution of ISAR images, but larger damage, (conceivably the source of much larger numbers of tiny fragments), could perhaps be detected.
It is to be hoped though, that some of the debris removal missions which have been proposed in recent years will soon be reaching orbit, and, (given the acknowledged threat which the rocket bodies pose), it is assumed that these will be prioritised for removal. Hence, it should, in the relatively near future, be possible to get “up close and personal” with a number of these valuable records of on-orbit debris activity.
Debris removal satellites are expected to have imaging sensors on board to allow them to conduct their missions effectively, and it is to be hoped that, prior to “capturing” their target objects, they might conduct a high-resolution imaging survey of their surface first in order to count the holes. The number of holes, and their relative sizes, should provide clues to the size distribution of the small fragments in orbit.
In the subsequent imagery analysis, allowance will obviously need to be made for the relatively similar-looking holes caused by micrometeorites. The relative flux of micrometeorites is variable over time, (meteor showers fluctuate in intensity), and although the majority of micro-meteorites are very small, (millimetres), they travel considerably faster than man-made objects due to their origin in highly elliptical, comet-like orbits.
Having captured their target objects, most debris removal companies currently plan to de-orbit the debris objects, causing them to burn up in the Earth’s atmosphere. Aside from recent concerns about the impact of aluminium from satellites on the upper atmosphere, the debris removal companies need to be persuaded not to do this; as there is another way in which the debris can be used against itself.
This solution does, admittedly, rely on some futuristic thinking, but a company in Australia (Neumann Space) is trying to develop a thruster system based on fuel rods made from the sorts of metallic materials from which satellites are constructed, (i.e. aluminium). The basic principle behind this technology is to ionise the metal using a discharge from a super-capacitor, and then to accelerate the ions out of the spacecraft using a strong electromagnetic field to create thrust.
It follows, therefore, that there is a large potential supply of “fuel” for thrusters waiting for them in low Earthorbit. We usually refer to this “fuel” as “space debris” at present, and it would be a shame to waste it by simply deorbiting it.
Now clearly, we can’t wait indefinitely for the metal-powered satellites to arrive, so what is needed in the interim is a space junkyard, where defunct satellites can be “deposited” and managed to ensure that they don’t collide with something. Fortunately, this concept exists already, in the form of the “Necropolis”, which was devised by Mark Hempsell and Roger Longstaff of the British interplanetary Society.
The operational principle behind this concept is that space tugs would deliver defunct satellites to the Necropolis and attach them rigidly to its truss structure. The Necropolis, a fully operational satellite in its own right, can then ensure that they do not collide with any other passing debris.
Studies of debris removal missions have shown that one of the limiting factors on their operation is the amount of fuel needed to transit from one debris object to another, since the target objects are generally in different orbital planes around the Earth. By adding some “robotic reprocessing” to the Necropolis concept, these space junkyards would in theory be able not only to take delivery of defunct satellite hardware, but also provide space tugs with fuel rods in return; providing the velocity change capability that the tugs need to capture their next target object.
Space hardware does, of course, degrade over time, but using solar powered ion thrusters and fuel rods derived from the existing debris population it is possible to imagine a population of long-lived, self-sustaining space tugs exploiting on-orbit debris resources to first characterise. and clean up, the near-Earth space environment.
Dr Stuart Eves
SJE Space Ltd.