A Holistic Approach To Space Safety

A very positive aspect of the new ESA Space Safety programme is that it brings together the three central elements of space tracking: Space Weather Monitoring; the detection and tracking of Near- Earth Objects; and orbit determination for man-made objects in Earth orbit.

Historically there has been a tendency to treat these three aspects of space situation awareness separately, but there are clear linkages between them.

For example, space weather has the potential to affect the orbits and behaviour of Near-Earth objects. The level of irradiation from the Sun varies in intensity slightly over the solar cycle, and also changes in its “hardness” – there is significantly more emission at UV and X-ray wavelengths at the peak of the solar cycle. As a consequence, there will be changes to the strength of the YORP and Yarkovsky effects which affect the rotation rates and orbital eccentricities of asteroids respectively. These effects will be small, of course, but as a consequence of the chaotic nature of orbits in the solar system, even small changes in an object’s ephemeris can lead to large differences in its subsequent trajectory.

To some extent, variations in the solar flux will also affect the rate of release of volatiles from asteroids. Although all asteroids were once thought to be largely volatile-free, a class of objects has now been discovered which display comet-like behaviour, in the sense that they are observed to form tails of material when they approach the Sun.

A further possible influence of space weather, (and more specifically, the solar wind), is on their surface charge. There is much evidence, (from the dynamics of planetary ring systems), that objects in the solar system acquire can charge from the solar wind, and this process introduces the possibility of a weak Lorentz force interaction.

One area of active research is to understand whether some of these physical mechanisms also apply to satellites. There is much evidence that they accumulate charge from the solar wind, and arcing events have been observed when different materials on the surface of satellites have acquired significantly different potentials. Recent measurements of the orbits of grave-yarded GEO satellites also suggest that their eccentricities are evolving faster than expected, possibly as a result of the Yarkovsky effect; possibly as a result of a Lorentz force interaction with the Earth’s magnetic field, (since the drift caused by their super-synchronous graveyard orbits causes them to cross magnetic field lines); and possibly some combination of these factors.

The presence of a significant debris population in GEO also requires an explanation. It could be the case that the YORP effect is spinning up these abandoned satellites, leading to blanketing materials and other structures being “centrifuged” off the parent spacecraft.

Other explanations for this debris population are also possible. It could be, for example, that cyclic thermal variations on defunct satellites that are no longer performing active thermal control are causing their blanketing to lose structural integrity and break off. A further hypothesis is that impacts, (either from natural micrometeorites or from slag generated by satellite propulsion systems) are slowly degrading these objects. More detailed surveillance in GEO, from a dedicated inspection mission, may be the only way to decide between these competing hypotheses.

The most significant interaction between the SSA pillars is the one that exists between space weather and the collection of SSA on LEO objects. As is well known, the effect of significant solar activity is to deposit energy in the Earth’s upper atmosphere, and this heating effect increases the drag on satellites in LEO. Unfortunately, there is the potential for this to become a “perfect storm”, in the sense that, just at the point where increased tracking is needed to maintain custody of the objects in the catalogue, other effects of the storm are conspiring to make this difficult.

For instance, another well-established effect of space weather is the scintillation that it can generate in the ionosphere. Among the systems adversely affected by such scintillation are the radars that are used to track targets in LEO. The severity and duration of the effects will depend on the radar frequency, but there is likely to be some degradation in the accuracy of the data collected by all radar systems for a time. Another factor determining the level of disruption is the latitude of the radar sites – during a major storm, enhanced auroras and scintillation effects are expected to reach mid-latitudes, and many of the current tracking facilities are situated there. It is possible that latitudes as low as the tropics may be affected by a very large storm, so even the new US Space Fence radar may not be immune.

Some tracking of LEO spacecraft is performed by optical sensors, (e.g. geodetic missions equipped with laser retroreflectors), and here too there could be disruption if the whole sky is glowing with shifting red and green auroras.

Furthermore, some LEO satellites determine their own positions using GNSS. Even assuming that, a) the GNSS satellites are healthy after the storm, and b) that the LEO satellites have also escaped the effects of single event upsets caused by the (enhanced) South Atlantic Anomaly, they may still be unable to report their positions to the ground due to the on-going disruption to RF communications in general.

One of the greatest ironies is that this disruption could also affect the space weather monitoring satellites stationed at the Lagrange points. Their initial warnings would probably arrive before the storm, but their subsequent measurements of the on-going fluctuations in the plasma environment might be lost in the ether.

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