An evasive manoeuvre to avoid a crash!
In my car, even at 100 kilometres per hour, I perceive that I’m travelling along quite quickly. In the nice saloon in our car pool, you can reach up to 160 kilometres per hour without it feeling that much faster. Of course, this is because you cannot perceive speed alone. You can notice accelerations, but when you are travelling at a constant speed, you can only tell how fast you are moving relative to other objects – or perhaps when the car begins to rattle. It is the same on the ISS. The speed of the Space Station is much greater – it needs to travel at 28,000 kilometres per hour so it doesn’t ‘fall down’.##markend##
However, the astronauts barely notice this enormous velocity – unless they look out of the window and see entire continents drifting by in just minutes and as long as nothing travelling at a different velocity crosses the path of the Station… Unfortunately, this happens from time to time; either natural micro-meteors or debris from previous space missions cross the trajectory of the ISS every now and then. To prevent a collision, the space agencies monitor the near-Earth region of space and have logged all the larger items of debris along with their orbit data. Hence, they can predict with relative certainty when there is a risk of a near-collision with the ISS. If one of the particles happens to enter the no-go area around the ISS with a probability of more than 0.01 percent in the opinion of the flight controllers, the ISS performs an evasive manoeuvre. This generally involves increasing its orbit altitude to 'kill two birds with one stone'.
This occurred again yesterday. A warning was given many hours in advance that part of a Russian rocket stage that had carried satellites into space almost two years ago was on a collision course with the ISS. However, the path of the fragment was hard to predict. The extremely tenuous remnants of Earth’s atmosphere were affecting its trajectory increasingly, leading to serious uncertainties about its path in the radar measurements carried out for each orbit of the Earth – first a repeated trend towards a better outlook, then a red alert for a relatively high probability of collision.
As a result of this unpredictable behaviour, it was decided that a Debris Avoidance Manoeuvre (DAM) was required for the ISS. This was a somewhat unusual manoeuvre, as the ISS had been placed in a non-standard orientation for the imminent arrival of a Progress spacecraft. It was orbiting ‘upside down’, so the standard plans in place for such eventualities could not be used. The manoeuvre needed to be precisely tailored to the situation; this was not a problem, but it was a lot of work, especially for our United States and Russian colleagues. Also, we could not use the thrusters on a supply spacecraft, as is our usual practice, because none were docked with the Station – one was still waiting to launch, the other was already en route to burn up in Earth’s atmosphere. Therefore, the Russian Service Module (SM) had to provide the necessary thrust. Normally we would prefer accelerating the Space Station to change its orbit. This time, we had to slow it down, lowering the path of the ISS slightly – by about 800 metres.
German ESA astronaut Alexander Gerst was kind enough to share with us what such a braking manoeuvre feels like on the ISS. Shortly before the burn, he asked us to switch on the cameras in Columbus – and what we saw then looked as if a small gravitational field had been switched on at the moment of ignition. Suddenly every free-floating object – including Alex – moved in one direction in the field-of-view of the camera, and this unexpected ‘force’ continued for the 30-second duration of the engine burn. Thanks to Alex, this also confirmed what was explained at the beginning of this blog – there is weightlessness despite the incredible speed of 28,000 kilometres per hour, but even a slight acceleration puts an end to this. Q.E.D.