Space is far from safe – danger appears in every orbit, as one of the Swarm satellites discovered a few weeks ago. The three spacecraft are part of a single mission from the European Space Agency (ESA), studying the Earth’s magnetic field. This very interest is what put them in danger.
Our planet’s magnetosphere responds to the effects of the Sun. Since the Sun has entered the very active phase of its 11-year-long cycle, that means more active space weather and more intense extreme ultraviolet light. The high radiation heats up the atmosphere, leading to increased density of the upper atmosphere, where satellites travel. This creates drag and can slow satellites enough to make them fall back down to Earth.
So, Swarm members Alpha and Charlie had to be moved to higher orbits (Bravo was fine, already at a higher altitude). Both satellites had to perform 25 maneuvers in the course of 10 weeks to get to safety.
Alpha was on its merry way back on June 30, when an alert came in: Eight hours later Alpha was possibly going to hit something.
It was unclear what the space junk was, just that it was human-made and that the likelihood of the impact was high enough that Alpha needed to get out of the way. The team got together and, four hours later, they were ready to move the satellite.
However, that was not all. The orbit-raising maneuver was also time sensitive, so the team had to schedule a new attempt as soon afterward as possible. Luckily, they were able to do that too, and Alpha was back on track just a few hours after the collision was going to take place.
Alpha and Charlie are kept relatively close to each other to deliver the correct observations, like the one about the magnetic waves on the surface of the Earth's core. So if they were no longer on similar orbits, the science would have to be stopped. Luckily, the team was able to shift together with only minor delays.
There is a lot of space debris around Earth, and ESA usually has to move each of its satellites twice per year. The alerts tend to be even more common, but fortunately, not every one results in the need for a collision avoidance maneuver. Each of these maneuvers needs to make sure they don’t put the satellite in an even more dangerous orbit, so they take a lot of planning.