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SpaceX declines to shift Starlink satellite to avoid collision

When European Space Agency (ESA) engineers realized there was a greater than normal chance that a new SpaceX Starlink satellite could collide with ESA’s already orbiting Aeolus satellite, they asked SpaceX to shift its orbit, only to have SpaceX decline.

According to Holger Krag, head of the Space Debris Office at ESA, the risk of collision between the two satellites was 1 in 1,000 – ten times higher than the threshold that requires a collision avoidance maneuver. However, despite Aeolus occupying this region of space nine months before Starlink 44, SpaceX declined to move their satellite after the two were alerted to the impact risk by the U.S. military, who monitor space traffic. “Based on this we informed SpaceX, who replied and said that they do not plan to take action,” says Krag, who said SpaceX informed them via email – the first contact that had been made with SpaceX, despite repeated attempts by Krag and his team to get in touch since Starlink launched. “It was at least clear who had to react. So we decided to react because the collision was close to 1 in 1,000, which was ten times higher than our threshold.”

As to why SpaceX refused to move their satellite, that is not entirely clear (the company did not respond to a request for comment). Krag suspected it could be something to do with SpaceX’s electric propulsion system, which “maybe is not reacting so fast” as the chemical propulsion on board Aeolus.

The article is clearly spun to make SpaceX look bad, though based on the stated facts the company shot itself in the foot quite ably. If their propulsion system could not have done the job as well as the other satellite, they should have simply said so and worked with ESA to get the issue fixed, rather than simply saying they would do nothing.

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  • Mike Borgelt

    1 in 1000? I can see why SpaceX didn’t bother.

  • avg_joe

    I think the story is spun. Too many things don’t make sense or were left out. Spacex launched 60 satellites. 57 moved from the 320km initial orbit to 550km. This has been established. Three failed to respond and 2 were to be used to test de-orbiting. To say that Spacex “refused” to alter the course of one of the 2 test units seems ludicrous. They were destined for destruction regardless. The remaining 3 in the 320km orbit were non-functional. More than likely it was one of these that were the problem and therefore, Spacex couldn’t move it (having no control). That’s not the same as “refusing”.
    The article suggests that Spacex risked blowing up a very expensive satellite and raining debris over the 320km orbit… to be obstinate? There was certainly no financial incentive to take such a risk and the avoidance maneuver would have made a great test.

    … unless this was one of the dead satellites and Spacex couldn’t move it and the original article was basic B.S.

  • mpthompson

    > The article is clearly spun to make SpaceX look bad…

    My take as well. Best to get the other side of the story before forming a strong opinion.


    “The two satellites were predicted to come within about four kilometers of each other”

    Four km radius is a tremendously large cross-sectional area (50 million square meters). How does 50 million square meters translates into a 1:1,000 chance of collision? I have to presume that satellites come this close to each other all the time. So, from the get-go, something doesn’t smell right.

    SpaceX’s four satellites at this level probably don’t make up a large percentage of the satellites in this region). So, why is ESA complaining about what appears to be an extremely low probability? And we have to consider the possibility that SpaceX was looking at precedence. Are they willing to move any of their thousands of future satellites every time someone else asks them too when SpaceX’s own algorithms say that there is not a sufficient risk of collision? SpaceX needs to clarify the situation.

  • Andi

    That’s an interesting exercise. Assuming that both satellites are spheres with radius r (meters), the center of one would have to come within 2r of the center of the other to have a collision. The area of this danger region is therefore pi*(2r)^2 square meters.

    A four km radius gives a cross-sectional area of pi * 4000^2 square meters

    In order to have a 1:1000 chance of collision, that would mean that [pi*(2r)^2] / [pi * 4000^2] = 1/1000.

    Solving for r gives r = 4000 / [ 2 * sqrt(1000) ] = 63.2 m radius.

    That’s an awfully big satellite!

  • Edward

    DougSpace asked: “How does 50 million square meters translates into a 1:1,000 chance of collision?” Andi did a calculation.

    It isn’t just an “area” concern, because the timing is also important. If one satellite arrives before the other, then they will also miss each other. It requires a calculation of a volume. Also, Andi, you need to make the calculation for radii of both satellites, not just one, but assuming that both are the same size then I think the calculation gives r = 44.7 m, which is still large.

    The rules, however, use specific ovoid dimensions, because orbital perturbations are difficult to predict and calculate. Much of what they use for chance-of-collision calculation is rule of thumb rather than comparison of satellite size. Part of the reason is that satellites rarely are spherical or cubical but have protrusions such as antennas and solar panels.

    Considering that there are quite a few thousand satellites in low Earth orbit, both active and dead, satellite operators are becoming more and more concerned with the consequences of collisions and the Kessler effect (or syndrome), as exaggerated in the movie “Gravity.” With so many objects in orbit, there are plenty of opportunities for collision, and even more if you include the tiny debris that cannot be tracked.

    An example is the collision in 2009 between an active Iridium and a dead Kosmos satellite.

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