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About a week ago, the 18th Space Control Squadron, US Air Force, relayed warning data to the European Space Agency.
The data indicated that there was a non-negligible collision risk between ESA's Aeolus satellite and Starlink44, an active SpaceX satellite, at 11:02 UTC on Monday, 2 September.
As days passed, the probability of collision continued to increase, and by Wednesday, August 28, ESA's Ops team decided to reach out to Starlink to discuss their options. Within a day, the Starlink team informed ESA that they had no plan to take action at that point. By Thursday evening, ESA's probability threshold for conducting an avoidance manoeuvre had been reached, and preparations were made to lift Aeolus 350 meter in orbit. By Sunday evening, chances of a collision had risen to 1 in 1000, and commands were sent to the Aeolus satellite, which triggered a total of 3 thruster burns on Monday morning, half an orbit before the potential collision. About half an hour after the collision prediction time, Aeolus contacted base, and normal measurement operations could continue.
What the SpaceX satellite was doing in ESA's Aeolus orbit is not clear.
ESA has taken the opportunity to point out that, given SpaceX plans to put up 20,000 of those things, handling monitoring and avoidance semi-manually, and by mail, is no longer practical.
(Score: 2) by quietus on Wednesday September 04 2019, @07:22PM (2 children)
What you are saying is that NASA can track 20,000 pieces of space debris the size of a softball and 500,000 pieces the size of a marble, can manoeuvre their satellites away from these, but has not really an idea where their satellites are at any moment to about, what, hundreds of meters?
Further, linky to the Air Force probability data, please.
(Score: 2) by The Shire on Wednesday September 04 2019, @09:37PM (1 child)
What I'm saying is that when NASA says they're tracking 20,000 pieces of debris they really mean they have a good ballpark idea of where it is, like within a few kilometers. This is why collisions are reported in terms of probability rather than being definitive. They only know the position of a satellite is somewhere within a given area - not exactly, nor do they have a high precision idea of its speed. Why would ESA move their satellite a 1/4 mile if they knew precisely where and how fast everything was.
(Score: 2) by quietus on Thursday September 05 2019, @10:46AM
Within a few kilometers, eh?
GPS and Galileo satellites transmit satellite orbit data as a set of 16 quasi-Keplerian parameters, known as the ephemeris. Two further parameters are used, both of which are considered constant: the Earth rotation rate, and the Earth's gravitational constant. Long story short, Keplerian motion/orbit is determined by solving Kepler's equation, a calculation which is solved iteratively, resulting in millimetric accuracy.
Satellites depart from pure Keplerian motion due to a combination of nonuniformity of the Earth's gravitational field, the gravitational fields of the Sun and Moon, solar radiation pressurre, and other effects. These are approximated by the remaining ephemeris parameters: the mean motion correction (resolution 10E-13 rad/s), rates of change of the inclination and longitude of the ascending node (both also with a resolution of 10E-13 rad/s), and six harmonic correction terms, 4 of which have a resolution of 10E-9 rad/s, and 2 of which have a resolution of 0.03125m.
Source: Principles of GNSS, inertial, and multisensor integrated navigation systems, second edition, Paul D. Groves, published by artech house (2013). Pages 330-339.
I notice you didn't provide the 'AF' data.