By Duncan Steel 14/10/2020


Two objects in low-Earth orbit may collide with each other on Friday, in a hyper-velocity impact which would lead to millions of fragments being left on-orbit, each potentially-lethal to functioning satellites. Fingers crossed (not that I am superstitious) that it is a miss, rather than a hit. One local media article is available here.

Above I wrote ‘objects’ because neither is an operational satellite. One is Cosmos-2004, a Russian military navigation satellite launched in February 1989, and long past its use-by date. It circles our planet at altitudes between 977 and 1020 km, rather higher than most satellites in so-called low-Earth orbit (LEO); for example, the International Space Station (ISS) orbits at an altitude just above 400 km. If we deserted the ISS, within a few years/a decade or so the tenuous atmospheric drag at that height would cause it to re-enter and break apart in a fiery display for all below. However, up at 1,000 km there is nothing much to disturb orbiting objects, and so they will persist there (unless something or somebody intervenes) for a long, long time: certainly millennia. This is part of the problem: we have polluted near-Earth space with defunct satellites and currently no-one doing much about clearing up the altitudes of concern. It’s the Tragedy of the Commons written across our skies.

As is often the case in science, we have multiple ways of referring to the same basic thing. Cosmos-2004 is labelled as 19826 in the so-called NORAD or Satellite Catalog Numbers (SCN), but the international standard descriptor is 1989-017A: the payload (hence A) in the 17th launch of 1989. Its orbital plane is tilted at 83.0 degrees to the equator.

The other orbiting object is the upper-stage rocket body from a Chinese launch in 2009 that placed two satellites in orbit between altitudes of 977 and 1211 km. These were Yaogan-8 (SCN 36121 or 2009-072A) and Xiwang-1 (SCN 36122 or 2009-072B). The former is an Earth-observation satellite still in operation; the latter is a smaller amateur radio communications satellite. The possible impactor – the rocket body – is labelled as SCN 36123 or 2009-072C, and is part of a Chang Zheng-4C launcher. Its orbital plane is tilted at 100.4 degrees to the equator.

There are various factors that make this possible collision of great concern. The masses of the two bodies (825 and 1040 kg respectively) means that they have the potential to spawn vast amounts of smaller debris as fragments and shards, each continuing in high-speed trajectories and, for the most-part, too small to be tracked from the ground. Of course, it is the large size of these two bodies that make a collision more likely.

I just mentioned ‘high-speed trajectories’: as they close on each other, the Russian satellite will be travelling at 7.3 km/sec, and the Chinese rocket body at 7.5 km/sec. Because they are speeding in almost-opposite directions, their relative velocity will be 14.66 km/sec (almost 53,000 kph, or around 500 times the top speed of a jetliner). At such a rate, a collision is highly-energetic (insert exclamation marks here), and one might anticipate the two objects been smashed into millions or billions of pieces, all potentially-lethal to a functioning satellite – or an astronaut.

The time of the possible collision we can compute to the nearest millisecond, as shown in the following graphic. As I write, on the evening of the 14th (NZDT), the orbital elements I have downloaded from the USSPACECOM database are about 20 hours old; over the next 36 hours, those will be updated with new observations being available, but at present the nominal miss-distance is 35 metres. That’s not much when the two car-sized objects are closing on each other at over 14 km per second.

The geometry of the possible collision between Cosmos-2004 and a CZ-4C rocket body on Friday October 16th. The altitude above the Earth’s surface (actually, the eastern coast of the Antarctic Peninsula) is near 1003 km; some other reports give a different altitude, but the figure depends upon whether the altitude is referenced to Earth’s equatorial radius, mean sea level, or the reference ellipsoid (I have used WGS84 here) at the latitude in question. It is this distinction between different definitions of altitude that leads many people into thinking that the top of Mount Cook/Aoraki is the point in New Zealand furthest from Earth’s centre, whereas in fact it is the peak of Mount Ruapehu.

Figuring that a movie showing the two orbiting bodies closing on each other might be of interest, I have produced one. It is just 19 seconds long, and not a large file (less than 12 MB). Here is a link. Enjoy, but just remember that we must be hoping that these two lumps of orbiting junk will have a close encounter, but not a high-speed meeting in space.

Addenda, October 16th:

Here is a report on the Stuff website about this close encounter.

Here is a report from the BBC also about this near-collision.

Later on October 16th: no collision apparent, good news!