Rise or Fall
2015 Sep 3
Thursday, Day 246
Source of elements, decays, etc
German Space Center on ROSAT
Center for Orbital & Re-entry Debris Studies
ROSAT Decay and Re-entry
ROSAT (Röntgen Satellite), an x-ray observatory, was launched in 1990 and air drag caused the orbit to decay to the point of re-entry. At 2400 kg, and containing physically large refractory components, individual debris items up to 400 kg mass could have made it through the Earth's atmosphere and reached the surface.
ROSAT's orbital inclination was 53°.0 so surviving debris could have impacted anywhere between 53° north and 53° south latitude.
As of Oct 20, there was a news story circulating that DLR, the manufacturer, thought the 1600 kg main mirror might survive and impact in one piece!
DLR, the manufacturer, gave the time range as 23 October 01:45 - 02:15. Spacetrack said 01:50 ±7 minutes. They were in agreement.
The track on this map (produced using Orbitron software) illustrates the Spacetrack version, the DLR orbit arc is a little longer. All that can be said is that re-entry occurred somewhere on the line.
DLR must be relieved because the path is almost entirely over the ocean. Any fragments would have hit the Earth's surface about 15 minutes after entering the upper layers of the atmosphere and be scattered up to 1000 kilometres or more beyond the actual atmospheric entry point, possibly taking them into China. As yet, there are no reports of impacts. The event took place in daylight so there was no possibility of the plasma trail being observed.
The plots below are based on data from Spacetrack and follow ROSAT through the last two months of its life. Spacetrack issued what seemed to be its final element set thirty hours before re-entry. However, Oct 23 around 15:00 UTC a further set appeared in the database with an epoch Oct 22 at 22:54 UTC.
Orbital period decreases steadily but with increasing rapidity. The small fluctuations are what makes re-entry prediction so uncertain. They may be due to minor errors in orbit determination, variations in atmospheric density in reaction to the Solar Wind, or orientation changes by ROSAT changing the mass/cross section ratio.
This is the increasing rate of decay as ROSAT dips deeper into the atmosphere - the final point is off the scale at a massive 0.1. Units are revs/day/day. Re-entry occured at the point the line became vertical.
Perigee and Apogee
Apogee and perigee decrease in a similar way to the period and they converge as the orbit tends towards being truly circular. At about 90 kilometres the atmosphere takes over, slows the satellite down. It effectively drops out of orbit and starts to burn up through frictional heating, creating glowing trails of plasma as it disintegrates.
Around September 3, the orbit became near-enough circular. After that, apogee and perigee effectively changed places in the Twoline model. This is more to do with the standardised calculation of orbit height from the Twoline Orbital Elements sets than it is with reality.
Perigee is traced in blue and apogee in red.
Argument of Perigee
When nothing is happening, ROSAT's argument of perigee increases slowly. An orbit change can produce a sudden change in argument of perigee but it immediately settles down again to the slow drift. The sudden change around September 3 is NOT, however, the result of a manoeuvre. It is the point where apogee and perigee effectively changed places (see the note above) and the location flipped by 180° as a result.
Eccentricity is very sensitive to small changes in orbit parameters. Its tendency is to reduce because air drag causes apogee to decrease at a greater rate than that of perigee unless they are very close to each other in value. The obvious change September 3 is due to eccenticity hitting zero and then starting to rise because the local atmospheric density at orbital altitude varies between different locations around the Earth.
Charts on this page are produced using JpGraph.
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