|The NASA Upper Atmosphere Research Satellite (UARS) was launched on Space Shuttle mission STS-48 and deployed on 15 Sep 1991. Its ten scientific instruments were designed to explore the photochemistry of the atmosphere. It is approximately 11 m long with a diameter of 4.5 m (plus a large solar panel) and has a mass of 5900 kg. Its orbital inclination is 57 degrees.|
Image: NASA artist's impression
The following prediction for the reentry of the UARS spacecraft was made using the Australian Space Academy orbital reentry model. The curve shows the predicted variation in altitude with time. The red dots are calculated mean altitudes of the satellite using orbital elements acquired from the 'celestrak.com' web site.
The plot and prediction in the graph below was made late 20 Sep 2011 UT
UARS currently has a moderate ballistic coefficient (effective mass to cross sectional area) of around 120 kg/m2. This may increase as the satellite encounters greater air density, with a resultant increase in lifetime before reentry.
The error in reentry models is typically 10% of the remaining life of the object. The reentry prediction also assumes no increase in solar flux or geomagnetic activity in the next few days. Solar conditions are such that the solar ten centimetre flux is in fact more likely to slowly decrease over this time period. This could cause another small increase in satellite lifetime.
The NASA Orbital Debris Program Office has conducted a reentry survival analysis using a program called ORSAT (Object Reentry Survival Analysis Tool). Their analysis predicts that around 26 individual objects will survive reentry and deposit a total mass of 532 kg on the Earth's surface (most likely in the ocean). The largest object will have a mass of about 160 kg and the smallest about 0.5 kg. Typical impact velocities are expected to range from 15 to 150 metre/sec. The debris field will extend over a narrow ellipse about 800 km in length. Objects with the smallest mass to area ratio will drop first, with the heavier and more compact objects the last to fall.
The plot and prediction below were made late Sep 22 UT
Continued high solar ten centimetre flux (F10.7) has caused this latest prediction to be brought forward.
24 September 2011
The NASA UARS web mission pages report that UARS reentered the lower atmosphere between 0329 and 0509 UT on September 24. Tracking of the spacecraft in its final hours were performed by the US military Joint Operations Space Center at Vandenberg Air Force Base in California, USA.
This reentry was substantially later than the last prediction made above. The reason was a drastic decrease in the effective area (Ae) that the satellite presented to its direction of motion. Up until early Sep 23, the satellite was rotating as it orbited, and the effective area in this case is the average cross-sectional area to the ram direction. The relatively high Ae that UARS displayed was in large part due to its large solar panel. As the satellite moved to lower altitudes, the atmospheric drag on the larger area structures acted to slow the rotation and orient the satellite so that it presented the lowest Ae to its motion. In fact the effective area of UARS decreased by a factor of 3 to 4 in its last orbits, and this was responsible for prolonging its life.
The map below shows the predicted ground tracks of UARS just before and during the stated reentry period. Debris falls may have occurred anywhere around the red trace (satellite was moving from left to right on the map), but probably not near the beginning nor ending of the trace. Reports of reentry sightings from Washington state (USA) and south-western Canada have been made.
Australian Space Academy