The following prediction for the reentry of space object USA 193 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 computed from observations of the Seesat-L network of observers.
USA 193 has a large ballistic coefficient (effective mass to cross sectional area) of around 245 kg/m2, compared with a more typical value of 100. This may be due to non-deployment of the solar panels on this satellite, a fact that appears to be confirmed by amateur imagery. This large coefficient means that the satellite is not decaying at as fast a rate as is typical for the average satellite at the same altitude.
The error in reentry models is typically 10% of the remaining life of the object. In this case 10% is about +/- 3 days. The prediction assumes a constant minimum level for the solar ten centimetre radio flux and the absence of any significant geomagnetic storms in the remaining time before reentry. The probability of these conditions being fulfilled is quite high. The other factor that can change the rate of decay is the orientation of the satellite. If the average orientation of the object changes significantly, the ballistic coefficient may change and this will lead to a different drag force.
Increases in either the solar F10.7 value or the geomagnetic activity (measured by the Ap index), will result in increases of the upper atmospheric density at low satellite heights. The resultant increased drag will increase the orbital decay of the satellite and decrease its remaining life in orbit.
As at 12 February new computations give reentry at 16 March +/- 2 days:
A discontinuity in the effective drag was noted in late January leading to an earlier reentry date than first predicted. The effective ballistic coefficient has been decreased to 215 kg/m3.
As at 19 February new computations give reentry at 15 March +/- 1 day:
The effective satellite ballistic coefficient has been slightly reduced to a value of 210 kg/m2 to better fit the data points. Because of the quality of the data fit to the curve, the estimated accuracy has been reduced to within one day of the estimated date. This of course assumes that the upper atmospheric environment remains quiescent, and that the satellite does not change its average or effective orientation due to increasing drag.
A satellite analyst, Ted Molczan, has recently drawn attention to the fact that the US FAA (Federal Aviation Administration) has issued a NOTAM (NOtice To AirMen) that defines a temporary restricted area west of the Hawaiian Islands. This restricted area extends in latitude from about 12.5 to 31.5 degrees north and in longitude from about 161.5 to 172.5 degrees west, and it is active from 02:30 to 05:00 UT on the 21st of February. It also happens that USA 193 will pass over this area at around 03:30 UT, and it is thus speculated that the US may conduct an antisatellite test against this satellite (as announced by US national security officials) during this period.
The above plot shows a partial orbit of USA 193 from 03:00 to 04:00 UT on Feb 21. The satellite is moving from east to west, and the red box shows the possible area around which it is speculated that the satellite may be destroyed by an ASAT missile.
The debris from such an event at the speculated location would be thrown into a wide range of orbits. However, because of the low altitude of the target satellite, it is expected that all debris would reenter the Earth's atmosphere within a very few weeks and pose no long term hazard to other satellites. Some debris might be expected to reenter within a short time, around the track that the satellite would have continued to follow. However, it is not possible to predict the exact distribution of the debris, let alone when and where specific pieces might reenter.
We can say that a successful event where both satellite and missile are destroyed by hypervelocity impact will cause a range of fragment sizes that are much more likely to burn up in the Earth's atmosphere on reentry (and thus not pose a ground hazard) than the original satellite.
On February 21, USA 193 was destroyed by an anti-satellite missile launched from a US Navy ship in the middle of the Pacific Ocean.
A US Department of Defense News Release issued on 21 February (UT), reads (in essence):
A network of land, air, sea and spaced based sensors confirms that the US military intercepted a non-functioning National Reconnaissance Satellite which was in its final orbits before entering the Earth's atmosphere. At approximately 03:26 UT on Feb 21, a US Navy AEGIS warship, the USS Lake Erie (CG-70), fired a single modified tactical Standard Missile-3 (SM-3) hitting the satellite approximately 247 kilometers over the Pacific Ocean as it traveled in space at more than 27,000 kph. USS Decatur (DDG-73) and USS Russell (DDG-59) were also part of the task force. The objective was to rupture the fuel tank to dissipate the approximately 453 kg of hydrazine, a hazardous fuel which could pose a danger to people on Earth, before it entered into Earth's atmosphere. Confirmation that the fuel tank has been ruptured should be available within 24 hours. Due to the relatively low altitude of the satellite at the time of the engagement, debris will begin to re-enter the Earth's atmosphere immediately. Nearly all of the debris will burn up of reentry within 24-48 hours and the remaining debris should re-enter within 40 days.
The US Navy released the image at left showing the launch of the SM-3
missile that destroyed space object USA-193 at 03:29 UT on Feb 21, 2008.
The missile is part of the US Aegis Ballistic Missile Defense System,
and the warhead was a Lightweight Exo-Atmospheric Projectile (LEAP)
kinetic warhead. This type of warhead carries no explosive, as it
has sufficient closing velocity to detroy its target by kinetic energy
alone. For more details see Satellite Breakup
Analysis.
Radar analysis of the debris seemed to indicate that there were no fragments "larger than a football".
A group of observers at the Prince George Astronomical Observatory in British Columbia observed some of the fragments of USA-193 at 03:43 UT as they burned up on entering the atmosphere. The PGAO is located at about 53.75o N, and 122.85o W, and 30 people were present observing the lunar eclipse occurring at the time. Brian Battersby of the Prince George Centre of the Royal Astronomical Society of Canada reported that:
"Many debris trails were witnessed moving from south-west to north-east at high altitude. One was especially bright and long lasting. I can recall about 6 bright trails and 15 fainter ones. The debris trails seemed to come in waves with the first wave being brighter than the debris that followed behind it. The trails seemed to be in a fan shape with the trails being wider apart in the north- east than they were in the south-west."Additional fragments are expected to enter in the next few days. Some may reenter and be observable in the sky over Australia. It is very unlikely that any piece of the satellite will reach the surface of the Earth.
Reentering space debris can generally be distinguished from meteors by the time (duration) over which they are observable. Even extremely bright meteors or fireballs rarely last for more than five seconds, and most are well under one second. Orbital reentries, on the other hand, are usually visible for well over ten seconds.
Not all reentering debris seen around this time however, will necessarily be from USA 193. A Molniya rocket was also due for reentry on 21 February.
Australian Space Academy
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