Patience, lots of luck needed to spot falling satellite

Armchair satellite trackers hoping to catch a glimpse of NASA's doomed Upper Atmosphere Research Satellite when it plunges back to Earth on Friday will need patience, access to the Internet, a clear sky, and a large helping of luck, experts say.

Even with last-minute updates from NASA and U.S. Strategic Command pinpointing when and where UARS will begin its final plunge, the sheer size of the planet, with its vast stretches of ocean and remote terrain, means the odds of catching a glimpse of the spacecraft's fiery demise--or of being anywhere near any falling debris--will be remote.

But as re-entry footage of the old Russian Mir space station and the more recent flaming fall of a European cargo craft show, satellite "decays" offer the public and experienced amateurs alike a chance to witness a fairly spectacular show.

The UARS re-entry "is nothing to be particularly worried about, and if you're very lucky and in the right place at the right time, you may see quite a nice little fireworks show from it. But it's highly improbable that you'll get even that much out of it," said Ted Molczan, a well-known satellite watcher whose computer analysis and predictions have helped sophisticated hobbyists around the world track down spy satellites and other challenging targets.

"You have to maintain reasonable expectations. And in this case, the right expectation is, 'I'm not going to see this.' On the other hand, if you stop there it becomes a self-fulfilling prophecy. So you do want to see it, you know you won't, but (if you make the effort) you might!"

The 6.3-ton UARS, launched from the shuttle Discovery in 1991 and decommissioned in 2005, is expected to fall back into the dense lower atmosphere sometime Friday. The bus-size spacecraft will tumble, break apart, and mostly burn up on the way down. But a NASA analysis using software designed to evaluate satellite decays indicates 26 chunks of debris, the largest weighing 330 pounds or so, likely will survive atmospheric friction to hit the ground somewhere along a 500-mile track downrange of the entry point.

For comparison, some 42.5 tons of wreckage from the shuttle Columbia hit the ground in a footprint stretching from central Texas to Louisiana when the orbiter broke apart during re-entry in 2003. No injuries or significant property damage were reported.

USSTRATCOM tracks some 22,000 objects in low-Earth orbit, ranging in size from about four inches across to the 900,000-pound International Space Station, roughly the size of a football field. But USSTRATCOM's radar network will not be able to pinpoint exactly where UARS will begin its destructive plunge--and thus where its debris might fall and where it might be visible--until just a few orbits before re-entry.

"As of Sept. 21, 2011, the orbit of UARS was 120 mi by 130 mi (195 km by 210 km)," NASA said in a brief update Wednesday. "Re-entry is expected Sept. 23, United States time. The time reference does not mean that the satellite is expected to re-enter over the United States. It is simply a time reference. Although it is still too early to predict the time and location of re-entry, predictions of the time period are becoming more refined."

Amateur satellite trackers hoping to catch a glimpse of UARS can find predictions for when the satellite will pass above their local horizon at Heavens Above, an authoritative source of tracking information for hundreds of satellites.

Veteran sky watchers typically look for satellites within a few hours of sunrise and sunset, when the observer is in Earth's shadow and satellites above one's horizon are still in sunlight. Heavens Above provides rise and set times for an observer's location, along with a sky chart showing the spacecraft's path across the sky. Depending on their size, satellites can appear as very dim, rapidly moving "stars" or, in the case of the International Space Station, rival Jupiter and Venus in brightness.

Normally, only sun-lighted dawn and dusk passes are listed by Heavens Above. But for the UARS re-entry, the Web site is listing all possible passes, even those that occur when the satellite is in darkness or broad daylight. If no passes are listed, the satellite never climbs above the viewer's horizon.

Molczan advised would-be UARS observers to monitor NASA's latest re-entry predictions as they become more refined and then to compare those times with the list of passes provided by Heavens Above. If the two are close, the observer might have a chance to see UARS' final moments, although one would still need a fair amount of luck.

Observers also can monitor Space Track (registration required) to obtain the raw data from USSTRATCOM. Space Track routinely provides decay predictions, including time, longitude, and latitude, for satellites expected to re-enter in the near future. The orbit-defining two-line elements, or "elsets," provided by Space Track can be plugged into satellite tracking software to generate up-to-date maps with a satellite's location shown in real time.

"The orbit's going to be changing pretty rapidly," Molczan said of the UARS trajectory. "One of the things that is maybe not always intuitive, the visible part of the decay, the dramatic part of it, is below 100 kilometers. That's really low, and that means you've got to be very close to the ground track to have it some reasonable elevation above your horizon.

"Most of us live in urban areas and when you're standing outside, a lot of your horizon is cut off by buildings, trees, that stuff. You've got to take into account that things below 15 or 20 degrees elevation, you're not going to see unless you take special effort to get into a place that's free of obstructions."

And once the UARS re-entry begins, the actual path of the satellite likely will deviate from the predicted trajectory.

"Of course, on the other hand, as long as it's anywhere above the horizon the decay when it happens is so prominent, how can you miss it?" Molczan said.

But even with decades of experience, Molczan has only witnessed one satellite decay--the re-entry of a Russian communications satellite in 2004.

"I live in a high-rise building and I went up to the roof, so I had a pretty good view," he recalled. "I knew exactly where to expect it. It came into view, it passed very close to Jupiter, which happened to be sitting there, it was about as bright as Jupiter already. And the point is, it was in the Earth's shadow so the only way I would be seeing it is if it were burning up.

"And it had a nice, long plasma trail behind it just like you see with re-entering shuttles. From 10 degrees (elevation) in the southwest, culminating at around 35 degrees and then reaching 10 degrees in the southeast, that took about 89 seconds. It was motoring along. This was the fastest I've ever seen a satellite move."

Below is a video animation of the UARS re-entry from Analytical Graphics.