The Upper Atmospher Research Satellite or UARS taught us much about the chemistry of the atmosphere and ozone layer during its 14 years of active life. Credit: NASA
You may have already gotten wind that the NASA’s Upper Atmosphere Research Satellite (UARS) will soon be coming back to Earth … in pieces. The 13,000 lb. satellite was launched from the space shuttle Discovery in 1991 to study the chemical makeup of the atmosphere with particular emphasis on the ozone layer. It also measured winds and temperatures in the stratosphere and monitored ultraviolet light emitted by the sun. UARS played a major role in confirming that chlorofluorocarbons or “CFCs” like those used in Freon, aerosol propellants and solvents lead to ozone depletion and the formation of the Ozone Hole over Antarctica.
After 14 years of good science and its fuel used up, NASA switched off the satellite in December 2005 after a final nudge toward Earth to begin the slow process of “decay” or return through our atmosphere. Since then its orbit has been slowly shrinking year by year. UARS originally orbited 375 miles high, but as of September 16 it swings ’round the planet in an ellipse measuring just 140 x 155 miles.
Rarified as it is, the upper atmosphere increases the drag on the satellite, reducing its speed and bringing it closer to Earth. A big increase in solar activity since early last week also plays a role. Ultraviolet radiation from flares and other solar activity heats and expands the outer atmosphere, increasing drag and hastening the decay of UARS’ orbit.
The Remote Manipulator System holds onto the Upper Atmosphere Research Satellite before being released by the space shuttle Discovery in September 1991. Credit: NASA/ Marshall Space Flight Center
At the moment, NASA scientists predict a fiery re-entry this coming Friday the 23rd, give or take a day. Because the satellite has no fuel left, the point of re-entry can’t be controlled on the way down, but since 2/3 of the Earth is ocean, it will more than likely land there. UARS is a large, heavy satellite weighing 6.3 tons and measuring 35 feet long by 15 feet in diameter. Most of it will vaporize when it plunges in fireball-like fashion through our atmosphere, however some 1,000 lbs. of satellite components are expected to land somewhere on Earth between the latitudes of 57 degrees north and 57 degrees south — the satellite’s overfly zone. That’s a very large area! And that’s why the chances of anyone getting hit by a piece are extremely remote.
There is a 1-in-3,200 chance a piece of debris could injure or kill a person, according to an assessment by NASA. “I hope [people] don’t get too concerned because this is something with a very low probability of anyone being hurt or anyone’s property being damaged,” said Nick Johnson, chief scientist for NASA’s orbital debris program at the Johnson Space Center in Houston. “Unless we build these things out of papermache, we can’t reduce the risk to zero,” he later added.
Illustration showing space debris and active satellites orbiting Earth. Geosynchronous satellites responsible for relaying communications around the world define the distinct outer ring. The dense inner circle are satellites in low Earth orbit. Credit: NASA
To further put your mind at ease, since the dawn of the Space Age some five decades ago, no human has been killed or even hurt by an artificial object falling from the heavens, though in 1997 a piece of debris from a U.S. Atlas II rocket brushed a woman’s shoulder in Oklahoma, the only known instance of a “grazing fall” on a human. Pieces of space junk are falling harmlessly all the time. Last year one satellite-related object per day burned up in our atmosphere according to Johnson.
Ever since UARS was decommissioned, it’s been a little more than another (albeit large) piece of space junk, hence the need to see it through to its ultimate demise. It’s too early yet to know where and exactly when it will land, so NASA will be posting regular updates at this site. I’ll also do the same. If you do see the fall and later find a piece, the agency asks that you not touch it, but contact a local law enforcement agency. I know the chances are small, but I bet many of us are hoping we’ll get a glimpse of the spectacle as UARS takes the final plunge.
10-second time exposure of UARS showing a small flare taken in June 2010 by Dutch satellite observer Marco Langbroek. Click to visit his satellite blog
UARS is an easy satellite to spot, sometimes appearing as bright as the brightest stars though typically it’s more like those in the Big Dipper. If you’d like to catch its last few passes in the morning sky, go to Heavens Above, log in and select the UARS link. For the Duluth, Minn. region, UARS will cruise low across the southwestern sky tomorrow (Monday) morning starting at 6:15 a.m. In tomorrow’s blog, I’ll update with all remaining passes as well as roll out new times to watch the International Space Station, which has once again returned to view at dawn.