Credit: NASA

A large defunct NASA satellite is expected to fall to Earth late Friday afternoon (eastern time), September 23, or early Saturday morning. As it makes the plunge, it will catch fire and break apart, but not all of the 6.5-ton spacecraft will burn up. Debris is expected to reach the surface, roughly 26 large pieces, but the exact location and time of re-entry are still unknown. NASA officials say predictions will become more defined within 24 to 36 hours, however, they do know that the satellite will not be passing over North America.

Debris from the bus-sized Upper Atmosphere Research Satellite (UARS) should fall across an area roughly 500 miles long, and has a 1-in-3,200 chance of hitting a person, which is considerably remote, says agency officials. The satellite's most likely landing spot is the ocean, which covers almost 75 percent of Earth. NASA estimates any "surviving components" of UARS will land within a zone between 57 degrees north latitude and 57 degrees south latitude--ranging from northern Canada to southern South America.

UARS was deployed in 1991 on a mission to study Earth's atmosphere, particularly the ozone layer, and was decommissioned in 2005. It has been falling faster than anticipated due to increased solar activity, which can cause Earth's atmosphere to heat and expand, increasing drag on low-flying spacecraft, according to Space.com. Due to it's unpredictable nature--it is essentially tumbling out-of-control--scientists won't be able to pinpoint the satellite's point of impact until about two hours before re-entry.

The U.S. Strategic Command at Vandenberg Air Force Base in California and NASA are closely watching the plummeting UARS using sophisticated modeling software. I previously wrote about these systems, which are also used to anticipate collisions between spacecraft and space junk, for Technology Review,

To foresee the paths of space junk so that collisions can be avoided, NASA developed one of the world's most sophisticated predictive models. Called Legend (for "low-Earth to geosynchronous environment debris"), the three-dimensional model simulates the routes of all trackable space objects and even factors in new debris from future crashes. To take uncertainty and randomness into account, hundreds of scenarios are generated using the Monte Carlo method, a set of algorithms that can calculate risk factors in a complex environment. With Legend, NASA scientists use the average of multiple simulations to estimate the number, size, and type of objects that will collide—and approximately how often. Unlike models used by the U.S. Strategic Command Joint Space Operations Center, which detects and tracks large objects and screens active satellites daily for possible collisions within 72 hours, Legend includes smaller fragments and looks far into the future.

In place since 2004, the NASA model is constantly fed with data gathered from the results of ground tests and spacecraft that have broken up in orbit; from telescopes and radars viewing the sky; and from analysis of crater-marked spacecraft surfaces that have returned to Earth. That means new simulations must be run continually. Legend enables scientists to calculate the consequences of a particular breakup or collision and helps them alert managers at the space station that a piece of debris could be in its path. The model also advises soon-to-launch satellites of areas to avoid and will guide scientists as they attempt to develop and launch debris removal technology for the first time.

While the falling satellite is a concern, it is not the first or the biggest spacecraft to come crashing to Earth. Other notable plummets include, NASA's Skylab in 1979, Space Shuttle Columbia in 2003, and one of the largest ever, Russia's Mir space station in 2001.

Updates from NASA:

As of 10:30 a.m. EDT on Sept. 23, 2011, the orbit of UARS was 100 miles by 105 miles (160 km by 170 km). Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time. Solar activity is no longer the major factor in the satellite's rate of descent. The satellite's orientation or configuration apparently has changed, and that is now slowing its descent. There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent. It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 to 18 hours.

A rendering of Boeing's new Space-Based Space Surveillance Satellite. Credit: Boeing

With 20/20 hindsight, catastrophes can appear inevitable. The subprime mortgage meltdown. The BP oil spill. To avert a different kind of catastrophe, the U.S. military is trying to gain 20/20 foresight on the looming space junk crisis, which I wrote about in the June issue of WIRED.

To get an unprecedented view of the space waste cluttering the heavens, the U.S. Air Force is scrambling to reschedule the launch of the first-ever Space-Based Space Surveillance Satellite. Currently, the military monitors space junk through a ground-based network of radar and optical sensors. But this would be the first time that the Pentagon would capture detailed views of the 500,000 pieces of orbiting trash by relaying photos of debris from space itself.

The one-ton spacecraft will also keep watch on other spacecraft that might pose an accidental--or purposeful--menace to any of America's many vital satellites.

The launch, initially set for July 8, was delayed after tests found software bugs in the lift-off vehicle. The rocket is now expected to blast skyward in mid-August from California's Vandenberg Air Force Base, says a spokesman for Boeing, and the lead contractor on the project.

The new eye in the sky will start snapping photos at a time when the threat posed by space waste seems to be growing. In late May, the Pentagon released an alarming report to Congress warning about future collisions among active satellite and zombiesats--like last year's unprecedented crash between the long-retired Cosmos-2251 and Iridium-33. That smash-up added 2,000 fresh new fragments to a fast-growing catalog of debris objects. The Pentagon raised the specter that a catastrophic chain reaction of crashes has the potential to devastate the $250 billion satellite services industry, crippling global communication and commerce.

Then, in mid-June, NASA was alerted to three pieces of junk that zoomed dangerously close to the International Space Station during a docking mission. After a string of similar close calls last year, NASA officials have already called space junk the top threat to the $100 billion space laboratory and its international crew of astronauts. The ISS, flying at just 220 miles high, happens to inhabit the most cluttered band of low-Earth orbit (LEO).

Following that, President Obama released an 18-page National Space Policy that catapults the space junk problem to the very top of the space agenda. But in a break with the past, Obama not only called for more "mitigation" and monitoring of debris but also urged space junk removal, something that has yet to be tried. His 2011 budget for NASA is the first to propose funding for debris removal projects.

While the space debris situation is already perilous, the problem would be compounded if a collision between objects of different nations leads to a misunderstanding. After all, one man's waste can be seen as another man's anti-satellite weapon. What might at first appear to be an accident could develop into an international imbroglio.

"The center of gravity of American military power is in space," says George Friedman, the CEO of Stratfor, an Austin, Texas-based geopolitical consulting firm, in the new issue of Smithsonian. Looking ahead deeper into the 21^st century, he warns that an enemy who wants to attack the U.S. would strike first by knocking out our satellites, in order "to blind us, to cripple us."

That does sound ominous, and it's all the more reason to pay far more attention to a different kind of UFO, the unintended flying object.