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.

The Wikileaks website has obtained diplomatic cables, which have been released to the U.K.'s Daily Telegraph, that suggest that anti-satellite tests conducted by China in 2007 and by the United States in 2008 were not merely "tests" but showcases of each country's space weapon or military powers. This is not entirely surprising, but the documents put in writing the some of the realpolitik involved with two competing super powers, i.e. my weapons are bigger and better than yours.

The Chinese intentionally shot down an aging weather satellite 530 miles above Earth in January 2007, which resulted in thousands of pieces of debris, exponentially compounding the space debris problem. The strike down garnered criticism from nations around the world, including the United States. Then in February 2008 the United States shot down a malfunctioning American spy satellite, a task it claimed it had to conduct because the satellite was carrying toxic fuel that could pose health concerns.

According to the Telegraph,

One month before the strike, the US criticised Beijing for launching its own "anti-satellite test", noting: "The United States has not conducted an anti-satellite test since 1985." In a formal diplomatic protest, officials working for Condoleezza Rice, the then secretary of state, told Beijing: "A Chinese attack on a satellite using a weapon launched by a ballistic missile threatens to destroy space systems that the United States and other nations use for commerce and national security. Destroying satellites endangers people."

The warning continued: "Any purposeful interference with US space systems will be interpreted by the United States as an infringement of its rights and considered an escalation in a crisis or conflict.

"The United States reserves the right, consistent with the UN Charter and international law, to defend and protect its space systems with a wide range of options, from diplomatic to military."

. . .

In secret dispatches, US officials indicated that the strike was, in fact, military in nature.

Immediately after the US Navy missile destroyed the satellite, the American Embassy in China received "direct confirmation of the results of the anti-satellite test" from the US military command in the Pacific, according to a secret memo.

The most recent cable in the collection was sent from the office of Mrs Clinton in January 2010.

It claimed that US intelligence detected that China had launched a fresh anti-satellite missile test. Crucially, Washington wanted to keep secret its knowledge that the missile test was linked to China's previous space strikes.

The cable, marked "secret" said the Chinese army had sent an SC-19 missile that successfully destroyed a CSS-X-11 missile about 150 miles above the Earth.

The leaked cables are interesting, but lack the muster to confirm the Telegraph's claim of "a secret 'star wars' arms race" between China and the U.S. (Given the diplomatic climate at the time, one might expect the U.S. embassy in China to be informed of the American satellite's destruction regardless of whether or not an ulterior agenda was playing out.) More to the point, the cables bring to life dangerous tensions between two powerful nations and continue the Wikileaks saga--that is of secrets and transparency, and how one begins to make sense of it all.

The nanosatellite called O/OREOS being worked on. Credit: NASA

Here's a video of the launch of O/OREOS:

"With O/OREOS we can analyse the stability of organics in the local space environment in real-time and test flight hardware that can be used for future payloads to address fundamental astrobiology objectives," said Pascale Ehrenfreund, O/OREOS project scientist at the Space Policy Institute at George Washington University, in a NASA press release.

Researchers will be able to make contact with the nanosatellite 12.5 hours after it reaches low Earth orbit. It's mission will last 6 months. During that time the satellite will conduct experiments autonomously and will receive commands from a ground station in California to which it will relay data daily.

O/OREOS will be conducting two experiments. One will characterize the growth, activity of health of microorganisms in a space environment, which includes exposure to radiation and weightlessness. A second experiment will monitor the stability and changes in different organic molecules as they are exposed to these space conditions.

The new nanosatellite adds to NASA's collection of loaf-of-bread-sized spacecraft. Last year the agency launched PharmSat to test antifungal drugs in orbit, and in 2006 it sent GeneSat to space to test how E. coli bacteria behave in space. "Secondary payload nanosatellites, like O/OREOS are an innovative way to extend and enhance scientists' opportunities to conduct research in low Earth orbit by providing an alternative to the International Space Station or space shuttle investigations," said Ehrenfreund.