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 scale model of Russia's next-generation spacecraft was demonstrated at the Paris Air and Space Show in June. Credit: Anatoly Zak

With the U.S. manned space program grounded following the last mission of the space shuttle, the Russian Soyuz spacecraft is the only avenue into space for NASA astronauts. And, in an unprecedented arrangement for NASA, U.S. taxpayers will now provide the Russian government with the extra cash it needs to build a new-generation manned vehicle to replace the 40-year-old Soyuz.

Just as in 1993, when the Russian space agency suddenly found itself in the driver's seat of the stalled U.S.-led space station program by providing crucial elements of the outpost from their own stillborn Mir-2 project, Moscow space officials can again hardly believe their luck. The retirement of the U.S. space shuttle before its replacement is ready means a lucrative deal for Russia to transport all crews to the International Space Station in the next several years.

However, as the Russian space agency's officials are celebrating this windfall, the leaders of the Russian space industry are far from resting on their laurels—they are pushing ahead with plans for a new spacecraft and launcher. However, behind the scenes, RKK Energia, the nation's chief manned spaceflight contractor, has embarked on a collision course with its parent agency—Roskosmos—over the future strategy.

"We've got an unfortunate situation with our next-generation spacecraft," says Aleksandr Derechin, deputy designer general at RKK Energia. "Roskosmos wants a large 23-ton spacecraft [to replace Soyuz], which would also need a new powerful rocket and the new launch site on the far-eastern fringes of the country." But for more than four years, this ambitious plan has become a heavy burden for the Russian space program, Derechin argues.

While the official schedule calls for the first launch of the brand-new Rus-M rocket from the yet-to-be built Vostochny Cosmodrome in 2015, and the first manned mission from this site in 2018, many industry experts consider this timeline wildly unrealistic. In a run-up to the 2014 Winter Olympics, the country may have to choose between multibillion-dollar investments in Sochi Olympic facilities or in the new space center. These experts believe that the current Russian strategy could push back the birth date of the Soyuz replacement by years, if not a decade. Critics point to the ongoing development of the Angara family of rockets, which was initiated at the beginning of the 1990s and has perpetually remained several years away from its maiden mission.

In the meantime, RKK Energia has watched nervously as several modestly priced commercial ventures for carrying astronauts into space have been fostered by NASA. Seeing the emergence of these "private" spacecraft as competition, RKK Energia has come up with its own fast-track strategy, one that would bypass the Russian space agency's grand space plan. The company has proposed to fly a streamlined 12-ton version of the new-generation manned spacecraft onboard an off-the-shelf Zenit rocket, from an existing launchpad in Baikonur, Kazakhstan.

Roskosmos has so far rejected this cheaper, faster approach, preferring to stick to the original plan insisted upon by the government. Despite this setback, RKK Energia's alternative launch vehicle based on the Zenit reappeared last month at the Paris Air and Space Show.

The Zenit, first introduced in 1985, is a two-stage rocket that uses liquid oxygen and kerosene and is capable of delivering up to 13 tons of payload into low-Earth orbit. The Zenit still remains a critical part of the Russian space fleet, and RKK Energia based many of its manned spacecraft designs on the capabilities of the Zenit-based rockets.

Anatoly Zak is a freelance writer and illustrator specializing in space exploration. He is the publisher of RussianSpaceWeb.com, ­ a resource on the history of and the latest developments in the former USSR space program.

NASA has released the first edition of its new bi-monthy newsletter that focuses on "happenings" in the agency's commercial spaceflight development program. The first newsletter is devoted to the progress made in the commercial crew development program, which recently awarded four companies money to develop spacecraft that can carry astronauts to space. The progress made by these companies--SpaceX, Boeing, Blue Origin, and Sierra Nevada Corporation--is small. But with the space shuttle's final mission scheduled for July 8, the pressure is on for these companies to work quickly and efficiently to meet their goals.

"The space shuttle's retirement gives commercial companies more incentive to push the development of their systems," says Craig Steidle, the president of the Commercial Spaceflight Federation. "They are excited about what's coming up, but the pressure is getting financial support, to make sure we have the money to allow them to do spaceflight demonstrations."

Steidle is optimistic that the commercial companies working on human spaceflight will meet their goals, and we will see the first astronaut launch to space on a commercial spacecraft in 2017.

Here's a round up of what these companies are up to:

Boeing is developing the CST-100 spacecraft, and perhaps achieved the greatest milestone for its spacecraft thus far by completing its delta Systems Definition Review--an analysis of the design and requirements of the spacecraft and its subsystems, including structures, thermal, electrical, propulsion, life support, software and avionics. According to the company's press release,

The Delta SDR enables a common understanding of the design baseline as the team progresses toward a system-level Preliminary Design Review (PDR), which will further mature the system design and ensure it meets all requirements. Under the second round of NASA's Commercial Crew Development Space Act Agreement, Boeing expects to complete its System PDR no later than early spring 2012.

Boeing is preparing to gather performance data on the spacecraft's launch abort system and service module fuel tank; evaluate vehicle ascent performance in wind tunnel testing; and build on earlier landing air bag and parachute demonstrations with more in-depth investigations.

In June Boeing will present a plan for identifying and mitigating potential spaceflight safety hazards for the spacecraft.

SpaceX meanwhile is developing the Falcon 9 rocket and the Dragon spacecraft, which are both test flight proven. With the new funding the company is focusing on the development of a launch abort system and improving the design of the crew systems. SpaceX completed its initial milestone, a kickoff meeting with NASA officials to review requirements and present design status updates. In July, the company will have to present data, documentation, and risk assessments to show that the launch abort system concept is technically sound.

Sierra Nevada Corporation is building the Dream Chaser, a reusable piloted spacecraft that will be launched on an Atlas V rocket. It also had initial kickoff meeting and Systems Requirement Review and will present test results on the aerodynamic and thermal performance of the airfoil for the Dream Chaser's tip fins.

Blue Origin's crew transportation system will be a reusable biconic space vehicle that has been launched on an Atlas V rocket and then on the company's own reusable booster system. After initial meetings the company improved the overall space vehicle design. The next step will be ground and flight tests of its pusher escape system for astronauts, and accelerating the engine design for the reusable booster system.