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.

Artist concept of NASA's new rocket, called the Space Launch System. Credit: NASA

Today NASA unveiled the design of a new rocket to help the agency meet a challenge from President Obama: send astronauts to an asteroid by 2025 and to Mars by the mid-2030s. Called the Space Launch System (SLS), the new heavy-lift launch vehicle will cost $18 billion, with its first test flight planned for 2017. It will be designed to carry the Orion Multi-Purpose Crew Vehicle for transport of crew and cargo.

The much anticipated announcement comes on the heels of the July retirement of the space shuttles, and is part of a plan laid out by the White House--the NASA Authorization Act of 2010--developed after Congress canceled a moon program, called Constellation, for the agency to focus on a vehicle to take astronauts to places like the moon and Mars while commercial companies focus on a rocket to transport crew to low Earth orbit.

The new rocket will include technology from the Space Shuttles and the Constellation program, which was building two rockets, Ares I and Ares V, and it will share a resemblance to the Saturn V, the first rocket to travel to the moon. "But it is difficult to compare rockets from one generation to the other" because of constant upgrades in technology and manufacturing techniques, said William Gerstenmaier, associate administrator for NASA's Human Exploration and Operations Mission Directorate, at today's press conference in Washington.

The SLS will use a liquid hydrogen and liquid oxygen propulsion system, which will allow NASA to reduce costs and leverage experience and existing technology, said NASA administrator Charles Bolden. The rocket will use five solid rocket boosters attached on either side of its core for the initial development flights, but NASA will hold a competition to replace these side-strapped boosters for more advanced designs. Gerstenmaier estimated the SLS thrust to be between 10 percent and 20 percent greater than that of the Saturn V.

Credit: NASA

Atlantis blasts off on the final shuttle mission. Credit: NASA.

Space Shuttle Atlantis lifted off today on its final voyage, bringing NASA's 30-year-old space shuttle program to an end. With the retirement of the shuttles, and no U.S. rocket capable of taking astronauts into space, experts are buzzing with one question: what's next for NASA? Here are a few important issues surrounding the agency and its plans going forward.

Relying on the Russians to send U.S. astronauts to space is the first piece of NASA's future plan. For many at the agency, this is a bitter pill to swallow. According to SpaceflightNow.com,

In the near term, "we're going to have a reverse brain drain," [Michael Griffin, former NASA administrator and architect of the Bush administrations moon program], told CBS News. "It used to be that people came from other places and other industries to work in the space program because of what it meant and what it was. And as it goes away, we're going to lose those people because talented folks go where there are tough problems. And that's not going to be good for the country."

Griffin's concerns are echoed by many critics of the Obama administrations' plan to retire the shuttles. The administration has also canceled the Constellation program (the moon program devised by the Bush administration), tasked the commercial space industry with developing new low Earth orbit transportation systems, and asked NASA to build a heavy-lift rocket for trips to an asteroid, the moon, and eventually Mars.

Former astronauts, Neil Armstrong, John Glenn, and Jim Lovell, believe NASA should keep flying the shuttles—an agency without a backup system for getting into space and bringing astronauts home is a violation of one of NASA's critical design criteria, these astronauts told the Associated Press. Armstrong and Lovell wrote a letter to the Obama administration saying an end to the shuttle program was a mistake,

Glenn said he doesn't disagree with Obama's plans, although he said he believes private spaceflight will take years longer than [NASA administrator Charles] Bolden predicts. What Glenn objects to is the gap between the shuttle and a future spacecraft. While the Soyuz is reliable, Glenn said NASA should always want an alternative in case of a "hiccup" in the Soyuz plans.

[...]

"Throughout the history of the manned spaceflight program we've always had another program to transition into [...] we had that and it got canceled and we don't have anything," launch manager Leinbach told his fellow workers at Kennedy Space Center. "Frankly as a senior NASA manager I would like to apologize that we don't have that."

The future is not all bleak, though, according to NASA administrator Bolden. During a press conference at Kennedy Space Center held on July 7, according to Space.com,

Private spaceflight firms will pick up NASA's slack before too long, ferrying humans to low-Earth orbit and back relatively cheaply and efficiently, [said Bolden].

And handing off that taxi service to commercial companies, Bolden added, will free the space agency up to do what it was meant to do: explore further afield in our solar system. So the nation is not abandoning human spaceflight, despite a pervasive public perception to the contrary, he said.

NASA is pouring money into the commercial industry while still hammering out a design for a heavy-lift rocket. David Mindell, professor of aeronautics and astronautics at MIT, told me in an interview last week that there is a lot of anxiety about the ending of the shuttle program and part of the problem is that NASA's leadership is not doing a good job at articulating clearly to congress and the public what they are trying to do.

Mindell believes that it is time for the shuttles to retire because "we can't afford two space programs." He adds that the shuttle became an unambitious program, while a lot of other exciting things are happening in commercial spaceflight. "SpaceX is doing things NASA tried to do at one-third the cost; private space will deliver something," he said.

A detailed plan for NASA's future will become clearer when Congress releases the agency's 2012 budget. According to Mindell, there is still one fundamental question that NASA needs to clearly answer, "why send people at all?"