Tiangong-2 space lab draws global praise, with the world’s first “cold” atomic clock on board

• NASA closely watches Tiangong-2 launch

• NASA closely watches Tiangong-2 launch. The space industry has paid close attention as China sent the Tiangong-2 lab into space. NASA is watching the Tiangong-2, as well as China's space program in general. It says the launch marks another step on China's long march to a permanent orbital outpost. Former...
  
  https://youtu.be/pRgAmhCZ9AM
  
  China on Thursday hurled its first Tiangong-2 lab into space, marking another step forward in the country's plans to establish a permanent station by the early 2020s.
  
  China's rapid development in space exploration within the past decade has impressed the world. Martin Barstow, director of Leicester Institute of Space & Earth Observation at the University of Leicester, told Xinhua in a recent interview that China's developing space program is another major milestone towards establishing a permanent presence in space.
  
  "The earlier success of the first space station (Tiangong-1) shows how the program is developing and the new space laboratory will continue to add to China's status as a major space power," the professor said.
  
  Former NASA astronaut Leroy Chiao, the first Chinese-American to be commander of the International Space Station, hailed Tiangong-2 as "another significant step for China's human spaceflight program."
  
  "China is moving in a very deliberate and orderly fashion to advance their space capability," Chiao said. "I think the technology is good, and they are moving to get more operational experience through TG-2, before the beginning of space station construction."
  
  Barstow also spoke highly of China's space capability, saying "China is already a key player in the international space industry," and Tiangong-2 will "enhance" its well-developed space capability.
  
  Gao Yang, director of Surrey Technology for Autonomous Systems and Robotics (STAR) Lab, said manned spaceflight is of indicative significance in space technology, and China's rapid development in this area is well-known.
  
  British Broadcasting Corporation (BBC) said in an article published on Thursday that "Beijing has made space exploration a national priority and is the third country, after the Soviet Union and the U.S., to put astronauts into space."
  
  INTERNATIONAL COOPERATION NEEDED
  
  In different interviews Xinhua carried with space experts, all mentioned the need for international cooperation in space exploration. Space station programs have always been a cradle for countries to work together, Gao said.
  
  Such collaboration has been vividly reflected in the Tiangong-2 mission, which carries, among a number of scientific experiments, an astrophysics detector that is the first space-science experiment built jointly by China with European countries.
  
  POLAR, dedicated to establishing whether the photons from Gama ray bursts (GRBs) -- thought to be a particularly energetic type of stellar explosion -- are polarized, was built largely with Swiss funding, and with the collaboration of Swiss, China and Polish scientists and support from the European Space Agency (ESA), according to the British journal Nature.
  
  POLAR project manager Nicolas Produit, who spoke to Nature, said U.S. law bars NASA from doing joint projects with China's space agencies, but the Chinese Academy of Sciences is discussing a number of other collaborative space projects with the ESA.
  
  Gregory Kulacki, senior analyst and China project manager at the U.S.-based Union of Concerned Scientists' Global Security Program, said that it is encouraging that China intends to solicit international participation in its space station project.
  
  "My hope is that the United States and China will, at an appropriate time in the future, find a way to cooperate in the peaceful exploration of space instead of competing to turn it into a battlefield, as they are now," he said.
  
  Chiao said international coperation is "a common point of interest that helps improve overall relationships. The International Space Station is a great example of that. Many nations came together to build the amazing facility, and we are working together to further science. This helps to improve overall relations between the member countries."
  
  Barstow believed that more and more countries are seeing the importance of space activity but this will not turn into a race. He said that to benefit smaller economies, a growth of space activity across the world will need to be nurtured by the major agencies like ESA, NASA, the Russian Federal Space Agency (Roscomos) and China National Space Administration (CNSA).
  
  CHINA'S AMBITIOUS SPACE PROGRAM
  
  China has been actively developing a three-step manned space program since the first decade of the 21st century.
  
  The program's first mission took place in 1999 with the launch of the Shenzhou-1 to examine the performance and reliability of the launcher and verify key technologies relating to capsule connection and separation, heat prevention, control and landing.
  
  The first step, to send an astronaut into space and return safely, was fulfilled by Yang Liwei in the Shenzhou-5 mission in 2003.
  
  The second step was developing advanced space flight techniques and technologies including extra-vehicular activity and orbital docking. This phase also includes the launch of two space laboratories -- effectively mini space-stations that can be manned on a temporary basis.
  
  The next step will be to assemble and operate a permanent manned space station.
  
  China will begin building a space station that is more economically efficient and uses more data than the current International Space Station (ISS), starting as early as 2017, chief engineer of China's manned space program Zhou Jianping told Xinhua on Thursday.
  
  With the ISS set to retire in 2024, the Chinese space station will offer a promising alternative, and it will make China the only country to have a permanent space station after the  ISS. -Xinhua
  
  

  China launched first 'cold' atomic clock aboard second space station

  
  
  Every clock on Earth is flawed. Even science’s most accurate atomic clocks are beholden to our planet’s gravitational pull, and end up slowing down ever so slightly over time. That’s why researchers from Shanghai decided to send one up into space.
  
  On Sept. 15, Chinese researchers launched a cold atomic clock into orbit around Earth, where it will only slow down by one second every billion years, as opposed to every 300 million years like the current gold-standard of atomic clocks. The Cold Atomic Clock in Space (Cacs), as it is called, will likely become humanity’s most accurate timekeeping device.
  
  Atomic clocks are largely used for calibrating extremely sensitive electronics, like global positioning systems (GPS), or conducting experiments in hyper accuracy-dependent disciplines like particle physics and geology. According to the South China Morning Post, the Chinese government intends to use Cacs to improve their own national GPS, which currently operates at levels below the system employed by the US.
  
  Atomic clocks were originally created to run on the exact measure of a second as agreed upon by the entire scientific community. Seconds used to be measured as a tiny fraction of a day. The trouble is, the average day includes a lot of variation, depending on where you are and the earth’s axial wobble, caused by its magnetic poles and, more recently, melting ice sheets.
  
  Scientists realized that the way that electrons (the tiny negatively charged particles that surround atoms) jump back and forth between different energy states in molecules or atoms, was a much more precise way of calculating a second. These oscillations end up appearing like vibrations that occur at a constant rate—as long as molecules or atoms are at a constant temperature. Since 1967, the official definition of a second has been “9,192,631,770 vibrations of a cesium 133 atom.”
  
  Laser-cooled “cold” atomic clocks are generally considered to be the most accurate clocks that exist—other clocks and watches, like the kind anyone could buy in a store, tend to slow down over time. Although it’s often just a couple of seconds, that inconsistency won’t do in a research setting.
  
  But alas, on Earth, even cold atomic clocks are prone to slowing down ever so slightly. Because of the force Earth’s gravity applies to atoms, the National Institute of Standards and Technology’s NIST-F2 atomic clock in Boulder, Colorado will slow down by a second every 300 million years.
  
  Cacs, which will orbit our planet gravity-free, will use rubidium atom vibrations to keep time, and will slow down much more slowly than Earth-bound atomic clocks. It’s also a lot smaller—about the size of the trunk of a car, whereas the NIST-F2 takes up an entire room.
  
  Cacs was sent into space on the second Chinese space laboratory, called Tiangong-2, launched from Jiuquan Satellite Launch Centre in Inner Mongolia. Although there are currently no humans aboard, the China National Space Administration plans to send two astronauts to the lab in October to conduct various experiments for a month, as the next step towards launching a full-fledged space station in 2020. - Quartz @ qz.com
  
  China launched its second space station, Tiangong-2, from Jiuquan Satellite Launch Center in northwestern China's Gobi desert on Thursday with the world’s first “cold” atomic clock on board – representing a pivotal step in China’s plan to become a major player in the modern-day space race.
  
  Two astronauts are set arrive at the station in October where they will spend one month completing experiments. Running the Cold Atomic Clock in Space, one of 14 different experiments planned for the station’s two-year orbital stint, aims to determine if escaping the effects of gravity increases the accuracy of the timepiece.
  
  “It is the world’s first cold atomic clock to operate in space ... it will have military and civilian applications,” Professor Xu Zhen, a scientist involved with the atomic clock project, told the South China Morning Post.
  

  China to launch world's first 'cold' atomic clock in space ... and it'll stay ...

  www.scmp.com › News › China South China Morning Post 

  
  
  Atomic clocks, some of which are so accurate that it would take billions of years to drift off by even a second, are used to conduct sensitive experiments across a variety of scientific fields and calibrate electronics used by global positioning systems.
  
  To keep time, these clocks rely on measuring the natural vibration rates of atoms and the scientific definition of a second: 9,192,631,770 vibrations of a cesium 133 atom. China’s cold clock will use a laser to slow down the atom and thus lessen the likelihood of the timekeeper missing one of the atom’s rotations and introducing an error, according to the South China Morning Post. Additionally China hopes that sending the cold clock into space and freeing the timekeeping atoms from their ties to gravity will make it more accurate than other atomic clocks currently keeping time around the world and make it capable of measuring fluctuations in microgravity.
  
  The station will serve as a laboratory for a variety of experiments across scientific disciplines that China expects to culminate in launching its own equivalent of the International Space Station (ISS).
  
  "The launch of Tiangong-2 will lay a solid foundation for the building and operation of a permanent space station in the future," Wu Ping, deputy director of China's manned space engineering office, said during a prelaunch briefing, according to Xinhua.
  
  China is not a member of the international consortium that operates the ISS (and isn't allowed to send its astronauts to the station), so it is planning to build its own permanent space station, which at an estimated 60 tons will be 380 tons lighter than the ISS.
  
  China's president, Xi Jinpin called on engineers and scientists to help advance the Chinese space program at a Space Day celebration earlier this year.
  
  "In establishing Space Day, we are commemorating history, passing on the spirit, and galvanising popular enthusiasm for science, exploration of the unknown and innovation, particularly among young people," Xi said at the celebration, according to The Economic Times. "Becoming an aerospace power has always been a dream we've been striving for.”
  
  China’s ambitious goals for space exploration in the coming years include plans to study the yet-unexplored dark side of the moon, and a mission to Mars that will not only orbit the Red Planet, but land and deploy a rover. Both missions are scheduled to launch in 2020.
  
  Dean Cheng, a Chinese space policy expert at the Heritage Foundation in Washington D.C., told New Scientist that Thursday's launch is about national pride – "a reminder that China has a manned space program, including the ability to put its own astronauts into space, something the Americans cannot do [without assistance from Russia]." - Source: Christian Science Monitor
  
  

  Astronauts given comfort upgrade on China's new space lab

  
  
  
  
  New space lab will be equipped with Bluetooth, better lighting, sound dampening, exercise equipment and other features
  
  China's newest space laboratory, Tiangong II, will provide more comfortable digs to astronauts living aboard than its predecessor, Tiangong I, the spacecraft's designers said.
  
  Zhu Zongpeng, chief designer of Tiangong II at China Academy of Space Technology, said designers aimed to create an astronaut-friendly environment in every regard when they refitted the space lab that was developed based on Tiangong I.
  
  "We considered many factors including the sound, lighting, inner decorations as well as support facilities. For instance, we installed a foldable, multifunctional table that can be used for dining and experiments. We also equipped the astronauts with Bluetooth headsets and Bluetooth speakers," he said.
  
  "A number of particulars were taken into account－the carpet in Tiangong I was replaced with floorboards. The light is softer and its brightness can be adjusted. Each astronaut has a bed lamp," Zhu added.
• 
  
  China's space lab Tiangong II roars into the air on the back of a Long March-2F rocket from the Jiuquan Satellite Launch Center in Northwest China, Sept 15, 2016. [Photo/Xinhua]
  
  The Tiangong II consists of two cabins with separate functions－the experiment cabin will be hermetically sealed and will act as the astronauts' living quarters, while the resource cabin will contain solar panels, storage batteries, propellant and engines.
  
  Liao Jianlin, a senior engineer at the academy who took part in Tiangong II's development, said the lab has about 15 square meters for astronauts to live and work, including a separate sleep section and waste storage area.
  
  He said engineers installed muffler devices in the spacecraft to ensure its inner sound is kept under 50 decibels. Environmental controls will keep the temperature within the experiment cabin between 22 C and 24 C and the humidity between 45 and 55 percent.
  
  In addition, Tiangong II has an air detector capable of checking for and dealing with more than 20 hazardous gases and microbes.
  
  Furthermore, designers placed exercise equipment in the space lab such as a treadmill, exercise bike and acupuncture point massager to help astronauts keep healthy, according to Liao.
  
  He said its communications systems also allow astronauts to receive and reply to emails and make calls to family and friends.
• 
     

  Space lab to cross five-year mark.

  
  China’s space lab Tiangong-2 may serve for more than five years and co-exist with its first space station, scheduled for completion around 2020, an expert at the space programme said.
  
  China successfully launched Tiangong-2 on a Long March-2F T2 rocket, blasting off from Jiuquan Satellite Launch Centre in the northwest Gobi Desert on Thursday.
  
  With a designed life of two years, Tiangong-2 was originally built as a backup to Tiangong-1, which completed its mission in March, said Zhu Congpeng, chief designer of Tiangong-2.
  
  “But we expect Tiangong-2 to serve for more than five years given the introduction of an in-orbit propellant technique for the first time,” Zhu said.
  
  In April 2017, China’s first cargo spaceship Tianzhou-1 will be sent into orbit to dock with the space lab, providing fuel and other supplies.
  
  “If the fuel-supply experiment goes well, we will become the second after Russia to master the in-orbit propellant technique,” Zhu said.
  
  While in space, the 8.6-tonne space lab will manoeuvre itself into orbit about 393km above Earth surface.
  
  “As it is higher than past space missions, the Tiangong-2 will be more cost-effective and have a longer lifespan,” said Zhu. — Xinhua
  
  

  Beyond borders

  This is the space station that China is installing in Neuquen, Argentina. The station will begin operation in March 2017 and will allow the tracking of different missions to space that the government is planning in the coming years, according to official sources.
  
  Related post:
  
  
  

  China successfully launched Tiangong-2 space lab

  https://youtu.be/2Ia25ls46WM https://youtu.be/goRzJM4PIEI China's Tiangong-2 successfully launched Crossover: China's Tia...

NASA satellite falls back to Earth !

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Dead satellite likely fell into Pacific Ocean--maybe

By: William Harwood, CNET

NASA's decommissioned 6.3-ton Upper Atmosphere Research Satellite, out of gas and out of control after two decades in space, plunged back into the atmosphere early Saturday, heating up, breaking apart, and presumably showering chunks of debris along a 500-mile-long Pacific Ocean impact zone.
Maybe.

U.S. Strategic Command radar tracking indicated re-entry would occur around 12:16 a.m. EDT (GMT-4) Saturday as the satellite was descending across the Pacific Ocean on a southwest-to-northeast trajectory approaching Canada's west coast. If re-entry occurred on or before the predicted time, any wreckage that survived atmospheric heating almost certainly fell into the Pacific Ocean.

NASA's derelict Upper Atmosphere Research Satellite fell to Earth Saturday, presumably into the Pacific Ocean west of Canada. But it's not yet a sure thing.

"Because we don't know where the re-entry point actually was, we don't know where the debris field might be," said Nicholas Johnson, chief orbital debris scientist at NASA's Johnson Space Center.

"If the re-entry point was at the (predicted time) of 04:16 GMT, then all that debris wound up in the Pacific Ocean. If the re-entry point occurred earlier than that, practically the entire pass before 04:16...is over water. So the only way debris could have probably reached land would be if the re-entry occurred after 04:16."

Johnson said amateur satellite watchers in the U.S. northwest and the Canadian southwest were "looking to observe UARS as it came over. Every one of those attempts came up negative. That would suggest that the re-entry did, in fact, occur before it reached the North American coast, which, again, would mean most of this debris fell into the Pacific."

But it's not yet certain and it's equally possible a delayed re-entry resulted in debris falling somewhere in northern Canada or elsewhere along the trajectory.

"We may never know," Johnson told reporters in an afternoon teleconference.

The centerpiece of a $750 million mission, the Upper Atmosphere Research satellite was launched from the shuttle Discovery at 12:23 a.m. EDT (GMT-4) on Sept. 15, 1991. The solar-powered satellite studied a wide variety of atmospheric phenomena, including the depletion of Earth's ozone layer 15 to 30 miles up.

The long-lived satellite was decommissioned in 2005, and one side of its orbit was lowered using the last of its fuel to hasten re-entry and minimize the chances of orbital collisions that could produce even more orbital debris. No more fuel was available for maneuvering and the satellite's re-entry was "uncontrolled."

As with all satellites in low-Earth orbit, UARS was a victim of atmospheric drag, the slow but steady reduction in velocity, and thus altitude, caused by flying through the tenuous extreme upper atmosphere at some five miles per second.

UARS' final trajectory as it neared the discernible atmosphere proved difficult to predict. The descent slowed somewhat Friday, presumably because the spacecraft's orientation changed. As the day wore on, the predicted impact time slipped from Friday afternoon to early Saturday.

Johnson said falling satellites typically begin breaking up at an altitude of around 50 miles. In the case of UARS, computer analysis indicated about 26 pieces of debris would survive to reach the surface, spread out along a 500-mile-long down-range footprint. Johnson said the heel of the footprint, the area where the lightest debris might fall, is typically 300 miles or so beyond the breakup point.

But so far, "we've got no reports of anyone seeing anything that we believe are credible," Johnson said.

Johnson told reporters last week he expected most of the satellite to burn up as it slammed into the dense lower atmosphere at more than 17,000 mph. But computer software used to analyze possible re-entry outcomes predicted 26 pieces of debris would survive to impact the surface, the largest weighing some 330 pounds. Impact velocities were expected to range from 30 mph to 240 mph.

"We looked at those 26 pieces and how big they are, and we've looked at the fact they can hit anywhere in the world between 57 north and 57 south, and we looked at what the population density of the world is," he said. "Numerically, it comes out to a chance of 1 in 3,200 that one person anywhere in the world might be struck by a piece of debris. Those are obviously very, very low odds that anybody's going to be impacted by this debris."

For comparison, some 42.5 tons of wreckage from the shuttle Columbia hit the ground in a footprint stretching from central Texas to Louisiana when the orbiter broke apart during re-entry in 2003. No one on the ground was injured and no significant property damage was reported.

An astronaut picture of the UARS satellite being deployed in 1991.

Photograph courtesy NASA

An astronaut picture of the UARS satellite being deployed in 1991.

Photograph courtesy NASA

Traci Watson for National Geographic News 

Defying predictions one last time, NASA's doomed UARS satellite dove through Earth's atmosphere late last night over the North Pacific Ocean, off the U.S. West Coast, the space agency says. (Also see "Space Debris: Five Unexpected Objects That Fell to Earth.")

As recently as Friday morning, U.S. officials had forecast that the Upper Atmosphere Research Satellite, or UARS, would fall out of the sky in the late afternoon or early evening Friday, eastern time.
(See "NASA Satellite Falling Faster Due to Solar Activity.")

But the satellite shifted position as it tumbled toward the planet, forcing scientists to throw out their earlier time estimates.

NASA said early Saturday that UARS fell out of orbit sometime between 11:23 p.m. and 1:09 a.m. ET.
Amateur satellite trackers in places such as San Antonio, Texas, and northern Minnesota reported catching glimpses of UARS as it made its final, doomed circles around Earth.

Though the spacecraft plummeted over the Pacific, it's still not clear exactly where debris from the satellite has landed. Pieces of the satellite will be strung along a debris "footprint" stretching 500 miles (800 kilometers).

So far there are "no reports of any damage or injury," NASA said via Twitter close to midday Saturday.
Satellite Pieces Not For Sale

UARS, which weighed more than six tons, was lofted into orbit by the space shuttle Discovery in 1991. The craft recorded data on Earth's atmosphere until it was switched off in 2005.

Some 1,100 pounds (500 kilograms) of debris from the satellite were projected to survive the superheated descent through the atmosphere. The biggest intact piece, NASA said, would probably be a 300-pound (140-kilogram) chunk of the spacecraft's structure.

NASA warned the curious not to touch any pieces of the spacecraft that may have made it to the ground, because of the risk of sharp edges.

The space agency also tried to head off sales of UARS remnants on Internet auction sites such as eBay.
"Any pieces of UARS found are still the property of the country that made it," NASA warned via Twitter this morning. "You'll have to give 'em back to U.S."

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Lifeless satellite falls back to Earth

(China Daily)

Junk left from colliding satellites floats through space in this computer-generated image. NASA confirmed that two communication satellites from the United States and Russia collided 800 kilometers above northern Siberia on Sept 10. [Provided to China Daily]

NASA assures public that there is little chance of getting hit by debris

WASHINGTON - Fragments from an old 6-ton NASA satellite hurtled toward Earth on Friday, while the exact site of the crash-landing remained a mystery into the final hours.

The US space agency has stressed that the risk is "extremely small" that any of the 26 chunks that are expected to survive the fiery re-entry into Earth's atmosphere will hit one of the planet's seven billion people a one in 3,200 chance.


"Re-entry is possible sometime during the afternoon or early evening of Sept 23, Eastern Daylight Time," NASA said on its website on Thursday night.

That would be early morning Saturday Beijing time.

"It is still too early to predict the time and location of re-entry with any more certainty, but predictions will become more refined in the next 24 hours."

The influence of solar flares and the tumbling motion of the satellite make narrowing down the landing a particularly difficult task, experts said as the Internet lit up with rumors of where and when it would fall.

The US Department of Defense and NASA were busy tracking the debris and keeping all federal disaster agencies informed, a NASA spokeswoman said.

The Federal Aviation Administration issued a notice on Thursday to pilots and flight crews of the potential hazard and urged them to "report any observed falling space debris to the appropriate (air traffic control) facility and include position, altitude, time and direction of debris observed," CNN said.

The satellite was launched in 1991 and was designed to provide data for better understanding Earth's upper atmosphere and the effects of natural and human interactions on the atmosphere. The satellite was deactivated in 2005 as it ran out of fuel and was left orbiting Earth.



Orbital debris experts say space junk of this size from broken-down satellites and spent rockets tends to fall back to Earth about once a year, though this is the biggest NASA satellite to fall in three decades.

NASA's Skylab crashed into western Australia in 1979.

The surviving chunks of the tour-bus sized satellite will include titanium fuel tanks, beryllium housing and stainless steel batteries and wheel rims. The parts may weigh as little as one kg or as much as 158 kg, NASA said.

Orbital debris scientists say the pieces will fall somewhere between 57 north latitude and 57 south latitude, which covers most of the populated world.

The debris field is expected to span 800 square kilometers.

Pang Zhihao, a researcher from the Chinese Research Institute of Space Technology, told Xinhua that the crash could have been avoided had the satellite been put into a higher orbit, or manipulated to drop in the South Pacific when it had abundant fuel.

Pang said most spacecraft will incinerate upon re-entering Earth's atmosphere, and the debris will mostly likely fall into the ocean or hit an uninhabited area.

NASA has also said that in 50 years of space exploration no one has ever been confirmed injured by falling space junk.

The craft contains no fuel and so is not expected to explode on impact.

"No consideration ever was given to shooting it down," NASA spokeswoman Beth Dickey said.
NASA has warned anyone who comes across what they believe may be debris not to touch it but to contact authorities for assistance.

Space law professor Frans von der Dunk from the University of Nebraska-Lincoln's College of Law told AFP that the United States will likely have to pay damages to any country where the debris falls.

"The damage to be compensated is essentially without limit," von der Dunk said, referring to the 1972 Liability Convention to which the United States is one of 80 signatories.

"Damage here concerns 'loss of life, personal injury or other impairment of health; or loss of or damage to property of States or of persons, natural or juridical, or property of international intergovernmental organizations,'" he said, reading from the agreement.

However, the issue could get thornier if the debris causes damage in a country that is not part of the convention.

"The number of countries so far theoretically at risk is rather large, so there may be an issue if damage would be caused to a state not being party to the Liability Convention," he said.

AFP-Xinhua

Huge Tumbling Satellite Could Fall to Earth Over US Tonight or Saturday, NASA Says

by Tariq Malik, SPACE.com Managing Editor

The Upper Atmosphere Research Satellite is in the grasp of the remote manipulator system end effector above the payload bay of the Earth-orbiting Discovery during STS-48 pre-deployment checkout procedures. CREDIT: NASA Johnson Space Center View full size image

This story was updated at 2:04 p.m. ET

A huge, dead satellite tumbling to Earth is falling slower than expected, and may now plummet down somewhere over the United States tonight or early Saturday, despite forecasts that it would miss North America entirely, NASA officials now say.

The 6 1/2-ton Upper Atmosphere Research Satellite (UARS) was expected to fall to Earth sometime this afternoon (Sept. 23), but changes in the school bus-size satellite's motion may push it to early Saturday, according to NASA's latest observations of the spacecraft.

 "The satellite's orientation or configuration apparently has changed, and that is now slowing its descent," NASA officials wrote in a morning status update today. "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." [Complete coverage of NASA's falling satellite]

NASA expects about 26 large pieces of the UARS spacecraft to survive re-entry through Earth's atmosphere and reach the planet's surface. The biggest piece should weigh about 300 pounds. The spacecraft is the largest NASA satellite to fall from space uncontrolled since 1979. [6 Biggest Spacecraft to Fall Uncontrolled From Space]

NASA officials have said the the chances that a piece of UARS debris hits and injures one of the nearly 7 billion people on the planet are about 1 in 3,200. However, the personal odds of you being struck by UARS satellite debris are actually about 1 in several trillion, NASA officials have said.

As of 10:30 a.m. EDT (1430 GMT) today, the UARS satellite was flying in an orbit of about 100 miles by 105 miles (160 kilometers by 170 km), and dropping.  NASA launched the UARS satellite in 1991 to study Earth's ozone layer and upper atmosphere. The satellite was decommissioned in 2005.

"Re-entry is expected late Friday, Sept. 23, or early Saturday, Sept. 24, Eastern Daylight Time," NASA officials wrote. "Solar activity is no longer the major factor in the satellite's rate of descent."

The sun has had an extremely active week, one that has included several solar flares. High solar activity can cause the Earth's atmosphere to heat and expand, which can increase drag on a low-flying satellite like UARS, making it fall faster.

Get a snapshot view of NASA's Upper Atmosphere Research Satellite (UARS), which will fall to Earth in 2011, in this SPACE.com infographic.
CREDIT: Karl Tate, SPACE.com Contributor

View full size image

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