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Chinese rocket Big segment set to fall to Earth


Large Chinese rocket tumbling toward toward unguided re-entry this … Sat, 08 May 2021 11:00:00 +0100-The main segment from the Long March-5b vehicle was used to launch the first module of China's new space station last month. At 18 tonnes it is one of the largest …

Big Chinese rocket segment set to fall to Earth

By Jonathan Amos
BBC Science Correspondent

image copyrightGetty Images
image captionThe rocket was launched to carry a Chinese space station section into orbit

Debris from a Chinese rocket is expected to fall back to Earth in an uncontrolled re-entry this weekend.

The main segment from the Long March-5b vehicle was used to launch the first module of China's new space station last month.

At 18 tonnes it is one of the largest items in decades to have an undirected dive into the atmosphere.

The US on Thursday said it was watching the path of the object but currently had no plans to shoot it down.

“We're hopeful that it will land in a place where it won't harm anyone,” US Defense Secretary Lloyd Austin said. “Hopefully in the ocean, or someplace like that.”

Various space debris modelling experts are pointing to late Saturday or early Sunday (GMT) as the likely moment of re-entry. However, such projections are always highly uncertain.

Originally injected into an elliptical orbit approximately 160km by 375km above Earth's surface on 29 April, the Long March-5b core stage has been losing height ever since.

Just how quickly the core's orbit will continue to decay will depend on the density of air it encounters at altitude and the amount of drag this produces. These details are poorly known.

Most of the vehicle should burn up when it makes its final plunge through the atmosphere, although there is always the possibility that metals with high melting points, and other resistant materials, could survive to the surface.

When a similar core stage returned to Earth a year ago, piping assumed to be from the rocket was identified on the ground in Ivory Coast, Africa.

The chances of anyone actually being hit by a piece of space junk are very small, not least because so much of the Earth's surface is covered by ocean, and because that part which is land includes huge areas that are uninhabited.

The BBC is not responsible for the content of external sites.View original tweet on Twitter

The zone of potential fall in this case is restricted still further by the trajectory of the rocket stage. It's moving on an inclination to the equator of about 41.5 degrees. This means it's possible already to exclude that any debris could fall further north than approximately 41.5 degrees North latitude and further south than 41.5 degrees South latitude.

China has bridled at the suggestion that it has been negligent in allowing the uncontrolled return of so large an object. Commentary in the country's media has described Western reports about the potential hazards involved as “hype” and predicted the debris will likely fall somewhere in international waters.

The Global Times quoted aerospace expert Song Zhongping who added that China's space monitoring network would keep a close watch and take necessary measures should damage occur.

But the respected cataloguer of space activity, Jonathan McDowell from the Harvard-Smithsonian Center for Astrophysics, US, said the situation did reflect poorly on China.

“It is indeed seen as negligence,” he told BBC News.

“This is the second launch of this rocket; the debris in Ivory Coast last year was from the previous launch, i.e. a basically identical rocket.

“These two incidents [the one now and the Ivory Coast one] are the two largest objects deliberately left to re-enter uncontrolled since Skylab in 1979.”

Fragments of the US space station Skylab scattered across Western Australia in 1979, attracting worldwide attention.

Hugh Lewis, who models space debris at Southampton University, UK, noted that more than 60 years of spaceflight had left a large legacy of junk in orbit. The responsibility for this litter rests on several countries, but principally Russia and the US.

“It's worth remembering that there are approximately 900 orbital rocket stages in low-Earth orbit, left behind by nearly every launch-capable nation and with a combined mass orders or magnitude greater than the one expected to re-enter the atmosphere this [weekend],” Dr Lewis posted on Twitter.

image captionLong March-5B rocket launched the Tianhe module on 29 April

Modern practice now calls for rocket stages to be de-orbited as soon as possible after their mission. In the case of large core segments, these would normally come straight back, within one orbit, falling into the ocean or on land (the US company SpaceX now propulsively lands its core stages so they can be used again).

For upper-stages that go into an orbit and may travel around the globe several times as they precisely position a payload, the preference is to include a re-ignitable engine that can steer the stage into a return at the earliest opportunity.

Usually, this would be over an ocean – potentially in the furthest place from land in the South Pacific, between Australia, New Zealand and South America.

Over an area of approximately 1,500 sq km (580 sq miles) this region is a popular graveyard of rocket elements and defunct satellites, where the remains of around 260 missions are thought to be scattered on the ocean floor.

media captionSpace junk map tracks 200 'ticking time bombs'

Additional reporting by Andreas Illmer

By Jonathan Amos
BBC Science Correspondent

image copyrightGetty Images
image captionThe rocket was launched to carry a Chinese space station section into orbit

Debris from a Chinese rocket is expected to fall back to Earth in an uncontrolled re-entry this weekend.

The main segment from the Long March-5b vehicle was used to launch the first module of China's new space station last month.

At 18 tonnes it is one of the largest items in decades to have an undirected dive into the atmosphere.

The US on Thursday said it was watching the path of the object but currently had no plans to shoot it down.

“We're hopeful that it will land in a place where it won't harm anyone,” US Defense Secretary Lloyd Austin said. “Hopefully in the ocean, or someplace like that.”

Various space debris modelling experts are pointing to late Saturday or early Sunday (GMT) as the likely moment of re-entry. However, such projections are always highly uncertain.

Originally injected into an elliptical orbit approximately 160km by 375km above Earth's surface on 29 April, the Long March-5b core stage has been losing height ever since.

Just how quickly the core's orbit will continue to decay will depend on the density of air it encounters at altitude and the amount of drag this produces. These details are poorly known.

Most of the vehicle should burn up when it makes its final plunge through the atmosphere, although there is always the possibility that metals with high melting points, and other resistant materials, could survive to the surface.

When a similar core stage returned to Earth a year ago, piping assumed to be from the rocket was identified on the ground in Ivory Coast, Africa.

The chances of anyone actually being hit by a piece of space junk are very small, not least because so much of the Earth's surface is covered by ocean, and because that part which is land includes huge areas that are uninhabited.

The BBC is not responsible for the content of external sites.View original tweet on Twitter

The zone of potential fall in this case is restricted still further by the trajectory of the rocket stage. It's moving on an inclination to the equator of about 41.5 degrees. This means it's possible already to exclude that any debris could fall further north than approximately 41.5 degrees North latitude and further south than 41.5 degrees South latitude.

China has bridled at the suggestion that it has been negligent in allowing the uncontrolled return of so large an object. Commentary in the country's media has described Western reports about the potential hazards involved as “hype” and predicted the debris will likely fall somewhere in international waters.

The Global Times quoted aerospace expert Song Zhongping who added that China's space monitoring network would keep a close watch and take necessary measures should damage occur.

But the respected cataloguer of space activity, Jonathan McDowell from the Harvard-Smithsonian Center for Astrophysics, US, said the situation did reflect poorly on China.

“It is indeed seen as negligence,” he told BBC News.

“This is the second launch of this rocket; the debris in Ivory Coast last year was from the previous launch, i.e. a basically identical rocket.

“These two incidents [the one now and the Ivory Coast one] are the two largest objects deliberately left to re-enter uncontrolled since Skylab in 1979.”

Fragments of the US space station Skylab scattered across Western Australia in 1979, attracting worldwide attention.

Hugh Lewis, who models space debris at Southampton University, UK, noted that more than 60 years of spaceflight had left a large legacy of junk in orbit. The responsibility for this litter rests on several countries, but principally Russia and the US.

“It's worth remembering that there are approximately 900 orbital rocket stages in low-Earth orbit, left behind by nearly every launch-capable nation and with a combined mass orders or magnitude greater than the one expected to re-enter the atmosphere this [weekend],” Dr Lewis posted on Twitter.

image captionLong March-5B rocket launched the Tianhe module on 29 April

Modern practice now calls for rocket stages to be de-orbited as soon as possible after their mission. In the case of large core segments, these would normally come straight back, within one orbit, falling into the ocean or on land (the US company SpaceX now propulsively lands its core stages so they can be used again).

For upper-stages that go into an orbit and may travel around the globe several times as they precisely position a payload, the preference is to include a re-ignitable engine that can steer the stage into a return at the earliest opportunity.

Usually, this would be over an ocean – potentially in the furthest place from land in the South Pacific, between Australia, New Zealand and South America.

Over an area of approximately 1,500 sq km (580 sq miles) this region is a popular graveyard of rocket elements and defunct satellites, where the remains of around 260 missions are thought to be scattered on the ocean floor.

media captionSpace junk map tracks 200 'ticking time bombs'

Additional reporting by Andreas Illmer


قراءة المزيد

Big Chinese rocket segment set to fall to Earth Sat, 08 May 2021 11:00:00 +0100-The launcher shed its four strap-on boosters about three minutes into the mission, but the two engines on the Long March 5B's core stage continued firing for about …

Large Chinese rocket tumbling toward toward unguided re-entry this weekend

May 07, 2021

A Long March 5B rocket lifts off with the Tianhe space station module. Credit: Xinhua

For the second time in a year, a large spent Chinese rocket stage intentionally left in orbit is heading for an unguided plunge back into Earth’s atmosphere Saturday or Sunday somewhere between 41.5 degrees north and south latitude.

The heavy-lift Long March 5B rocket stage took off April 28 with the Tianhe core module for China’s space station. The Long March 5B — one of the most powerful rockets in the world — tracked downrange to the southeast from the Wenchang launch base on Hainan Island, China’s southernmost province.

The launcher shed its four strap-on boosters about three minutes into the mission, but the two engines on the Long March 5B’s core stage continued firing for about eight minutes, doing all the work to place the 54.4-foot (16.6-meter) Tianhe space station module into orbit.

Most rockets have an upper stage to finish the job of deploying payloads into orbit. On those launchers, the first stage does not attain enough velocity to orbit the Earth. SpaceX recovers its first stages for reuse, while other launchers have booster stages that fall back to the ground hundreds of miles downrange from the launch site.

Many launch operators design their upper stages to reignite their engines at the end of their missions to guide the rockets to a controlled re-entry over a remote stretch of ocean. Rather than designing the huge Long March 5B core stage t0 remove itself from orbit using engines or thrusters, Chinese engineers left the rocket in space after finishing its mission.

The Long March 5B’s core stage has a mass of about 21.6 metric tons (23.8 tons) and measures about 98 feet (100 meters) long and 16 feet (5 meters) in diameter.

As of late Friday, the rocket was circling Earth in an orbit ranging in altitude between 92 miles and 145 miles (149-by-234 kilometers), according to U.S. military tracking data. The effects of aerodynamic drag from the upper layers of the atmosphere are gradually putting the brakes on the rocket’s velocity, which reduces the core stage’s altitude.

And the drag will only increase as the rocket gets closer to Earth. Some time Saturday or Sunday, the atmosphere will effectively capture the core stage when it no longer has the speed required to remain in orbit.

“The Long March 5B re-entry is unusual because during launch, the first stage of the rocket reached orbital velocity instead of falling downrange as is common practice,” the Aerospace Corp. said. “The empty rocket body is now in an elliptical orbit around Earth where it is being dragged toward an uncontrolled re-entry.”

The rocket will plunge back into atmosphere at nearly 17,000 mph (28,000 kilometers per hour), and most of the structure will burn up during re-entry. Friction generated from the rocket encountering air molecules will cause temperatures to build up to thousands of degrees.

But some fragments from the rocket could survive re-entry and fall back to Earth’s surface. While the rocket’s empty hydrogen and oxygen propellant tanks might burn up, denser components from the two main engines or strong pressure vessels could withstand the re-entry heating.

“The general rule of thumb is that 20–40% of the mass of a large object will reach the ground, but the exact number depends on the design of the object,” wrote Marlon Sorge, principal engineer at the Aerospace Corp.’s Center for Orbital and Re-entry Debris Studies. “In this case, we would expect about five to ten metric tons.

“Generally, for an upper stage, we see small and medium tanks survive more or less intact, and large engine components,” Sorge wrote. “The large tanks and the skin of this core stage are likely to come apart. We will also see lightweight material such as insulation fall out. The melting point of the materials used will make a difference in what remains.”

Exactly when and where the rocket will fall back into the atmosphere remained uncertain late Friday.

A forecast from the U.S. military’s 18th Space Control Squadron issued late Friday predicts the Long March 5B core stage will re-enter the atmosphere during a 12-hour period beginning around 11 p.m. EDT Saturday (0300 GMT Sunday).

The European Union’s Space Surveillance and Tracking Consortium said late Friday it expected re-entry of the Long March 5B between about 8:30 p.m. EDT Saturday and 8:30 a.m. EDT Sunday (0030-1230 GMT Sunday).

A map shows the expected ground tracks of the Long March 5B rocket’s core stage during the 12-hour re-entry window predicted by the European Union’s Space Surveillance and Tracking Consortium. Credit: EU SST Consortium

But uncontrolled re-entries are difficult to predict, and the forecast could change as ground-based radars continue tracking the Long March 5B rocket body in orbit. Changes in atmospheric drag and solar activity could move the time of re-entry earlier or later.

The error bars on the re-entry prediction will grow smaller as the time nears.

The rocket’s orbital track is inclined 41.5 degrees to the equator, meaning any debris from the rocket will fall in an area bounded by New York or Rome in the northern hemisphere, and Wellington, New Zealand, in the southern hemisphere.

“In this region, most part of the Earth surface is covered by ocean or uninhabited areas, so the statistical probability of an impact on the ground in populated areas is low,” the EU SST Consortium said. “These predictions however come with uncertainties as the object is uncontrolled, and a better estimation will only be possible a few hours before the actual re-entry.”

A one-minute error in predicting the re-entry time changes the location of potential falling debris by nearly 300 miles, or about 500 kilometers, according to the Aerospace Corp.

The consortium said the Long March 5B rocket stage is “one of the largest pieces of debris re-entering in the near past” and “deserves careful monitoring.”

“The probability that a piece of space debris will land on a city or a densely populated area is usually relatively small,” Sorge wrote. “What makes this re-entry particularly noteworthy is that it will occur between 41.5 deg N and 41.5 deg S latitudes, where the vast bulk of the world’s population lives.

“However, the statistical risk to any one person of being struck by falling space debris is so low that a colleague of mine jokes that if re-entry predictions put his house directly under the path, he’d go out with a camera and watch.”

The rocket’s plunge back to Earth this weekend is the second unguided re-entry of a massive Long March 5B core stage in less than a year.

The first launch of China’s Long March 5B rocket in May 2020 also left the core stage in orbit. The rocket dropped back into the atmosphere over the North Atlantic Ocean on May 11, 2020, minutes after a pass over the northern United States, including New York City.

Bits of wreckage from the Long March 5B rocket last year fell on Cote d’Ivoire, but no injuries were reported.

The Long March 5B core stage that re-entered the atmosphere last year was the most massive object to make an uncontrolled re-entry since the Soviet Union’s Salyut 7 space station in 1991, according to Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics who tracks global satellite and launch activity.

The Long March 5B core stage is more massive than other notable satellites that have plunged unguided back into Earth’s atmosphere in the last decade, such as China’s Tiangong 1 space lab, Russia’s failed Phobos-Grunt Mars probe, and NASA’s UARS atmospheric research satellite. It’s about one-quarter the mass of NASA’s Skylab space station, which made headlines when it fell to Earth over Australia in 1979.

Dead satellites and old rocket stages regularly re-enter the atmosphere, but re-entering objects with masses of more than a few tons are rare.

Additional Long March 5B rockets are set to launch new modules next year to assemble China’s space station. Unless China changes the design of the rocket’s core stage, there will be more uncontrolled rocket re-entry events after those missions.

Wang Wenbin, a spokesperson for the Chinese Foreign Ministry, said in a press conference Friday that it is “common practice” for upper stages of rockets to burn up while re-entering the atmosphere. He later incorrectly referred to the Long March 5B rocket body as an upper stage, and said that “most of its parts will burn up upon re-entry, making the likelihood of damage to aviation or ground facilities and activities extremely low.”

But no other launcher in the world leaves such a massive component to fall back to Earth in an uncontrolled manner.

“Controlled re-entries, particularly for a large object, require considerable planning and will have a significant impact on the design and payload capacity of the stage,” Sorge wrote. “Nevertheless, this is a preferred approach in international standards and is rapidly becoming a global norm.”

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Follow Stephen Clark on Twitter: @StephenClark1.


قراءة المزيد
– May 8, 2021
Chinese rocket Big segment set to fall to Earth