Large Chinese booster rocket set to fall back to Earth today – Spaceflight Now

Large Chinese booster rocket set to fall back to Earth today – Spaceflight Now

Large Chinese booster rocket set to fall back to Earth today – Spaceflight Now
This map shows the ground trace of the Long March 5B core phase during the two-hour reentry window beginning Saturday morning. Re-entry and debris imprint could occur anywhere along the route. Photo credit: Aerospace Corp.

The 22-ton core stage of a Chinese rocket is expected to fall back to Earth sometime Saturday, the third time in two years China has allowed such a large booster to re-enter the atmosphere uncontrolled. Unguided re-entry poses a small but avoidable risk to the world’s population, space debris experts said.

The Long March 5B rocket was launched on July 24 with the Wentian module bound for China’s Tiangong space station, carrying one of the heaviest payloads to be put into orbit in recent years. The Long March 5B rocket’s nearly 100-foot (30-meter) core stage fired its two hydrogen-powered engines for about eight minutes to launch the Wentian module into orbit.

Four strap-on boosters burned their fuel and were jettisoned minutes after liftoff, crashing into the South China Sea. But the design of the Long March 5B, one of the most powerful operational rockets in the world, means its core stage accelerates to orbital velocity.

Most launch vehicles carry an upper stage to complete the task of launching a payload into orbit and allow the booster to fall back to earth in the ocean or be salvaged for reuse, as SpaceX does with its Falcon 9 rocket.

Early Saturday, the Long March 5B core stage was forecast to reenter the atmosphere in a period between 1615 GMT (12:15 p.m. EDT) and 1815 GMT (2:15 p.m. EDT), according to a forecast by Aerospace Corp., a state-funded, non-profit research institute based in California.

The rocket’s orbit takes it between latitudes 41.5 degrees north and south during each one-and-a-half hour orbit around the Earth. The land between these latitudes is home to about 88% of the world’s population.

“On a global scale, it’s a low risk, but it’s an unnecessary risk and it can affect people, that’s why we’re talking about it,” said Ted Muelhaupt, a consultant at Aerospace Corp. and an expert on space debris re-entry.

It’s impossible to predict exactly when and where the rocket will reenter the atmosphere, but surviving debris will likely fall in a long, narrow footprint hundreds of miles long and up to tens of miles across. The rocket wreck will most likely fall into the sea or unpopulated areas.

This is the third time that China has exited an in-orbit Long March 5B nuclear stage to return to Earth unguided. The 2020 uncontrolled re-entry of the Long March 5B core first stage spread debris over Ivory Coast. Last year’s re-entry of the Long March 5B occurred over the Indian Ocean and no debris was found.

The window of uncertainty as to when the rocket will reenter the atmosphere is largely due to unknowns about the rocket’s orientation and the ever-changing density of the upper atmosphere, driven by solar activity causing atmospheric expansion or contact Mülhaupt.

The window shrinks as the time of re-entry approaches. Five days before reentry, experts estimated the window with an error of plus or minus a day. By Saturday morning, just hours before reentry, the error reduced to plus or minus an hour.

China’s Long March 5B takes off from Hainan Island’s Wenchang launch base on July 24. Photo credit: CASC

Aerodynamic drag will eventually slow the rocket’s speed enough for Earth’s gravity to pull back into the atmosphere, where most of the booster stage will burn up. Muelhaupt estimates that about 4 to 9 tons, or 20 to 40 percent of the rocket’s dry mass, will survive the searing heat of reentry and reach the surface.

Abandoned rocket bodies and dead satellites regularly re-enter the atmosphere. Around 50 man-made objects weighing more than a ton are uncontrolled re-entering the atmosphere every year, according to Muelhaupt.

But the Long March 5B core stage will be the sixth largest object to reenter the atmosphere, excluding the space shuttle, Muelhaupt said.

The Aerospace Corp. estimates the chance that any part of the Long March 5B core phase will kill or injure a person at 1 in 230 to 1 in 1,000, meaning there is a 99.5% chance of no casualties in reentry.

But US government policy requires space mission managers to ensure the risk of death or injury from re-entry does not exceed 1 in 10,000. The risk of damage from the re-entry of the Long March 5B is estimated to be at least 10 times the standard risk threshold for US space missions.

“When it does, it will certainly cross the 1 in 10,000 threshold, which is the generally accepted guideline,” Mülhaupt said. “And one of the reasons we’re paying special attention to this is that in May 2020, in the first test launch, this debris dropped debris in Africa.”

The risk of reentry for a single person is even lower — 6 in 10 trillion, according to the Aerospace Corp estimate.

“In reality, there are a number of things you can do about these types of things, especially if you are thinking ahead of your mission,” said Marlon Sorge, executive director of the Aerospace Center for Orbital and Reentry Debris Studies.

For example, designers can choose materials that are more likely to burn upon re-entry, reducing the risk of remaining debris hitting the Earth’s surface.

“With the rocket bodies, they’re just so big that it doesn’t really matter what you do during the design phase in terms of what you make them out of. You have huge chunks of metal where the engines are,” said Sorge.

“But there are other approaches you can take if you think, and one of those is controlled re-entry,” Sorge said. “Basically, once you’re done delivering your payload, you turn your rocket around, fire the engine, and drive it back into the ocean, usually somewhere where there’s no population. You do, and you’ve pretty much mitigated the risk right there. And that’s one of the things the US government is doing to mitigate these types of risks.”

After the last launch and re-entry of the Long March 5B last year, NASA Administrator Bill Nelson said China “failed to uphold responsible standards regarding its space debris.”

“Space nations must minimize the risks to people and property on Earth from the re-entry of space objects and maximize transparency regarding these operations,” Nelson said in a statement last year.

Wang Wenbin, a spokesman for China’s Foreign Ministry, told a news conference last year that it is “common practice” for the upper stages of rockets to burn up as they re-enter the atmosphere. He misidentified Long March 5B’s missile body as an upper stage and said that “most of its parts will be burned up on re-entry, making the likelihood of damage to airborne or ground-based facilities and activities extremely low.”

But no other launch vehicle in the world leaves such a massive component in orbit to fall back to Earth. Dead satellites and old rocket stages regularly re-enter the atmosphere, but objects with masses greater than a few tons are rare.

“Why are we worried? Well it caused property damage last time (a Long March 5B reentered),” Muelhaupt said this week. “So people have to prepare.

“And besides, it’s not necessary,” he said. “We have the technology not to have this problem. Every time you see a Falcon 9 land, that core phase isn’t going to fall randomly anywhere. Intentionally throwing things into the ocean when they are big enough to cause harm is the practice we want to encourage.”

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Follow Stephen Clark on Twitter: @Stephen Clark1.

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