HELSINKI, FINLAND — In what seems to be a global disaster caused by a science project gone awry in a science fiction movie, the mission launcher for China’s first module that was launched recently for its space station into orbit is slowly and unpredictably heading back to Earth.
The variant of China’s largest rocket, known as Long March 5B, successfully launched the 22.5-metric-ton Tianhe module from Wenchang and after the Tianhe separated from the core stage of the launcher following 492 seconds of flight; it has directly entered its planned initial orbit.
However, this core stage is now also in orbit and is likely to make an uncontrolled reentry over the next few days or week as growing interaction with the atmosphere drags it back to Earth. If so, it will be one of the largest instances of uncontrolled reentry of a spacecraft and could potentially land on an inhabited area.
In most cases, expendable rocket first stages do not reach orbital velocity and they reenter the atmosphere and land in a pre-defined reentry zone. But other larger second stages perform deorbit burns to lower altitude to reduce time in orbit and lower chances of collisions with other spacecraft or to immediately reenter the atmosphere.
With the threat of China’s mission launcher reentering the earth atmosphere, scientists have speculated that the Long March 5B core would perform an active maneuver to deorbit itself, but that appears not to have happened. At a Wenchang press conference on Thursday, Long March 5B launch vehicle’s commander-in-chief Wang Jue stated (Chinese) that China’s largest rocket had seen improvements over the first launch, but a possible deorbit maneuver was not stated.
Still, ground-based radars used by the United States. military to track spacecraft and other objects in space have detected an object and it has cataloged the object as the Long March 5B rocket body. Now designated 2021-035B, the roughly 30-meter-long, five-meter-wide Long March 5 core stage is in a 170 by 372-kilometer altitude orbit traveling at more than seven kilometers per second.
The first launch of the Long March 5B likewise saw the first stage reach orbit and make an uncontrolled reentry six days later. Reentry occurred over the Atlantic Ocean and had the event taken place 15 to 30 minutes earlier, debris not destroyed by the heat of reentry could have landed on US soil. The incident drew criticism from then NASA Administrator Jim Bridenstine.
To date, where and when the new Long March 5B stage will land is impossible to predict but what is certain is that the decay of its orbit will increase as atmospheric drag brings it down into denser traction. The speed of this process depends on the size and density of the object and variables include atmospheric variations and fluctuations, which are themselves influenced by solar activity and other factors.
The high speed of the rocket body means it orbits the Earth roughly every 90 minutes and so a change of just a few minutes in reentry time results in reentry point thousands of kilometers away.
The Long March 5B core stage’s orbital inclination of 41.5 degrees means the rocket body passes a little farther north than New York, Madrid and Beijing and as far south as southern Chile and Wellington, New Zealand, and could make its reentry at any point within this area.
The most likely event will see any debris surviving the intense heat of reentry falling into the oceans or uninhabited areas, but the risk remains of damage to people or property.
Spaceflight observer Jonathan McDowell told SpaceNews that the previous Long March 5B launch saw the most massive uncontrolled reentry in decades and the fourth biggest ever.
“The Long March 5B core stage is seven times more massive than the Falcon 9 second stage that caused a lot of press attention a few weeks ago when it reentered above Seattle and dumped a couple of pressure tanks on Washington state,” McDowell noted.
He expressed hope that China had enhanced the core stage to perform a controlled deorbit after separating from Tianhe.
Holger Krag, head of the Space Safety Programme Office for the European Space Agency, says from their experience, there is an average amount of mass of about 100 tons re-entering in an uncontrolled way per year. “This relates to about 50-60 individual events per year.”
“It is always difficult to assess the amount of surviving mass and number of fragments without knowing the design of the object, but a reasonable “rule-of-thumb” is about 20-40% of the original dry mass,” Krag pointed out.
Components made of heat-resistant materials, such as tanks and thrusters made stainless steel or titanium, can reach the ground. Surviving objects will fall vertically after deceleration and travel at terminal velocity.
The largest and most famous incident was the 1979 reentry of NASA’s 76-ton Skylab, whose uncontrolled reentry scattered debris across the Indian Ocean and Western Australia.
A night time reentry could make for spectacular viewing, as with a recent reentry of a Falcon 9 second stage, with debris fortunately not causing harm.
China’s 8-ton Tiangong-1 Spacelab made a high-profile uncontrolled reentry in 2018, while the successor Tiangong-2 was deorbited in a controlled manner in 2019. (Sourced online by TCabrera/AIMTVN)