This is what happens when spacecraft fall from the sky

Imagine 3 African elephants falling towards your house while on fire. That’s about the weight of the 22-ton Long March-5B rocket launcher that landed in the Indian Ocean near the Maldives on 9 May 2021.

The Chinese rocket was launched on 29 April, before falling into an uncontrolled re-entry into the atmosphere. Because of its size, and the nature of its re-entry, it was difficult to track for the ten days since the launch. The possible crash zone included most of the planet’s surface, causing people to follow the news about the rocket’s re-entry in a panic. 

The international space travel community were unhappy with the risk the re-entry posed. The United States of America’s (US) National Aeronautics and Space Association (NASA) called the incident irresponsible. Jonathan McDowell, an astrophysicist, called the uncontrolled re-entry a reckless gamble. The chance of the rocket hitting the ocean instead of any land mass may have been 70%, but many believe that the risk was just too great to ignore. 

China’s response

The foreign ministry spokesperson of China, Hua Chunying, said in a press release that China is being held to the unfair standards of other space-faring nations such as the US. She said that China did their best to control the situation by tracking the rocket’s trajectory as closely as possible, and frequently releasing statements on the situation as far in advance as they could. They also continually updated projections of the re-entry with the help of international mechanisms.

She claimed that Western media romanticised a similar case of a SpaceX rocket falling towards the states of Washington and Oregon with headlines like: “Shooting stars lighting up the night sky,” but heavily criticised China for the same thing. 

Safety measures and contingencies for re-entries

Prof Attie Jonker, an associate professor at the North-West University’s (NWU) Faculty of Mechanical Engineering, and an expert in light structures and aircraft design, said that uncontrolled re-entries are just that: uncontrolled. They’re often caused by a collision, or some sort of unforeseen problem, and control centres can do nothing more than watch and try to predict the trajectory of the object. 

In all cases, the initial trajectory of any object being launched into space is carefully calculated, planned and controlled, and re-entry into the atmosphere is only uncontrolled from the moment control centres can no longer alter the flight path, he said. He equates launching anything into space to firing a bullet from a rifle. A good marksman can direct the bullet to the best of his ability and have a pretty good idea of where it will go, but the moment the bullet leaves the barrel, the marksman is no longer in control. 

He also said: “Additionally, one must take into account that most boosters from before the SpaceX era had uncontrolled re-entries”. Space travel is still in the experimental phase for China, while the US and other major space-faring nations and companies have had the chance to perfect their methods and missions. This was not without accidents on their part, and in the early days of NASA’s space flight programme, they too had several accidents and uncontrolled re-entries.

Recent major instances of uncontrolled re-entries

This isn’t the first time a piece of space debris made an uncontrolled re-entry that posed a major risk. In 1978, some debris from the Skylab space station crashed over Australia, after making an uncontrolled re-entry. In 2003, 100 tons of loose space debris broke apart over Texas from the space shuttle Columbia. In 2018, China’ Tiangong 1 space station also exploded, with another Long March B5 launcher making an uncontrolled re-entry in 2020. 

Controlled and uncontrolled re-entries

Whether the criticism of China was fair or not, the risk of uncontrolled re-entries, as well as the options of controlled re-entries or lower orbit, remain. Controlled re-entries deploy thrusters, parachutes and calculated use of atmospheric pressure and friction to slow down. In other words, they use thrusters to both slow down their speed to below the speed of sound, and to control their direction. 

Then, they use parachutes and air brakes to further slow down the object. Air brakes are flaps and mechanisms that extend to increase drag (friction) to slow down an object. This can consume as much fuel on re-entry to achieve as it does in the initial launch. 

Uncontrolled re-entries are re-entries of satellites, objects or launch debris that do not have these mechanisms in place, and were often intended to be sent into low orbit, but something went wrong and caused them to change trajectory. They are then pulled into the atmosphere by earth’s gravity, and without the mechanisms in place to direct or slow them down, they often end up increasing speed before re-entering the atmosphere and then the friction caused by air slows them down. 

This also heats them up significantly, since they don’t have the heat shields specifically designed for re-entry, and many of these objects explode into pieces before either burning out in the lower atmosphere, or landing. These objects typically land in the ocean, which covers about 70% of the globe’s surface, but some have been known to hit populated areas. Upon re-entry most space debris lose between 10 to 40% of their mass.

Sending a launcher into low orbit is often the safest and most cost-effective method, as it doesn’t require the extra fuel and mechanisms for a controlled re-entry, and reduces the risk of harm, should even a controlled re-entry go wrong. This mechanism involves calculating the trajectory of the launch, and once the rocket launcher detaches, using the leftover fuel to direct it into a safe orbit around the earth, just high enough that it won’t be pulled back in by earth’s gravity. 

Due to increased costs of double the fuel and added mechanisms to prepare these objects for controlled re-entry, it is more economically viable to send an object into low orbit than to have a controlled re-entry for something such as a rocket launcher or satellite. Controlled re-entries are only necessary to return crew, shuttles and space stations to the earth that need to be salvaged for scientific reasons.  

Tracking uncontrolled re-entries to minimize risk

Objects with a mass of more than 800 kg can break up into as many as 300 fragments. Around 8.1% of object fragments actually make it to the surface of the earth. The rest burn up in the atmosphere. 70% of the world is covered in ocean, leaving a 30% chance for space debris from an uncontrolled reentry hitting any land mass. 

The majority of land is uninhabited, leaving the risk of being hit by space debris at less than one in a billion, according to the European Space Agency. Though there was a risk to the uncontrolled re-entry of the Long March rocket launcher, the actual risk to human life or property was significantly low enough that China felt free to take the risk, despite the criticism they received for taking the risk at all. 

Case Rijsdijk, one of the astronomers from the Astronomical Society of Southern Africa, told threestreamsmedia that it is possible to track uncontrolled re-entries with very sophisticated equipment, and the knowledge that an object’s orbit is collapsing. He also said that sending an object into lower orbit without the facilities to direct it to a safe landing zone, should it make an uncontrolled re-entry, is very irresponsible.