Rocket debris risk downplayed
China has defended its space programme and described the chances that a falling rocket could cause damage as ‘‘extremely low’’, as fragments from the spent booster are projected to reach Earth.
Chinese officials were ‘‘closely observing’’ the re-entry of the Long March-5B rocket and would ‘‘promptly report news about the situation to the outside world’’, Foreign Ministry spokesman Wang Wenbin said.
China has drawn criticism after the launch last month of the rocket ferrying the core module of the Tianhe space station, with plans to let the booster fall back through the atmosphere and land where it may.
Because the booster will be reentering Earth’s atmosphere at about 29,000kmh, it is almost impossible to predict where the trail of debris will hit until hours before re-entry, which is expected to occur this morning, within about an 11-hour margin of error, according to California-based Aerospace Corp.
Most satellites and other manmade objects are small enough that they burn up in the atmosphere. The Chinese rocket, however, is bigger, so there are concerns that pieces will survive to hit the ground.
The US Air Force Space Track Project has estimated that the debris will crash in a remote desert outside Mary, Turkmenistan, although researchers have warned that the projected site could be wildly off-base.
Part of the uncertainty stems from the rocket’s tumbling motion as it passes through the mesosphere, or an outer layer of Earth’s atmosphere. Changes in air density will slow it down at different rates that are tricky to predict. Modelling how the mesosphere could affect the rocket’s speed is also complicated by constantly fluctuating conditions on the Sun.
Wang said the rocket was designed so that most of its components would burn up in the atmosphere during re-entry. ‘‘This is standard international practice. The probability of causing harm to aviation activities and the ground is extremely low.’’
When an object falls out of orbit, how much debris makes it to the ground depends on the object’s size, shape and mass, and the melting temperatures of the materials used. Even the biggest satellites produce a comparatively tiny amount of debris.
Although researchers say the chances of the debris hitting populated areas are minuscule, some have called the Chinese mission irresponsible because the Long March, at 21 tonnes and almost 30 metres long, would be one of the largest objects to ever re-enter Earth’s atmosphere on an uncontrolled trajectory.
A controlled re-entry would involve firing thrusters to slow an object down enough to enter Earth’s atmosphere over a predetermined remote area or ocean. It is not an exact science, but it reduces the chance of debris hitting populated areas. Chinese officials did not make the necessary preparations for a controlled re-entry.
Last year, debris from another Chinese Long March rocket landed in Africa, drawing a rebuke from then-Nasa Administrator Jim Bridenstein.
Chinese state media this week reacted angrily to the international scrutiny, saying its launch was being unfairly maligned.
The Tianhe station, which is expected to be the only operational space station after the retirement of the International Space Station in the next four years, has been a point of national pride for China.
‘‘Hyping of the so-called China space threat ... it’s an old trick used by hostile powers every time they see technological breakthroughs in China, as they are nervous,’’ the state-run Global Times quoted Song Zhongping, a former Chinese military officer and popular TV commentator, as saying.