China defended its space program and described the chances that a falling rocket would cause damage as “extremely low” on Friday, roughly a day before fragments from the spent booster are projected to reach Earth.
China has drawn criticism after its 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 the Earth’s atmosphere at about 18,000 mph, it’s almost impossible to predict where the trail of debris will hit until hours before reentry, which is expected to occur late Saturday night Eastern time, within about an 11-hour margin of error, according to the California-based Aerospace Corp.
Objects orbiting Earth stay in space through a constant balance. While gravity tugs them toward Earth, their forward momentum keeps them in space. Over time, minuscule amounts of air drag slow an object, eventually knocking it out of orbit.
Most satellites and other man-made objects are small enough that they burn up in the atmosphere. The Chinese rocket, however, is bigger — so concern exists that pieces will survive to hit the ground.
As of early Friday, the U.S. Air Force’s Space Track Project estimated the debris will crash in a remote desert outside Mary, Turkmenistan, although researchers warned that with roughly a day left until reentry, 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 slow it down at different rates that are tricky to predict. Modeling how the mesosphere could affect the rocket’s speed is also complicated by constantly fluctuating conditions on the sun.
The rocket’s gradual loss of momentum means it’s approaching Earth’s surface at just 0.3 mph.
Wang said Friday that the rocket is designed so that most of its components will burn up in the atmosphere during reentry. “This is standard international practice,” he said. “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, mass and the melting temperatures of all 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 indeed minuscule, some have called the Chinese mission irresponsible because the Long March husk, at 21 metric tons and almost 100 feet long, would be one of the largest objects to ever reenter the Earth’s atmosphere on an uncontrolled trajectory.
A controlled reentry would entail firing thrusters opposite to the direction of travel, slowing an object down enough to enter Earth’s atmosphere over a predetermined remote area or ocean. It’s not an exact science, but it reduces the chance of debris impacting populated regions.
Chinese officials did not make the necessary preparations for a controlled reentry.
As part of its design, the Chinese sent the Long March into low orbit, where it has been drifting for days at high speed, as opposed to other rocket launches that have let the booster fall away more quickly and along a more predictable arc under the Earth’s gravitational pull.
Astrophysicists have expressed concern that episodes like this, featuring potentially hazardous corner-cutting, could become more frequent in the absence of stricter international law.
“There is at least a liability convention that says if your country’s space object crashes in my country, I can demand compensation, but I have to give the object back to you,” said Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics. “What there isn’t at the moment is what I call a ‘reckless driving law.’ You only get to complain if it actually crashes. [China] should not have left this big thing in orbit, but if they play the odds and get lucky and it crashes in the ocean, we don’t really have any comeback.”
Although the chances of actual human injury are astronomically small, close calls do happen: A farmer in eastern Washington state found part of a pressurized helium container from a SpaceX rocket launch in April. Last year, debris from another Chinese Long March rocket landed in Africa and drew a rebuke from Jim Bridenstine, the NASA administrator at the time.
Chinese state media this week reacted angrily to the international scrutiny, saying its launch was being unfairly maligned. State media reporters juxtaposed how U.S. media outlets covered China’s “out-of-control space junk” in contrast to the SpaceX wreckage, which was described as creating “a dazzling light show” over Pacific Northwest skies.
The Tianhe space 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, which has also completed a flurry of successful lunar and Mars missions and has spoken of putting humans back on the moon.
“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 Global Times quoted Song Zhongping, a former Chinese military officer and popular television commentator, as saying.
U.S. defense officials said this week that they were tracking the Long March debris and had no plans to shoot it down.