The 21-ton Long March 5B rocket stage was left as a part of the Wentian space station module, which was launched on Sunday and docked with the Tiangong space station.
However, the booster remains hurtling around the planet and while scientists believe it will crash to Earth on 31 July they cannot be sure exactly when it will happen.
“The predictions are highly sensitive to the modeling assumptions including how we think the Sun will affect the earth’s atmosphere which affects how quickly an object falls out of orbit”, wrote researchers from the Aerospace Corporation’s Center for Orbital Reentry and Debris Studies (CORDS) which has been tracking the rocket.
“The Sun’s activity, like solar flares, is one of the main uncertainties affecting our ability to accurately predict reentries. The Sun is pouring out a lot of energy which heats the Earth’s atmosphere.”
The rocket’s chaotic descent comes as the Sun is two years into its 11-year solar cycle, which increases the chance of giant explosions such as solar flares or coronal mass ejections, spewing light, energy and solar material into space.
These eruptions are capable of releasing 100,000 times more energy than all the power plants on Earth generate throughout a year and have often been the cause of trouble for satellite launches.
Solar weather has also been causing satellites to drop out of their orbits, scientists believe, noting that over the last half of the decade the satellites have been sinking about two and a half kilometers a year - but the sink rate between December and April has been 20 kilometers per year.
“If the amount of energy changes even a little, as with a solar flare, the atmosphere will expand or shrink — changing how much it pulls on a reentering object and affecting the timing. Predicting exactly what the sun will do is notoriously difficult and a major uncertainty in reentry predictions”, the researchers continue.
It is not yet known how much of the rocket will crash to Earth, but generally between 20 and 40 per cent of the craft will become debris. However, foreign ministry spokesman Zhao Lijian has said that this rocket uses a design that means most components will be destroyed during reentry, with a very low probability of causing harm to aviation and the ground.
This is the third time that China has left a rocket to descent uncontrolled to Earth. In May last year, a rocket that carried a central part of the Tianhe space station module shot around the Earth so quickly that it was impossible to say where it would land. The debris circled the Earth once every 90 minutes.
If the rocket had re-entered the atmosphere above a populated area, the result would have been akin to a small plane crash scattered over 100 miles. The year before, a similar prototype craft came within 13 minutes of hitting New York City.