Just last week, China sent the first module of its new space station to orbit. Today, space agencies around the world are anxiously watching the sky, as the rocket used for the journey is falling back to Earth. But we don’t know where, or when.

This will likely be the heaviest object to make an uncontrolled reentry into Earth’s atmosphere in over 20 years, according to experts at the European Space Agency, but we can’t know for sure. The Chinese government has a habit of keeping certain information under wraps, especially when it involves military matters, advanced tech, or the Uyghurs, so we simply don’t know how heavy their Long March 5 rocket (the one that’s making the reentry) actually is.

Either way, specialists and sensors around the world are keeping an eye on the situation as it unfolds, and, hopefully, the craft won’t fall on anyone’s head — or on something important.

The Great Leap Back Down

Whether or not this was initially intended is still unknown — there is some debate raging around the development process of the rocket — but the Long March 5 relies on uncontrolled reentries by design. That, by itself, isn’t unheard-of. Many rockets in the past have employed similar reentry approaches.

What is causing a lot of headaches for the global space community is that the rocket relies on uncontrolled reentries and we know next to nothing about its characteristics. Most importantly, we don’t know its mass, which makes calculating its behavior through the atmosphere impossible. In turn, this means we can’t predict when or where it’s going to finally come down with any degree of accuracy.

Reusable rockets, like the ones being tested by Musk’s SpaceX rely on controlled reentry, giving them the ability to change speed and course while flying back down to the surface.

“[The CZ-5B’s] design is not described in detail in public sources but it is estimated to be cylindrical with dimensions of 5 x 33.2 meters (16.4 x 108.9 feet) and a dry mass of about 18 metric tons (19.8 tons),” the ESA wrote for Deutsche Welle.

Right now, rocket’s core is tumbling through low orbit and is expected to start its descent through the atmosphere in the coming days.

The core is the part of the rocket that actually deployed the space station module to orbit. It was expected to start making a controlled reentry into the atmosphere after disengaging from the rocket proper and finishing its mission, however, that didn’t happen.

Ground radar picked up on the core afterwards, as it was travelling at speeds in excess of 15,840 mph (25,490 km/h). It was designated ‘object 2021-035B’ by the U.S. military, and you can see it being tracked here.

This event was not received well by the international community, especially given that this isn’t the first rocket from a Chinese spacecraft to make an uncontrolled reentry to Earth. The last time this happened, in 2017, the Tiangong-1 space station luckily landed in the Pacific Ocean, and nobody was hurt. But there are no guarantees that the same good luck will help us again. As such, several agencies and experts have called for tighter regulation regarding space traffic, especially on the matter of reentry.