This weekend, a Chinese rocket booster, weighing nearly 23 tons, came rushing back to Earth after spending more than a week in space—the result of what some critics, including NASA Administrator Bill Nelson, have attributed to poor planning by China. Pieces of the rocket, dubbed Long March 5B, are believed to have splashed down in the Indian Ocean near the Maldives, and no one was injured.

But the event has shown the potential dangers that come from humanity's expanding presence in space, said Hanspeter Schaub, professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences.

Schaub is an engineer with an eye for the myriad bits of junk that circle our planet—from meteors the size of grains of dust to manmade rocket stages as big as school buses. As humans launch more objects into space, he said, this debris may increasingly threaten the safety of satellites and human astronauts in orbit. In 2009, a decommissioned Russian satellite crashed into an active satellite called Iridium 33, sending a cloud of shrapnel hurtling around the planet.

Since the launch of Sputnik, the Earth's first artificial satellite, in 1957, more than 41,500 tons of manmade objects have been placed in orbit around the sun, the Earth, and other planetary bodies. Since that time, the majority of objects, such as rocket bodies and large pieces of space debris, re-entered the Earth's atmosphere in an uncontrolled way, posing a potential hazard to people and infrastructure. Predicting the re-entry date and time is a challenging task, as one needs to specify the density of the upper Earth atmosphere that strongly depends on solar activity which, in turn, is hard to predict. Earth atmosphere can become very heated due to solar activity which causes it to expand, and a satellite can decay in its orbit and fall back to the Earth due to the effect known as atmospheric drag.

Two companies, OneWeb and SpaceX, are racing to put fleets of thousands of communication satellites into orbit. In March, they had their first near miss. Avoidance maneuvers were successful, but how many more close calls will they face in the future?

SpaceX has already launched over a thousand of its Starlink global broadband internet satellites, and competitor OneWeb has lofted 146 of its own. Both companies—and several others—are actively prepping for dozens of more launches and thousands of more satellites.

But while space is a big place, orbits are a precious resource, especially with so many satellites already up and so many more planned. near misses are unavoidable, as both companies found out on March 30th, when they received several "red alerts" from the US Space Force's 18th Space Control Squadron, warning of a possible collision.

The red alert came just five days after OneWeb launched 36 satellites from Russia. While the OneWeb constellation orbits at a higher altitude than Starlink, they must pass through those orbits to get to their operational location.