The coronavirus pandemic has accustomed people to the cruel reality of “superspreader” events, when an unusually contagious virus, like the one behind COVID-19, infects many people at once.

Now the world is preparing for a superspreading contingency of another sort. On Thursday night, two decommissioned spacecraft could collide, spraying dangerous space junk across the sky. The encounter is expected to occur at 8:56 p.m. ET, according to estimates from LeoLabs, a sky-mapping startup based in Menlo Park, Calif.

The star-crossed machines are an old Soviet satellite formerly used for navigation and a discarded part of a Chinese rocket. The approximately car-sized artifacts are set to cross paths more than 600 miles above the South Atlantic, off the coast of Antarctica.

LeoLabs puts the odds of a smashup at roughly 10%. Because the crafts are orbiting earth at blazingly fast speeds (they complete a full orbit about every 90 minutes), projecting ahead compounds margins of error in the company’s models, creating uncertainty. Other factors, such as atmospheric conditions, like the nudging effects of solar radiation, limit precision.

Even though Daniel Ceperley, LeoLabs’ chief executive, pegs the probability of a smashup at about 1-in-10, he deems the situation “extremely high risk.”

“In addition to being exceptionally close, these objects are also very large—and I mean, massive,” Ceperley tells Fortune. “That’s a big problem because if they hit, they’ll turn into a lot of fragments of debris,” which will, he says, “spread out into a shell around earth.”

According to its most recent reports, the European Space Agency estimates that, as of February, there were more than 34,000 pieces of space debris greater than 10 centimeters in size circling Earth.

Jonathan McDowell, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics, says that, if Thursday’s collision indeed happens, it will cast vast amounts of shrapnel into low-earth orbit, the region of the upper atmosphere inhabited by many commercial and government satellites. The quantity of space junk could increase by 10-to-20%, he estimates.

More space junk means more future collisions. As that region of the sky gets polluted, it amplifies an “already significant” risk of collisions, he says.

That would have big consequences for people who increasingly rely on data beamed from space. Space is becoming a fertile frontier for businesses and governments alike.

“It would not be surprising if one or more of these active satellites were damaged or destroyed by the resulting debris over the next few years,” McDowell says. “If such collisions continue” over the course of decades, he says, “then key orbital regimes could become unusable.”

The possible collision will likely not be visible to the naked eye. Ceperley says his firm will be using its three radar arrays—in Texas, Alaska, and New Zealand—to observe the aftermath of the crossing, scanning for any signatures of fragments that would indicate a dreaded bullseye.

Then, Houston, we would have a major problem.