Two inactive satellites orbiting Earth may collide high above Pittsburgh tonight at 6:39 p.m. EST. The odds of a collision, recently pegged at 1 in 100, now sit at 1 in 20, according to LeoLabs, a company that runs a ground-based radar array that monitors collision risks for objects in low-Earth orbit.

The two defunct satellites — the Infrared Astronomical Satellite (IRAS) and the Gravity Gradient Stabilization Experiment (GGSE-4) — were initially expected to zip past each other with about 40 feet (12 meters) to spare. But now, revised calculations show a potential collision might be more likely than we first thought.

“Since we learned that GGSE 4 has a deployed 18m [59 foot] boom and we do not know which direction it is facing relative to IRAS, this changes the assumptions used in computing collision risk,” LeoLabs said in a tweet thread today.

The next tweet continues: “Adjusting our calculation to account for larger object sizes (by increasing our combined Hard Body Radius from 5m to 10m), this yields an updated collision probability closer to 1 in 20.”