Or no time at all. “Oumuamua, the strange little object that visited our solar system in 2017, was discovered not before but after its closest approach to Earth. Although ‘Oumuamua is an interstellar object and does not come from the Oort cloud, the suggestion still holds; One of these objects could sneak up on us and the Earth would be defenseless.

One way to prepare for these objects is to better understand their fundamental properties, including their size and composition. To this end, my colleagues and I are working on characterizing new long-period comets. The largest known star, Bernardinelli Amber, was discovered just three years ago and is about 120 kilometers across. Most known comets are much smaller, from one to a few miles, and some smaller ones are too faint for us to see. But newer telescopes are helping. In particular, the Rubin Observatory’s decade-long Legacy Survey of Space and Time, starting in 2025, could double the list of known Oort cloud comets, which currently stands at about 4,500.

The unpredictability of these objects makes them a challenging target for spacecraft, but the European Space Agency is preparing a mission to do just that: Comet Interceptor. Launch is scheduled for 2029 and the probe will park in space until a suitable target emerges from the Oort cloud. Studying one of these ancient and pristine objects could provide scientists with clues about the origins of the solar system.

As for the comets that are now near Earth, it’s okay to look up. Unlike the comet in the DiCaprio film, these two will not crash into Earth. The closest Tsuchinshan ATLAS that will reach us is about 44 million miles (70 million kilometers); C/2024 S1 (ATLAS), about 80 million miles (130 million kilometers). Sounds like a long shot, but in space it’s a near miss.

James Wray is Professor of Earth and Atmospheric Sciences at the Georgia Institute of Technology

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