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 originate in the Oort cloud, the statement still applies; 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 basic properties, including their size and composition. With this goal in mind, my colleagues and I are working to characterize new long-period comets. The largest known, Bernardinelli-Bernstein, discovered only three years ago, has a diameter of about 120 kilometers. Most known comets are much smaller, from one to several kilometers, and some smaller ones are too faint to see. But newer telescopes help. In particular, the Rubin Observatory's decade-long Legacy Survey of Space and Time, due to launch in 2025, could double the list of known Oort cloud comets, which now 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. With a launch planned for 2029, the probe will park in space until a suitable target emerges from the Oort Cloud. Studying one of these ancient and pristine objects could give scientists clues about the origins of the solar system.
As for the comets now near Earth, it's fine to look up. Unlike the comet in the DiCaprio movie, these two won't crash into Earth. The closest Tsuchinshan ATLAS will get to us is about 70 million kilometers; C/2024 S1 (ATLAS), approximately 80 million miles (130 million kilometers). Sounds like a long way, but in space it's almost an accident.
James Wray is a professor of earth and atmospheric sciences at New York University Georgian Institute of Technology
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