A few days ago, an inscrutable interstellar interloper made its closest approach to Mars, where a fleet of international spacecraft seek to unravel the red planet's ancient mysteries. Several of the probes encircling Mars took a break from their usual activities and turned their cameras toward space to catch a glimpse of an object named 3I/ATLAS, a rogue comet that arrived in our Solar System from interstellar space and is now barreling toward perihelion—its closest approach to the Sun—at the end of this month. This is the third interstellar object astronomers have detected within our Solar System, following 1I/ʻOumuamua and 2I/Borisov discovered in 2017 and 2019. Scientists think interstellar objects routinely transit among the planets, but telescopes have only recently had the ability to find one. For example, the telescope that discovered Oumuamua only came online in 2010. Detectable but still unreachable Astronomers first reported observations of 3I/ATLAS on July 1, just four months before reaching its deepest penetration into the Solar System. Unfortunately for astronomers, the particulars of this object's trajectory will bring it to perihelion when the Earth is on the opposite side of the Sun. The nearest 3I/ATLAS will come to Earth is about 170 million miles (270 million kilometers) in December, eliminating any chance for high-resolution imaging. The viewing geometry also means the Sun's glare will block all direct views of the comet from Earth until next month. The James Webb Space Telescope observed interstellar comet 3I/ATLAS on August 6 with its Near-Infrared Spectrograph instrument. Credit: NASA/James Webb Space Telescope Because of that, the closest any active spacecraft will get to 3I/ATLAS happened Friday, when it passed less than 20 million miles (30 million kilometers) from Mars. NASA's Perseverance rover and Mars Reconnaissance Orbiter were expected to make observations of 3I/ATLAS, along with Europe's Mars Express and ExoMars Trace Gas Orbiter missions. The best views of the object so far have been captured by the James Webb Space Telescope and the Hubble Space Telescope, positioned much closer to Earth. Those observations helped astronomers narrow down the object's size, but the estimates remain imprecise. Based on Hubble's images, the icy core of 3I/ATLAS is somewhere between the size of the Empire State Building to something a little larger than Central Park.