Discovered on July 1, the comet known as 3I/Atlas will pass within 269 million kilometres of our planet on Friday, the closest it gets on its grand tour of the solar system.
NASA continues to aim its space telescopes at the visiting ice ball, estimated to be between 440 metres and 5.6km in size.
But it is fading as it exits, so now is the time for backyard astronomers to catch it in the night sky with their telescopes.
The comet will come much closer to Jupiter in March, zipping within 53 million kilometres.
It will be the mid-2030s before it reaches interstellar space, never to return, said Paul Chodas, director of NASA's Center for Near Earth Object Studies.
It is the third known interstellar object to cut through our solar system.
Interstellar comets like 3I/Atlas originate in star systems elsewhere in the Milky Way, while home-grown comets like Halley's hail from the icy fringes of our solar system.
A telescope in Hawaii discovered the first confirmed interstellar visitor in 2017.
Two years later, an interstellar comet was spotted by a Crimean amateur astronomer.
NASA's sky-surveying Atlas telescope in Chile spotted comet 3I/Atlas while prowling for potentially dangerous asteroids.
Scientists believe the latest interloping comet, also harmless, may have originated in a star system much older than ours.
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