Asteroid 1998 QE2 will make its closest pass to Earth on May 31 at 1:59 p.m. PDT.

Scientists are not sure where this unusually large space rock, which was discovered 15 years ago, originated from. But the mysterious sooty substance on its surface could indicate it may be the result of a comet that flew too close to the sun, said Amy Mainzer, who tracks near-Earth objects at Jet Propulsion Laboratory in La Cañada Flintridge. It might also have leaked out of the asteroid belt between the orbits of Mars and Jupiter, she said.

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We will know more after the asteroid zips closer to Earth and scientists using the Deep Space Network antenna in Goldstone, Calif., and the Arecibo Observatory in Puerto Rico can get a better look at it. Astronomers at both observatories plan to track it closely from May 30 to June 9, according to a JPL release.