Other than its impressive size for a near-Earth asteroid, little was known about JO25. It was discovered in 2014 by the Catalina Sky Survey in Arizona, part of NASA and the University of Arizona's Near Earth Object (NEO) Observations Project, who were really only able to discern its size and orbit – and that it has a perplexingly reflective surface.
But as the asteroid approached Earth, we learned more about it. It has a "peanut"-esque shape. It's larger than previously estimated, with the size upped from 2,000 feet across to 3,280.
In addition to Slooh, NASA and the National Science Foundation announced their intentions to observe the mysterious asteroid with their radio telescopes. The Minor Planet Center has designated JO25 as a Potentially Hazardous Asteroid, as all space rocks at least one kilometer (0.6 miles) across that come within 4.3 million miles of Earth are.
It also means that there's a non-zero chance of it striking Earth one day – although that chance is still extremely low. Even if JO25 slams into Earth on its next close approach, that won't be until the 26th century or so.
"Although there is no possibility for the asteroid to collide with our planet, this will be a very close approach for an asteroid this size," NASA said in a statement. "Astronomers plan to observe it with telescopes around the world to learn as much about it as possible."
JO25 is the largest asteroid to make a close approach to Earth in 13 years. The last asteroid to outsize it was the 3.1-mile Toutatis, which in 2004 came even closer to Earth than JO25 did.
In August 2027, the asteroid 1999 AN10 will come within 250,000 miles of Earth. AN10 is slightly smaller than JO25 at 2,600 feet across, and will at one point be closer to our planet than the Moon is.