While a great many space rocks are located within the Asteroid Belt, these objects do not exist exclusively between the orbit of Mars and Jupiter. In fact, they are all over the place, and now astronomers have discovered the asteroid with the shortest year.

2019 LF6 is part of the “Atira” family of asteroids, a group of just 20 objects whose orbits are completely within the Earth’s orbit. 2019 LF6 is about 1 kilometer (0.62 miles) in diameter and orbits the Sun in 151 days getting a bit further than Venus at times and closer than Mercury at others.

"You don't find kilometer-size asteroids very often these days," said Quanzhi Ye, a postdoc at Caltech who co-discovered the object with Professor Tom Prince, JPL, and George Helou, both also at Caltech.

"Thirty years ago, people started organizing methodical asteroid searches, finding larger objects first, but now that most of them have been found, the bigger ones are rare birds," Ye continued. "LF6 is very unusual both in orbit and in size – its unique orbit explains why such a large asteroid eluded several decades of careful searches."

The object was discovered as part of the Zwicky Transient Facility, or ZTF, an instrument at the Palomar Observatory that can scan the sky very quickly to look for objects that shouldn’t be there. And that is crucial to discover new asteroids in the Atira group because they can only be found in the first half an hour after sunset or before sunrise.

For this reason, Ye, together with Wing-Huen Ip of the National Central University in Taiwan, developed a dedicated observation campaign known as Twilight. With that, they also discovered the previous record holder for the shortest year, 2019 AQ3, which orbits the Sun every 165 days.

"Both of the large Atira asteroids that were found by ZTF orbit well outside the plane of the Solar System," explained Prince. "This suggests that sometime in the past they were flung out of the plane of the Solar System because they came too close to Venus or Mercury."

So far, ZTF has found 100 near-Earth objects (NEOs) and about 2,000 new asteroids in the Main Belt between Mars and Jupiter. The team hopes that more Atira objects will soon be found by Twilight. They also hope that NASA will move forward with NEOCam, a proposed mission that will discover a lot more NEOs and assess if they are dangerous to Earth.