(CNN) A massive asteroid has eluded astronomers because of its unusual orbit -- until now.

Astronomers have spotted 2019 LF6, which is about a kilometer wide and boasts the shortest "year" of any known asteroid, circling the sun about every 151 days, according to the California Institute of Technology

This rare rocky body is one of only 20 known Atira asteroids , those whose orbits fall entirely within that of the Earth.

"You don't find kilometer-size asteroids very often these days," said Quanzhi Ye, a postdoctoral scholar at Caltech who discovered 2019 LF6 via the Zwicky Transient Facility, a camera at the school's Palomar Observatory that scans the sky for objects. "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."

It's difficult to spot the asteroids because astronomers only have about 20 to 30 minutes before or after sunset to find them, Ye said.

"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," Ye said.

In its orbit, 2019 LF6 swings out beyond Venus and at times comes closer to the sun than Venus, which circles it every 88 days.

The ZTF team has discovered one other Atira asteroid, 2019 AQ3 , which orbits the sun about every 165 days.

Besides the Atira asteroids, ZTF has identified about 100 near-Earth asteroids and about 2,000 orbiting in the Main Belt, between Mars and Jupiter.