On July 8, 2019, Caltech astronomers announced their discovery of an unusual asteroid with the shortest year known for any asteroid. The rocky body, dubbed 2019 LF6, is about half a mile (1 km) in size and circles the sun roughly every 151 days.
2019 LF6 is one of only 20 known Atira asteroids, which are objects whose orbits fall entirely within Earth’s path around the sun. That is, their orbit has an aphelion (farthest point from the sun) smaller than Earth’s perihelion (nearest point to the sun). In its orbit, 2019 LF6 swings out beyond Venus and, at times, comes closer to the sun than Mercury, which circles the sun every 88 days.
You don’t find kilometer-size asteroids very often these days. 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. 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.
2019 LF6 was discovered via the Zwicky Transient Facility, or ZTF, a state-of-the-art camera at California’s Palomar Observatory that scans the skies every night for transient objects, such as exploding and flashing stars and moving asteroids. Because ZTF scans the sky so rapidly, it is well-suited for finding Atira asteroids, which have short observing windows. Ye said:
We only have about 20 to 30 minutes before sunrise or after sunset to find these asteroids.
The ZTF team has discovered one other Atira asteroid so far, named 2019 AQ3. Before the astronomers spotted 2019 LF6, 2019 AQ3 had the shortest known year of any asteroid, orbiting the sun roughly every 165 days. Tom Prince is a physics professor at Caltech and a senior research scientist at JPL. Prince said:
Both of the large Atira asteroids that were found by ZTF orbit well outside the plane of the solar system. 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.
Bottom line: Asteroid 2019 LF6 orbits the sun roughly every 151 days, the shortest year yet known for an asteroid.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.