spaceSpace and PhysicsspaceAstronomy

The Fastest Orbiting Asteroid In The Solar System Has Been Discovered


Dr. Alfredo Carpineti

Senior Staff Writer & Space Correspondent

clockAug 24 2021, 16:59 UTC

Artist impression of 2021 PH27 with the Sun and Mercury. Image Credit: Carnegie Institution of Science

Astronomers have discovered a new asteroid, dubbed 2021 PH27, which is an object of records. It has the shortest year of all other known asteroids, orbiting the Sun in just 113 days. The only known object on a quicker orbit is the innermost planet, Mercury. It was discovered by Carnegie’s Scott S. Sheppard in evening twilight images taken by Brown University’s Ian Dell'Antonio and Shenming Fu.

The object is in an eccentric orbit going closer to the Sun than Mercury and then further away than Venus. Given its proximity to the Sun, due to the mass of our star, the asteroid’s orbits shift following the laws of general relativity.  2021 PH27 is the solar system object that experiences the largest examples of such relativistic effect. And that’s not all.


“2021 PH27 gets so close to the Sun that its surface temperature gets to around [500 °C] 900 degrees Fahrenheit at closest approach, hot enough to melt lead,” Sheppard said in a statement.

The asteroid, estimated to be around one kilometer (0.6 miles) across, is in an unstable orbit. The team estimates that within a few million years it will be destroyed. That might happen in various ways: a collision with Mercury or Venus, it might be pushed into the Sun, or it might be just flung out.

orbit 2021 PH27
The Orbit of asteroid 2021 PH27. Image Credit: Carnegie Institute for Science.

“Most likely 2021 PH27 was dislodged from the Main Asteroid Belt between Jupiter and Mars and the gravity of the inner planets shaped its orbit into its current configuration,” Sheppard said. “Although, based on its large angle of inclination of 32 degrees, it is possible that 2021 PH27 is an extinct comet from the outer Solar System that ventured too close to one of the planets as the path of its voyage brought it into proximity with the inner Solar System.”


More observations will be necessary to fully understand the asteroid origin, but they won’t be easy. The discovery itself wasn’t easy in the first place. Objects so close to the Sun, are only visible when the Sun sets or rises. Dell’Antonio and Fu used the powerful Dark Energy Camera on the National Science Foundation’s Blanco 4-meter (13-foot) telescope in Chile, to look at what might be moving close to the Sun.

The first hint of the object was spotted by Sheppard on August 13. It was followed up over the next few nights from the Chilean observatories as well as from observatories in South Africa when weather conditions in the South American area weren’t ideal.

asteroid 2021 PH27
Observations of 2021 PH27. Image Credit: Ian Dell'Antonio and Shenming Fu.

“Because the object was already in the Sun’s glare and moving more toward it, it was imperative that we determine the object’s orbit before it was lost behind our central star,” explained Dave Tholen of the University of Hawaii, who measured the fast-moving asteroid’s position on the sky and predicted where it would be the night after the initial discovery. “I surmised that for an asteroid this size to remain hidden for so long, it must have an orbit that keeps it so near to the Sun that it is difficult to detect from Earth’s position.”


The Las Cumbres Observatory’s extensive network of global 1-meter telescopes was crucial to deliver the final observations from South Africa and allowed the team to calculate the orbit of 2021 PH27.

“Although telescope time is very precious, the international nature and love of the unknown makes astronomers very willing to override their own science and observations to follow-up new interesting discoveries like this,” said Sheppard. “We are so grateful for all of our collaborators who enabled us to act quickly on this discovery.”


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