The Sun has a new nearest asteroid neighbor. A team including University of Hawaiʻi Institute for Astronomy (IfA) astronomer David Tholen has discovered an asteroid with the shortest known orbital period around the Sun – only 113 days. Of all the known objects in our solar system, only the planet Mercury has a shorter orbital period.

Team leader Scott S. Sheppard, an IfA graduate now at the Carnegie Institution for Science, discovered the asteroid in images taken on August 13, during evening twilight near the Sun, using the Dark Energy Camera (DECam) on the Victor M. Blanco 4-meter Telescope at Cerro Tololo Inter-American Observatory (CTIO), a program of NSF’s NOIRLab in Chile. Astronomers from Brown University, Ian Dell’antonio and Shenming Fu, were using DECam for an unrelated science program. In collaboration with Sheppard, they went from studying some of the most distant objects in our universe to some of the closest, using the first few minutes of twilight to take images near the Sun. Sheppard was able to look through the images within hours to find the asteroid, named 2021 PH27, peeking through twilight. As the Sun sets in the sky, this twilight time is the only efficient way to look for asteroids interior to Earth’s orbit that are near Mercury and Venus.

As the object was already in the glare of the Sun, and getting ever closer, it was imperative to determine the object’s orbit before it became hidden behind the Sun. Tholen, an expert in characterizing asteroid orbits, measured the fast-moving object’s position, and predicted where it would be the night following the discovery. These predictions allowed new observations on the second night, using both DECam and the nearby Magellan telescope.

The new data clearly showed that this object was very near the Sun, and relatively large for an inner Solar System asteroid. Tholen surmised that for an asteroid of 1 km in size to have remained undetected until now, it most likely has an orbit that keeps it near the Sun, making it hard to find from Earth. But the exact orbit for 2021 PH27 remained unknown, and a third night of observations was needed to not lose the object to the Sun’s glare.

With the threat of cloudy weather for the third night, further observations were taken with DECam and Magellan. Additionally, Marco Micheli, an IfA graduate now at the European Space Agency, used the extensive Las Cumbres Observatory Global Telescope to observe the asteroid from Chile and South Africa. Combined, these new observations confirmed that 2021 PH27 does indeed orbit close to the Sun, with the shortest orbital period of any known asteroid.

Understanding the population of asteroids interior to Earth’s orbit is vital for completing the census of near-Earth asteroids. Some of the most likely Earth impactors approach us during daylight, and cannot easily be discovered by most surveys, which typically observe at night. Comparing the actual population of objects found closer to the Sun than Earth and Venus, with those predicted to exist based on the known population farther out, is important to better understand how asteroids move on unstable orbits in the inner Solar System, and from where they may have originated.

Tholen explained, “One of the reasons why it is hard to achieve the goal of finding 100% of all NEOs larger than a kilometer is because some have orbits that help “hide” them from Earth-based observers. Objects like this one – orbiting entirely interior to the Earth’s orbit – are difficult to find and track. There are likely more such objects that have yet to be discovered, and we need a better census of them to estimate what might be the threat of Earth impact.”

Luckily, this particular asteroid poses no threat to Earth, because it never gets this far from the Sun. However, it does cross the orbits of both Mercury and Venus, so its orbit is dynamically unstable and it is possible that someday this asteroid will impact one of those planets or the Sun. It likely originally formed in the more dynamically stable Main Asteroid Belt, between Mars and Jupiter, and was dislodged relatively recently. It became a Near Earth Object (NEO) before gravitational perturbations from the inner planets shrank its orbit to be near the Sun. Its high 32 degree inclination suggests that it could also be an extinct comet that ventured too close to one of the inner planets. Future observations will allow astronomers to better determine 2021 PH27‘s origins.

In addition, internal stresses on objects close to the Sun – such as deformation by gravity and solar heating – will cause asteroids with fragile compositions to erode or break up as they approach the planets and the Sun. Thus, the fraction of all asteroids found closer to the Sun than Earth and Venus will give us insights into the strength and composition of these objects.

This newly-discovered asteroid will soon pass behind the Sun and be unobservable from
Earth until early next year, at which time additional observations of the asteroid will be able to refine its orbit to the precision needed to give the asteroid an official name.