The announcement came of the discovery of asteroid 2021 PH27, which has the shortest orbital period among the known asteroids with about 113 Earth days. Astronomer Scott S. Sheppard of the Carnegie Institution of Science identified it in data collected by the Dark Energy Camera (DECam), an instrument designed for cosmological research that on several occasions has also proved useful for other astronomical research. In this case, the images were captured by Ian Dell’Antonio and Shenming Fu of Brown University on August 13, 2021. The orbit of 2021 PH27 is probably not stable over the long period, and studying this asteroid will help to understand the movements of objects with orbits close to that of the planet Mercury.
Observing asteroids approaching the Sun enough to pass inside Mercury’s orbit is difficult due to sunlight. 2021 PH27 is one of them, although when it moves away from the Sun it also crosses the orbit of Venus. In essence, its orbit is very elliptical, certainly a consequence of the gravitational pushes it received. In the part closest to the Sun, its orbit is affected by relativistic effects just like Mercury’s orbit.
It’s possible that asteroid 2021 PH27 orbited in the asteroid belt between Mars and Jupiter until a gravitational pull from some planet sent it into the inner solar system. An alternative possibility is that it could be an extinct comet that passed who knows how many times through the inner solar system, losing more and more of the ice it contained.
After the discovery on August 13, asteroid 2021 PH27 was observed again the next day with the DECam but also with the Magellan Telescope. The top image (CTIO/NOIRLab/NSF/DOE/DECam/AURA/S.S. Sheppard (Carnegie Institution of Science)) shows 2021 PH27 in two different positions just a few minutes apart in the red and blue dots.
The spread of the news made it possible to obtain observations also with other telescopes in various parts of the world. The data obtained made it possible to ascertain that it’s the asteroid with the fastest known orbit so far with a year of 113 Earth days. In the solar system, Mercury is the only object that orbits the Sun even faster.
An elongated orbit like the one of asteroid 2021 PH27 is likely unstable in the long period because gravitational influences could lead it to crash into Mercury, Venus, or the Sun. Another possibility is that the gravitational influences end up acting as a kind of cosmic slingshot that will one day push it outside the solar system.
In addition to astronomical interest, there’s a practical interest in asteroids such as 2021 PH27. For several years, astronomers have been trying to create a census of asteroids, particularly the ones that come close to Earth and can be potentially dangerous. 2021 PH27 is far from Earth but its current orbit shows that an asteroid can be pushed around the solar system. For these reasons, the studies of 2021 PH27 will continue, although astronomers will have to wait until 2022 because very soon it will pass behind the Sun from the Earth’s point of view.