A newly discovered asteroid has the second shortest orbit we know in the entire solar system, scattered only by Mercury.
It is called PH21 2021 and takes only 113 days to complete a circuit around the Sun, over an unstable elliptical orbit that crosses the orbital paths of Venus and Mercury.
This means that it is extremely close to the Sun at its nearest point, or perihelion, with a surface close enough to achieve creamy temperatures of up to 480 degrees Celsius (900 Fahrenheit).
It also means that the asteroid’s time is limited: in a million years, it will either be abandoned from its current trajectory, or it will be annihilated in a collision with one of the two planets or the Sun.
Although its useful life (at least in its current orbit) is short, at least on a cosmic time scale, PH21 2021 and other objects in the inner solar system can reveal information about the evolution of our planetary system.
The orbital path of 2021 PH27. (Katherine Cain and Scott Sheppard / Carnegie Institution for Science)
That is, if we can find out where it came from.
“It is likely that in 2021 PH27 was displaced from the main asteroid belt between Jupiter and Mars and the gravity of the inner planets set up its orbit in its current configuration,” said astronomer Scott Sheppard of the Carnegie Institution for Science.
“Although, based on its wide 32-degree tilt angle, 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 his journey was brought closer to the inner Solar System. “
Asteroids that are in Earth’s orbit (known as Atira asteroids when their orbits are completely contained on Earth) are often very difficult to see, as they are usually very close to the Sun in the sky.
This means that the best time to detect them is just before sunrise, or just after sunset, when the asteroid is illuminated by the Sun, but does not exceed it.
This is how astronomers Ian Dell’Antonio and Shenming Fu of Brown University imagined 2021 PH27 on August 13, 2021, with the National Science Foundation’s 4-meter white telescope in Chile.
In his observations, taken just after sunset, Sheppard found the asteroid. David Tholen, of the University of Hawaii, who often collaborates with Sheppard, used these images to calculate where the asteroid would be the next night.
“Because the object was already in sunlight and moving more toward it, it was imperative to determine the object’s orbit before it got lost behind our central star,” Tholen said.
“I assumed that for an asteroid of this size to remain hidden for so long it must have an orbit that keeps it so close to the Sun that it is difficult to detect from the position of the Earth.”
This little moving point is the 2021 PH27. (Ian Dell’Antonio and Shenming Fu / Brown University)
The data obtained over the next two nights allowed the researchers to calculate a precise orbit and learn a few things about the 2021 PH27.
We know, for example, that its size is about 1 mile in diameter. We also know that because it flies so close to the Sun, it experiences very strongly the effects of its gravitational field as described by General Relativity, in particular the rotation of its orbit, a phenomenon known as apse precession.
Prior to the discovery of the 2021 PH27, Mercury had the strongest apse precession in the solar system; but, although Mercury’s orbital period is shorter (only 88 days), its orbit is much rounder, meaning it does not approach anywhere as close to the Sun as the asteroid.
2021 PH27 is about to spin behind the Sun, which means we won’t be able to see it until early next year. At this time, researchers hope to take further observations that will help further refine its orbit, which in turn will reveal more details about where the asteroid came from and what its future might be.