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The recently discovered asteroid 2021 PH27 is about 1 km (3,280 feet) in diameter and orbits the Sun in just 113 days – the shortest known orbital period for an asteroid and the second shortest for any object in our solar system after Mercury.
Asteroid 2021 PH27 was imaged inside Mercury’s orbit and has been colored red and blue to show the two different times it was imaged on August 13, 2021 – just three minutes apart. Image credit: CTIO / NOIRLab / NSF / DOE / DECam / AURA / SS Sheppard, Carnegie Institution of Science.
2021 PH27 has a semi-major axis of 70 million km (43 million miles, 0.46 AU), giving it an orbital period of 113 days in an unstable elongated orbit that intersects the orbits of Mercury and Venus.
This means that in a few million years, it will likely be destroyed in a collision with one of these planets or the Sun, or it will be thrown from its current position.
2021 PH27 was discovered by Dr Scott Sheppard of the Carnegie Institution for Science in images taken by Brown University astronomers Ian Dell’Antonio and Shenming Fu on August 13, 2021.
“Most likely 2021 PH27 has been dislodged from the main asteroid belt between Jupiter and Mars and the gravity of the inner planets has shaped its orbit in its current configuration,” Dr. Sheppard said.
“Although, based on its large tilt angle of 32 degrees, it is possible that 2021 PH27 was an extinct comet from the Outer Solar System that ventured too close to one of the planets as the trajectory of her journey brought her closer to the inner solar system.
An illustration of the 2021 PH27 orbit. Image credit: CTIO / NOIRLab / NSF / AURA / J. da Silva, Spaceengine.org.
Because 2021 PH27 is so close to the Sun’s massive gravitational field, it experiences the most significant general relativistic effects of any known object in the solar system.
This is seen in a slight angular deviation from its elliptical orbit over time, a movement called precession, which occurs at about one minute of arc per century.
The observation of the precession of Mercury intrigued scientists until Albert Einstein’s theory of general relativity explained its orbital adjustments over time. The precession of 2021 PH27 is even faster than that of Mercury.
“2021 PH27 gets so close to the Sun that its surface temperature reaches 482 degrees Celsius (900 degrees Fahrenheit) at the closest approach, hot enough to melt lead,” said Dr. Sheppard.
The asteroid will soon pass behind the Sun and will be unobservable from Earth until early next year, when observers can refine its orbit with the precision needed to give it an official name.
The discovery of 2021 PH27 is reported in the Minor planet electronic circular.
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SS Sheppard et al. 2021 PH27. MPEC 2021-Q41
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