Solar System's fastest-orbiting asteroid spotted, flies closer to the Sun than Mercury

Boffins ponder where space rock came from

Astronomers have discovered the Sun's fastest-orbiting asteroid yet, a one-kilometre-wide rock that completes a lap of our star every 113 Earth days.

That's "the shortest orbital period of any known asteroid in the Solar System," according to the US National Science Foundation's NOIRLab.

The space boulder, known as 2021 PH27, was clocked on August 13 by Scott Sheppard of the Carnegie Institution for Science in the US. He noticed the object in images taken by Ian Dell’antonio and Shenming Fu, of Brown University, using the 570-megapixel Dark Energy Camera on the Victor M. Blanco telescope in Chile.

Rather than live in the asteroid belt between Mars and Jupiter, PH27 was observed much closer to our star. In fact, during its orbit, PH27 gets closer to the Sun than Mercury: the rock's shortest distance to the star is just 20 million kilometres (12 million miles), at which point its surface has the lead-melting temperature of 480°C (900°F). PH27's 113-day orbit has a semi-major axis of 70 million kilometres (43 million miles), and it crosses the orbits of Mercury and Venus. Mercury gets as close as 29 million miles (47 million kilometres), and has an orbital period of 88 days.

David Tholen of the University of Hawaii calculated PH27’s path around the Sun from its position in the images. Over the next few days, as word of the exciting strange rock emerged, astronomers at other observatories used Tholen's calculations to point their telescopes at where the asteroid ought to be, and they too sighted PH27.

“Though telescope time for astronomers is very precious, the international nature and love of the unknown make astronomers very willing to override their own science and observations to follow up new, interesting discoveries like this,” Sheppard said on Monday.

The race was on to confirm Sheppard and Tholen's findings before the space rock disappeared from view.

“Because the object was already in the Sun’s glare and moving more toward it,” Tholen said, "it was imperative that we determine the object’s orbit before it was lost behind our central star. 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.”

Since PH27 lives so close to the Sun, and has a relatively low mass, it experiences general relativistic effects more so than any other known Solar System object, according to the folks at NOIRLab.

Its orbit is also unstable, and astronomers reckon in a few million years or so it’ll either be destroyed in a collision with Mercury or Venus, burn up from getting too close to the Sun, or be kicked out of its orbit by other bodies.

“Most likely 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˚, it is possible [it] 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.”

“Understanding the population of asteroids interior to Earth’s orbit is important to complete the census of asteroids near Earth, including some of the most likely Earth impactors that may approach Earth during daylight and that cannot easily be discovered in most surveys that are observing at night, away from the Sun,” he added.

Astronomers will be able to get a better look at the asteroid when it isn’t blocked by the Sun from Earth in 2022. ®

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