The near-Earth asteroid responsible for the spectacular annual Geminid meteor shower has been captured doing something truly unexpected.
Scientists studying the changing light from 3200 Phaethon have concluded that the rocky body is spinning faster and faster on its axis, shaving off about 4 milliseconds each year. It might not seem like much, but the spins of asteroids don’t usually change at all.
Figuring out why Phaethon is behaving this way could give us new insight into a class of asteroids considered “potentially hazardous” — those that fly past Earth as they orbit the Sun.
Phaethon currently poses no danger to Earth, but at 5.8 kilometers (3.6 miles) in diameter it is large enough to cause a small amount of pain if it were to hypothetically hit. Also, the asteroid’s trajectory brings it close enough that a sufficient change in its 524-day orbit could make us rethink our concerns.
It’s also a strange thing. The asteroid’s orbit falls close to the Sun like that of a comet, for example. It also has a dusty tail and happens to be one of two asteroids that produce meteor showers (most also come from comets).
And yet, unlike a comet, it appears to have no ice. Scientists have referred to it as a “rock comet”.
Oh, and it’s surprisingly blue. Most asteroids are reddish or gray.
Phaethon’s unusual characteristics have made it the target of a future landing mission, DESTINY+ (Demonstration and Experiment of Space Technology for INTERplanetary voiYage with Phaethon flyby and dust science), led by the Japan Space Agency. So scientists have been working to learn more about the strange rock, to better plan how to find it.
Phaethon’s brightness changes as it rotates, which means we were able to characterize its rotation period over time, narrowing it down to 3.6 hours. But we need accurate data if we’re going to put a probe into this thing, so planetary scientist Sean Marshall of the Arecibo Observatory in Puerto Rico was working to refine Phaethon’s size, shape and rotation when he noticed something thing
He presented his team’s findings last week at the 54th Annual Meeting of the Division of Planetary Sciences of the American Astronomical Society.
“The shape model predictions don’t match the data,” says Marshall.
“The times when the model was brightest were clearly out of sync with the times when Phaethon was actually observed to be brightest. I realized that this could be explained by Phaethon’s rotation period changing slightly sometime before the 2021 observations, perhaps from comet activity when it was near perihelion in December 2020.”
A closer look at the full data set, spanning the period 1989 to 2021, revealed that the change could be explained by a gradual and steady acceleration, losing 4 milliseconds of rotation period per year. Year after year, the change doesn’t make much of a difference, but as the decades go by, it’s become much more prominent.
In fact, a team of researchers noticed a discrepancy in the rotation period in 2016, when they realized their data was out of sync with 1989. At the time, the researchers didn’t have enough information to explain the difference Now, that discrepancy seems to have been resolved.
This makes Phaethon one of only 11 asteroids with accelerated rotations, among the thousands of asteroids whose rotations have been characterized. This may be the result of mass loss; outgassing in comets produces a spin effect, and a study last year found that Phaethon can outgas sodium.
A small Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect may also apply. It’s when the heat of a star influences the speed of rotation of a small body, like an asteroid.
More work will be needed to figure out what, exactly, is going on with Phaethon; but knowing that the rate of rotation is changing, and the rate at which it is changing, is an excellent finding in itself.
“This is great news for the DESTINY+ team,” says Marshall.
“Constant change means Phaethon’s orientation at the time of the spacecraft’s flyby can be accurately predicted, so it will know which regions will be illuminated by the Sun.”
The team presented their findings at the 54th annual meeting of the Division of Planetary Sciences of the American Astronomical Society.