An international team of astronomers has discovered a pair of asteroids that split off from their parent body a mere 300 years ago. The duo is exceptional because it is the youngest known “asteroid pair” by at least a factor of ten, it passes close to Earth’s orbit, and it has properties that are hard to explain given its young age.
The majority of asteroids in our solar system reside in the area between the orbits of Mars and Jupiter known as the main asteroid belt. Closer to home, scientists have identified additional asteroids known as Near Earth Asteroids (NEAs), whose orbits bring them within the vicinity of Earth.
In 2019, scientists using the Pan-STARRS1 survey telescope in Hawaii and the Catalina Sky Survey in Arizona each discovered a new NEA, identified as 2019 PR2 and 2019 QR6. The largest of the two measures about one kilometer in diameter and the other half that size, and they were found to have very similar orbits around the Sun. Further study by a team led by Petr Fatka of the Astronomical Institute of the Czech Academy of Sciences, confirmed that this was an asteroid pair-two asteroids that separated from a single parent asteroid within the past several million years-currently separated by about one million kilometers.
Several telescopes were used for the follow-up observations, including the 4.3-meter Lowell Discovery Telescope (LDT) in northern Arizona. These observations revealed very similar surface properties of both asteroids, further supporting the hypothesis of their common origin.
Fatka says, “Thanks to the measurements performed with the LDT, it is clear that 2019 PR2 and 2019 QR6 come from the same parent object and their high orbital similarity is not coincidental.”
The majority of asteroid pairs likely form by rotational fission, when a spinning asteroid (which is basically a rubble pile) reaches a critical speed at which debris flies off and forms one or more new bodies, maintaining highly similar orbits as the parent body.
Based on multiple modeling techniques and additional observations-including recovery of previously unnoticed detections made with the Catalina Sky Survey 14 years before the discovery-the team determined that the pair separated only 300 years ago, making it the youngest known pair of asteroid pairs.
Fatka says, “It’s very exciting to find such a young asteroid pair that was formed only about 300 years ago, which was like this morning-not even yesterday-in astronomical timescales.” The previous record holders were at least ten times older.
How did They Form?
This young age led to difficulty in reconciling the pair’s formation history. Standard models of asteroid pair formation by rotational fission couldn’t fully explain the properties of 2019 PR2 and 2019 QR6; something else had to be going on to explain their current separation.
The team then developed new models that assume the original body was a comet, whose jets of gas could push their orbits into the configuration seen today. This provided a viable origin story for these objects, but as team member Nicholas Moskovitz of Lowell Observatory explains, “In the present day, the bodies don’t display any signs of cometary activity. So it remains a mystery how these objects could have gone from a single parent body, to individually active objects, to the inactive pair we see today in just 300 years.”
To answer this question, more observations will have to come. However, this will have to wait for more than a decade. Fatka explains, “To have a better idea about what process caused the disruption of the parent body, we have to wait until 2033 when both objects will be within the reach of our telescopes again.”
