Binary asteroids,
Didymos and Dimorphos
The Hera spacecraft is targeting at a binary asteroid 65803 Didymos, which is a small asteroid about 780 meters in diameter and is paired with an even smaller satellite, Dimorphos. The binary asteroid Didymos and Dimorphos orbit the Sun with a period of about 770 days (Figure 1). They are classified as a Potentially Hazardous Asteroid (PHA) because they have the potential to approach the Earth and cause a significant damage if they could impact the Earth.
Figure 1: Orbit of asteroid Didymos (white) from Jet Propulsion Laboratory/Small-Body Database Lookup.
Now let’s look at the binary asteroid more closely. Ground-based radar observations provide us with their approximate shapes (Figure 2). When the DART and Hera spacecrafts arrive at the asteroids, we will see them in detail. The satellite Dimorphos orbits 1.2 km away from the primary body Didymos with a period of about 12 hours. Because Dimorphos rotates once during this period, it is always facing the same side to Didymos. It is just like the relationship between the Earth and the Moon. The spacecraft DART will collide with the satellite Dimorphos to change their mutual orbit. The DART impact is expected to change the orbit period of Dimorphos by several tens of seconds. The result of the asteroid deflection test allows us to estimate how much momentum the impact will transfer to the asteroid. This will be an important insight for future planetary defense.
Figure 2: Shape of the binary asteroid Didymos-Dimorphos, from Naidu et al. (2016, AIDA workshop).
So how did the binary asteroid form? Answering that question is one of the key science goals of the Hera mission. 15% of near-Earth asteroids larger than 200 meters are known to have satellites, such as Didymos (Margot et al., 2002). For a certain percentage of binary asteroids to exist, a mechanism is needed to maintain the stability of such asteroid systems over a long period of time. The first rendezvous with the binary asteroid by Hera will provide important clues to understand how they formed and evolved.
Many asteroids larger than several hundred meters are thought to be rubble piles made of fragments of larger parent bodies. Asteroid 25143 Itokawa is one of the rubble pile asteroids, which was explored by Japan’s Hayabusa spacecraft. In Figure 3, you can find that few asteroids larger than several hundred meters in size are spinning faster than a period of 2.2 hours. When the critical rotation period (i.e., the spin barrier) is exceeded, the centrifugal force due to rotation becomes greater than the gravitational force, and the asteroid cannot keep its shape. Because the rotation period of Didymos (~2.3 hours) is slightly longer than this limit, it is thought that the satellite Dimorphos may have been made of material ejected from Didymos by a rapid rotation.
Figure 3: Rotation period of asteroids vs. size, modified from Hestroffer et al. (2019).
Stay tuned for the DART and Hera missions, and the nature of the asteroid brothers.
Masanori Kanamaru,
Hera JAPAN
March 1, 2022