On April 20, the Lucy spacecraft made a close flyby of the asteroid Donaldjohanson. The images it transmitted showed a world resembling a dumbbell or a peanut. Donaldjohanson consists of two parts connected by an isthmus.
For scientists, as well as anyone interested in astronomy, this shape is not something fundamentally new. Such bodies are called contact binaries and are quite common in the Solar System. In this article, you will learn how they formed and about their most famous representatives.
How contact-double bodies are formed
As is often the science case, the existence of contact bodies was predicted even before they were discovered. In 1971, planetary scientist Allan Cook suggested that the extremely elongated asteroid Hector was a “composite” object.
The hypothesis was confirmed in the late 1980s, when radar imaging performed with the Arecibo telescope revealed that the near-Earth asteroid Castalia was a contact-double. Since then, astronomers have discovered many such bodies, not just asteroids. Scientists also know about contact comets and Kuiper belt objects.
But how are contact bodies formed? It is believed that they are the product of the evolution of dual systems whose components have approximately the same size (the average diameter of one body cannot exceed the diameter of the other by more than 2-2.5 times). In the process of their rotation around each other, progressive energy dissipation occurs. This leads to their slow convergence and, eventually, to a collision. If it occurs at a low speed, not exceeding a few meters per second, the bodies merge to form an isthmus connecting them.
Source: NASA/JHUAPL/SwRI/James Tuttle Keane
It is worth noting that rapidly rotating contact-double bodies are weakly coupled because their rotation speed around the center of mass is approximately equal to the first cosmic velocity. If the rotation of such an object accelerates under the influence of some external factors, it may again split into two independent parts.
Prevalence of contact-double bodies
How common are contact-double bodies? They are quite common. Thus, according to an estimate made in 2014, about 10%-15% of near-Earth asteroids with a diameter of more than 200 meters are “composite”.
However, this figure may be an underestimate. Based on the results of the analysis of radar data collected by the Arecibo telescope for the period from 2017 to 2019, researchers found that 10 out of 33 near-Earth asteroids with a diameter of more than 200 meters (i.e., 30%) were contact asteroids. Although the sample size is small and therefore cannot be statistically significant, this indicates that there are even more such bodies than previously thought.
And it’s not just about asteroids. The results of observations of Kuiper belt objects show that a significant part of them (from 10% to 50%, depending on the group) may also be contact.
And the Cassini mission data shows that some of Saturn’s irregular moons are also contactable. We are talking about small bodies that did not form together with the planet, but at some point were captured by its gravity. They were either originally contacted or assembled from fragments already in orbit around Saturn. Similarly, many of the irregular satellites of other giant planets are also very likely to be contact objects.
The most famous contact-double bodies
Now let’s talk about contact celestial bodies studied by spacecraft at close range. Let’s start with asteroids. To date, interplanetary probes have already visited a considerable number of contact bodies. These include, for example, the asteroid Itokawa, which was studied by the Japanese Hayabusa mission in 2005-2006.
Asteroid Toutatis is also a contact asteroid. In 2012, it was visited by the Chinese probe Chang’e-2.
Of the three asteroids visited by the Lucy mission during its flight, two are contact asteroids. In addition to the aforementioned Donaldjohanson, the small asteroid Selam, a satellite of the asteroid Dinkinesh, also turned out to be a contact.
Many of the comets studied by spacecraft are also contact-double comets, such as 103P/Hartley or 19P/Borrelly. But the most famous “composite” tailed guest is certainly the comet Churyumov-Gerasimenko, studied in detail by the Rosetta mission.
The farthest object in the Solar System visited by the spacecraft also turned out to be in contact. We are talking about the Arrokoth coiperoid. In 2019, the New Horizons probe made a close flyby of it. It turned out that Arrokoth consists of two parts. Interestingly, one of them is not a traditional spherical shape, but rather resembles a pancake. It is believed that this is the shape of the ancient planetesimals that gave rise to all the current planets in the Solar System.
This, as well as the small number of craters, made it possible for scientists to declare that Arrokoth is the most pristine celestial body explored by the spacecraft. And since the very first coiperoid turned out to be contactable, we can confidently state that such objects are widespread not only in the vicinity of the Earth but also on the outskirts of the Solar System.
