How do asteroids get their moons?

It might be the same way Earth did: with a cosmic crash.

If you had to make a rough guess, which set of Solar System objects do you think has the most combined moons — the eight planets, with their majestic rings and satellite systems, or the asteroids, with their weak gravitational fields? As of 2019, the answer is a little counter-intuitive: The asteroids are winning, with over 200 known natural satellites compared to the planets’ 185. Add in other minor bodies, like trans-Neptunian objects, and the asteroids’ total rises from 200 to over 350.

From left to right, Mimas, Enceladus and Tethys, three of Saturn’s 62 moons, as seen by Cassini. Image credit: NASA/JPL-Caltech/Space Science Institute

This might not be a completely surprising result; after all, we know of several hundred thousand asteroids, but only eight planets. Still, it’s worth thinking about how these asteroids end up with moons at all. Do they form in situ from the solar nebula? Could they form through giant impacts, like Earth’s Moon? Are they captured through three-body interactions? Is any of this even possible for bodies as small as asteroids?

The asteroid owes its name to Hector, the leader of the forces of the Troy during the Trojan war. The choice is a nod to the fact that Hektor is a Trojan asteroid. Also seen here is Hector’s son — Astyamax, born as Scamandrius. Image credit: Jastrow

Some asteroids are now beginning to give us new insight into possible moon formation processes. Consider the case of the Jupiter Trojan 624 Hektor. In 2006, 99 years after its discovery, the asteroid was found to host a tiny moon, Skamandrios. With a tight orbit around Hektor, the moon proved difficult to track, and it took five years to pin down its orbit. When the data was finally put together, though, simulations showed that its orbit might not have changed for billions of years — and suggested a surprising origin for both Hektor and Skamandrios.

The problem with air

The Subaru Telescope, the Keck Observatory, and the NASA Infrared Telescope Facility dot the summit of Mauna Kea in Hawaii. Image credit: Robert Linsdell, CC BY 2.0

The astronomers (Marchis et al. 2014) used the Keck Observatory to collect most of their data, and for good reason. The twin 10-meter telescopes there are some of the largest and most sensitive eyes on Earth, partly because they use a mechanism called adaptive optics (AO). Ground-based telescopes suffer to varying degrees from turbulence in the atmosphere, which makes it difficult to obtain high-resolution images. By detecting that turbulence, though, and correcting for it accordingly, astronomers can combat the problem.

Adaptive optics systems often use a guide star to measure turbulence by comparing real-time data with what the star is expected to look like. The Keck Observatory uses a different system, though, for studying dim targets: lasers. One powerful laser on each telescope points at the sky and ionizes sodium atoms 60 miles up in the atmosphere. The atoms light up enough to be as useful as a natural guide star, though not brightly enough to overwhelm faint objects.

A powerful laser mounted on the Keck II telescope on Mauna Kea. Image credit: Paul Hirst, CC BY-SA 2.5

Once the turbulence has been measured — and adaptive optics systems do this at an incredible rate — computers apply corrections to the telescope’s mirrors. This is often accomplished with special deformable mirrors that typically change shape with the help of an array of tiny actuators working in sync, although methods with magnetic fields of piezoelectric materials are used in some telescopes.

Precise corrections with adaptive optics systems makes it possible for some ground-based telescopes to have resolutions comparable to powerful space-based telescopes that don’t have to deal with atmospheric effects. AO turns out to be a necessity for detailed observations of asteroids by Earthbound observers. In fact, the discovery of Skamandrios would have been impossible without it.

Orbits as windows to the past

Figure 1, Marchis et al. Kp- and H- band images from a period of over five years show glimpses of Skamandrios orbiting Hektor. The moon looks like no more than a smudge.

When you look at Hektor — something that’s quite hard to do, because it’s small and far away — the first thing you might notice is its shape. As with most asteroids, it’s not round, because it’s nowhere near massive enough to be in hydrostatic equilibrium. Even for an asteroid, though, Hektor is unusual. Variations in its brightness show that it must be elongated. There are a few shapes that could explain its light curves: a simple convex blob, a bilobed object like 67P/Churyumov–Gerasimenko, or even a contact binary.

For the purposes of studying Skamandrios’ orbit, knowing Hektor’s shape is really important. The moon orbits only 7.8 Hektor-radii away, meaning that a major source of orbital perturbations is the asteroid itself. Accurate orbital modeling, of course, is thus impossible if we don’t know the primary’s mass distribution. Fortunately, the adaptive optics data supported the bilobed model, and the team chose that as the most likely configuration.

Figure 2, Marchis et al. Three possible configurations for Hektor were considered, but the bilobed model is thought to be the most promising.

After gathering the Keck observations of Skamandrios, the astronomers used an algorithm called Geniode-ANIS to fit a number of quantities: the moon’s orbital parameters, the shape of the primary, and the so-called degree two gravity harmonics, which are part of the expansion of the deviations from the gravitational field of a sphere. The Keck astrometry, in addition to yielding the sizes and masses of the objects in the system, led the team to some interesting conclusions:

• The system is tidally stable over timescales of billions of years, indicating that Skamandrios’ orbit may not have changed much since it formed.
• The moon’s orbital eccentricity is 0.31, surprisingly large, and its orbital inclination is 50.1°. These values are stable because perturbations due to Hektor’s shape don’t lead to a transfer of angular momentum that could lead to oscillations in eccentricity or inclination.
• The orbit is close to two unstable orbit-spin resonances, 1:10 and 2:21. If it had drifted towards either one, the moon would likely either collided with Hektor or been ejected from the system.

Putting the pieces together, the astronomers concluded that the orbit of Skamandrios was primordial, originating from the same event that formed Hektor. The high inclination ruled out a formation scenario involving the accretion of loose debris from a tidally disrupted body, which had been considered as a possible model for creating multiple-asteroid systems. Instead, Marchis et al. suggested that Hektor’s bilobed structure arose from a slow collision between two small objects, which joined together as a contact binary. As the rotation of the combined object increased, ejecta would have spun off, gradually coalescing into a small moon.

Figure 3 (A), (B), (D) and (E), Marchis et al. Simulations of the orbit of Skamandrios show small oscillations over the course of years, but the moon is still quite stable.

This is quite reminiscent of the giant impact hypothesis I mentioned earlier, which holds that a protoplanet collided with a young Earth, spewing material into a disk around the planet that drew itself together into the body we now know as the Moon. The analogy isn’t perfect, but the evidence that moon-forming impacts can happen on many different scales.

The discovery of Skamandrios is just one more example of how adaptive optics continues to make possible revolutionary developments in astronomy. AO has been a driving force behind our understanding of populations of asteroids and other minor Solar System bodies. It’s helped us discover moons around hundreds of asteroids, and as advances in optics progress, it’s certain that we’ll only keep finding more.