Hidden among millions of massive objects in the Asteroid Belt is a rare astronomical gem: an asteroid with two moons of its own.
Asteroid 87 Sylvia, named for the Roman mythical mother of twins Remus and Romulus, is an oblong rock averaging 286 kilometers in diameter. It’s twin moons are, of course, named Remus and Romulus.
Sylvia is an exceptionally low density asteroid. At least a quarter and probably more than half of its interior is empty space. At about 385 kilometers from tip to tip, there are plenty of weaving catacombs through 87 Sylvia to explore. Thanks to the light gravity of the asteroid, the caves can be traversed quite easily with huge leaps and bounds.
Though you can jump tremendous distances, you don’t have to worry about accidentally launching yourself into space. With an escape velocity of 138 meters a second, your giant leaps will carry you far, but will always bring you back to the surface. (Though… how gently depends largely on the nature of your leap. Please be cautious and utilize proper safety equipment for any bouncing space spelunking.)
Now that you know where to go, we can address the question of when.
Thanks to its twin moons, you can end your leaping cave exploration by finding a spot on the asteroid’s porous surface to watch a spectacular double (double!) eclipse.
A “double eclipse” sounds like it would be a rarity, but not on 87 Sylvia. Due to the asteroid’s relatively high rotation speed, a double eclipse is a surprisingly frequent occurrence. 87 Sylvia completes a rotation about once every five hours, Remus orbits its parent asteroid about once every day-and-a-half, and Romulus orbits 87 Sylvia about once every three-and-a-half days.
So, about once every three (Earth) years, Remus and Romulus come together in alignment between 87 Sylvia and the Sun. Each moon makes a nearly perfect eclipse of the other, as they each would look like they’re the same size from the surface (the farther one, Romulus, being larger than the inner moon, Remus).
This is not like a terrestrial eclipse. Romulus and Remus are relatively small (about 14 and 7 km in diameter, respectively), but they’re so close that they’ll appear to be huge to you, about twice as big as the Moon appears from Earth. The Sun, meanwhile, looks much, much smaller so far out.
Remus and Romulus totally devour the Sun in an eclipse. And with the light of the Sun converted into a soft halo around apparently massive twin spheres, the full sky is visible.
You won’t see other asteroids. Despite popular imaginings of the Asteroid Belt, most asteroids are so far from each other that no other asteroids are visible from each other’s surfaces. In fact, a third of the mass of the Belt is in one object, the dwarf planet Ceres.
But you’ll see so, so many stars. You have to travel great distances on Earth and wait a very long time to see all the stars above and below the poles. The fast rotation of 87 Sylvia means you only have to wait a few hours to see every star in the sky. With no sun and no atmosphere, the double eclipse on 87 Sylvia gives you the clearest star-filled sky in the inner solar system.
Well, except for one very big object.
87 Sylvia is part of the Cybele Group, on the outer edges of the Belt, which means if it’s nearby in its orbit, Jupiter would dominate the sky. When they’re on the same side of the solar system, Jupiter is almost three times closer to 87 Syliva than it is to Earth. You should even be able to make out Jupiter’s biggest moons with just the naked eye.
While sitting in your viewing spot, if you reached down to pick up a rock or part of asteroid, it would likely crumble away. 87 Sylvia is dark, ancient rubble pile of the primordial pieces of the Solar System’s creation held together by the collective gravity of those rocks.
The Asteroid Belt is a strange place, and a window into its star system’s early days.
As the giant planets moved all about – and Jupiter causing the most damage – over 99 percent of the Asteroid Belt’s original mass was lost in the first 100 million years of the Solar System’s history.
It was a chaotic time. At one point there were two large, rocky planets passing around the Sun at the same distance. (They eventually ran into each other and created the Earth and the Moon.) Other planets fell into the Sun or were ripped free. No doubt there are entire planets from the early days of our Solar System out wandering between the stars, unknown lost worlds flung from the Sun’s gravity.
Eventually it all stabilized. Relatively speaking, of course. Though it appears stable, the Solar System is constantly changing. 87 Sylvia likely got its moons in a collision with another asteroid.
The truth is the chaos never stopped. The Solar System is so old, so massive, so complex, that we are only experiencing a moment of apparent stability. The Asteroid Belt is a reminder of that. Every time Jupiter passes by it stirs up the rocks.
Eventually all of this apparent order will scatter. But for now, as in every moment of a solar system’s evolution, there are perfectly balanced wonders to find.
Like an asteroid with two moons.
Asteroid Diversity Points to a “Snow Globe” Solar System – Harvard-Smithsonian Center for Astrophysics
Asteroid Belt Between Mars And Jupiter Is A ‘Melting Pot’ Of Diverse Celestial Objects – International Business Times
Asteroids, Meteorites, and Comets by Linda T. Elkins-Tanton
First Asteroid Trio Discovered – Space.com