We already knew the asteroid 130 Elektra was special. Astronomers previously discovered it had two moons, making it a rare triple asteroid system. Now a third moon may have been found, making it even more uncommon — the first-known quadruple asteroid in the solar system.
Elektra was first discovered in 1873, orbiting in the asteroid belt between Mars and Jupiter. Oblong-shaped and 160 miles across on its longest side, it is a relatively large asteroid and completes an orbit of the sun every five years.
In 2003, the first moon was discovered orbiting Elektra, and in 2014 a second. The discoveries were interesting, but not unusual — more than 150 asteroids are known to have one or two moons, in the same way planets can have moons that are gravitationally bound to them. “Multiple moons can be found around large asteroids,” said Bin Yang, an astronomer from the European Southern Observatory in Chile who discovered Elektra’s second moon. A NASA mission, DART, is on target to collide with one such asteroid’s moon later in the year.
But until now, an asteroid with three moons has eluded astronomers. Anthony Berdeu from the National Astronomical Research Institute of Thailand and colleagues used images from the Very Large Telescope (V.L.T.) in Chile to take a closer look at Elektra, and they found evidence for a previously hidden moon inside the orbits of the other two.
“This is the first asteroid with three moons,” Dr. Berdeu said. “We are pretty confident. It’s quite exciting.”
Their results were published Tuesday in the journal Astronomy & Astrophysics.
At a paltry one mile across, the moon would be slightly smaller than its siblings at 1.2 and 3.7 miles across. It swings around Elektra once every 16 hours at a distance of only 220 miles. To an observer standing on the third moon’s surface, Elektra would loom large in the sky.
Dr. Berdeu says he was able to find the moon using a new algorithm to eke out its extremely faint light in images taken by the V.L.T. The data reduction techniques employed by the algorithm allowed for a sharper image of Elektra and its surroundings.
Dr. Yang, who was not involved in this paper, said that she and other astronomers had “been trying to look for quadruple systems for a while,” and that her team also saw tantalizing hints of this third moon in their studies of 130 Elektra. This discovery would be a “very exciting result,” she said, although further observations will be needed to confirm the moon’s existence.
Alan Fitzsimmons, an astronomer from Queen’s University Belfast who was also not involved in the paper, says the moons are most likely chunks of Elektra that were broken off in a collision when another object smashed into the asteroid in the past. “They all look like they’re from the same material,” he said.
Further study of this system could reveal the stability of such multi-moon asteroids. This third moon’s orbit is misaligned to the other two, something that’s “very strange,” Dr. Berdeu said. Dr. Yang said that she thought the system was unstable and that “the inner moons may eventually fall back” onto Elektra.
It could also tell us more about the formation of multi-moon asteroids. “This new finding will inspire modelers to look at asteroid impact formation, and try to set a limit on how many moons an impact can form,” Dr. Yang said. “How many moons can a system really sustain?”
Further studies are expected to unearth more quadruple systems too. New telescopes, such as the Extremely Large Telescope currently being built in Chile, will have the observing power necessary to more easily spot these multi-moon asteroid systems.
And astronomers may not stop at quadruple asteroids. “There is no limit to what we can find,” Dr. Berdeu said. “We expect to find more quadruple systems, and why not quintuple or sextuple.”