The asteroid 130 Elektra has three moons, not two as previously thought, making it the center of the first quadruple asteroid system ever discovered. Or to put it another way, it has as many moons as all four inner planets in our Solar System combined, quite the achievement since it is smaller than 47 American states.
Elektra is a main belt asteroid, with an orbit lasting 5.5 years. At 200 kilometers (120 miles) across, it is no giant, but it’s large enough to have been spotted in 1873 and tracked ever since. Its first moon was discovered in 2003, but that was nothing unusual – it's common for asteroids to have a moon. A second companion was discovered in 2014. However, since the slightly larger asteroid Sylvia had already been shown to have two moons nine years earlier, this wasn’t particularly notable either.
Now in Astronomy and Astrophysics, a team led by Dr Anthony Berdeu of the National Astronomical Research Institute of Thailand has reanalyzed data collected three days after the second moon discovery and announced the presence of S/2014 (130) 2, a third moon. Eventually, a catchier name may be officially recognized.
The first moon, S/2003 (130) 1, is 6 kilometers across and orbits Elektra at a distance of around 1,300 km (810 miles). Although this is a close orbit, it's still distant enough to make it easy to spot. The second and third satellites are similar in size to each other (2.0 and 1.6 kilometers wide respectively). The reason S/2014 (130) 1 was detected from the 2014 observations immediately while S/2014 (130) 2 took another seven years to find is that the latest discovery orbits closer to Elektra, just 344 kilometers away on average. Being 15,000 times fainter, its light is easily lost in Elektra’s glare.
Indeed S/2014 (130) 2 is so close to Elektra that irregularities in the parent body’s shape (Elektra is far too small to be round) distort its already quite eccentric orbit. More remarably, the orbits of the inner two moons overlap.
Some asteroid moons are caused when collisions knock a chunk off the parent body that never gets far enough away to escape its gravity. That’s likely to be the case for the first two moons, which both have similar spectra, and therefore composition, to Elektra itself. The same analysis has yet to be done on S/2014 (130) 2.
Dr Berdeu developed the data processing system used to find S/2014 (130) 2 for his PhD and was applying it to Jupiter's moons when he noticed the same telescope had been observing Elektra around the same time. Having proved its worth, the technique could be deployed to find more moons around other asteroids.
NASA already has one mission named after a Beatles song on its way to visit some asteroids. So far no plans have been announced to explore this fab four.