The tiny moon Phobos wasn’t the target for the Hubble Space Telescope. Mars was. Last May, the famous space telescope was tasked with observing Mars near opposition. That is when Mars was on the opposite side of the Earth from the Sun.
Like this: Sun → Earth → Mars. The opposite of opposition is conjunction. Which looks like this: Mars → Sun → Earth.
Over 22 minutes, Hubble captured 13 separate exposures. Phobos was in each of them. And astronomers used the opportunity to make a time-lapse video showing a portion of the moon’s orbital path.
It looks more like a tiny star, but that’s Phobos. It measures just 16.5 miles by 13.5 miles by 11 miles and is one of the smallest moons in the solar system.
The mini moon was discovered by Asaph Hall in 1877 at the U.S. Naval Observatory in Washington D.C. Hall made a one-two discovery punch that year. Six days earlier, he found the smaller Martian moon, Deimos.
Phobos does stand alone in one aspect over anything else in the solar system. The time-lapse above shows the moon moving in a hurry. It takes just 7 hours and 39 minutes for Phobos to complete a lap around Mars. And does it faster than Mars can complete a rotation at 24 hours, 39 minutes. Making it the only natural satellite in the solar system to orbit its planet quicker than the planet’s day, according to NASA.
Take our moon for example. It takes 27 days to orbit Earth once. How about Titan? 16 days to get around Jupiter one time. Enceladus? About 33 hours. All longer than it takes their parent planet to rotate one time (a day).
And Phobos little moon brother takes just over 30 hours to orbit Mars.
Where Phobos and Deimos come from is still up for debate. Scientists believe the two moons are made of the same materials as asteroids. But that doesn’t mean they were born as asteroids, according to NASA. Both moons cruise around Mars in almost perfect circular orbits. If the pair were captured asteroids, scientists would expect their orbits to be much more erratic.
Phobos is slowly breaking apart
Every hundred years, Mars tugs Phobos about 6.6 feet closer. Tack on a few dozen million years and the slow tug is going to spell bad news for Phobos. NASA believes the long grooves seen on Phobos today are the early signs of what will play out in about 30 to 50 million years.
“We think that Phobos has already started to fail, and the first sign of this failure is the production of these grooves,” said NASA’s Terry Hurford in 2015.
When the end nears for Phobos, scientists believe the moon’s dusty layer will be stripped away first creating a temporary ring around Mars. After that, smaller chunks of Phobos will likely break off and slam into Mars’ surface creating a new set of craters near the planet’s equator.
That will leave just Deimos as the sole moon around Mars. It’s tucked away in a much higher orbit, and won’t face Phobos’ fate.
Image credits: NASA