Considering its only 22km across, Phobos is one very interesting moon. It orbits closer
to its planet than any other moon in the solar system. Its orbit takes it so close to Mars,
only 6000km from the planet’s surface, that it orbits the planet faster than a Martian
day. In fact, standing on Mars, you’d see Phobos rise and set twice in one day. It’s
so close to the planet that you must be standing near the equator to see it, anything above
70° latitude means the curvature of the planet would block your view of the moon.
Being this close to the planet means that the tidal forces exerted on it are comparatively
large, so much so that scientists believe that Phobos only has around 50 million years
to live before it eventually is pulled apart completely and its debris crashes into the
planet. You can see signs of this process on the moon, in what scientists are calling
stretch marks. The moon is thought to be made of rubble, and with powerful tidal forces
exerted on it, fractures have started to appear as the rubble is pulled apart. Because of
its close orbit, its orbit is also degrading, or spiralling inwards, by 2cm a year. When
Phobos is eventually pulled apart, a small portion of the debris may remain in orbit
around Mars for a few million years, meaning Mars would have rings for a time.
But from a hypothetical standpoint, is there anything that could be done to stop Phobos’
demise? It’s only 22km across, surely all you would need to do is stick a rocket on
it and boost its speed a little to give it a higher orbit? Well, let’s say we wanted
to increase Phobos’ speed by 1mm/s. In order to accelerate something in space, we need
to throw some mass in the opposite direction. Knowing that Phobos’ mass is roughly 1 x
10^16kg, gives us our required change in momentum (10^13 m/s kg). Next, we need to figure out
how fast we are going to hurl this mass away from Phobos, let’s say 20km/s, which gives
us the amount of propellant we require, 500,000,000kg. (10^13/20000 = 5 x 10^8kg). Shooting that
much mass away from Phobos at that speed requires a lot of energy, 10^16 J (E = ½mv²) (0.5
x 5 x 10^8 x 20000²). That means to increase the orbital velocity by only 1mm/s, we would
need to shoot out the equivalent of an Empire State building with the energy of a nuclear
bomb. As it happens though, we would just need to
counter the orbital decay of Phobos, which is slowing down the orbit of Phobos by roughly
0.002mm/s per year giving us our drop of 2cm a year. We can counteract this by using roughly
0.03 kg of propellent per second over the course of a year. This still adds up though,
that’s equal to 1 million kg per year, plus the energy required to shoot it out.
So, could we eventually save Phobos? Maybe. It would require a lot of resources though
just to counteract its orbital decay, let alone raise its orbit. A 22km moon doesn’t
seem very big, but it is still 2000 times heavier than all the coal mined on Earth each
year. Thanks for watching! And big thank you to
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