Bringing down Phobos

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The moon Phobos is a potential threat for future Mars colonists. With the currently thin Mars atmosphere, the orbit of Phobos will be safe for several million years, but what if a human colony starts some terraforming some day, which increases the density and height of the atmosphere substantially, possibly causing air friction on Phobos? Therefore, a first step to prepare Mars for colonization can be the Bring down of Phobos.

Collision with an asteroid

With a small amount of fuel it might be possible to accelerate a small object from the asteroid belt toward Mars. The trajectory is controlled by maneuvering rockets during the journey. It ends in a collision with Phobos in an appropriate angle in order to reduce orbital speed of Phobos, which finally results in crashing Phobos onto the surface of Mars.

The idle velocity of the asteroid relative to the asteroid belt is 0. The journey of the asteroid is started with a small initial kick. Nearly all final velocity comes from the potential energy of the different solar orbits of Mars and asteroid belt, their orbital speed difference and the relative orbital speed of Phobos.


First of all the potential threat is over. A future colony does no longer have to fear a falling moon and can thrive.

The technology of crashing an asteroid in a potentially dangerous cosmic body may be useful for repelling threat from earth, as well. The knowledge from this Phobos project is valuable far beyond the Mars colonization.

The crash has a global impact on the Mars atmosphere and climate. Dust that is evaporated into the atmosphere temporarily reduces the sunlight on the surface. On the other side a huge amount of water steam causes a greenhouse effect in medium-term, which will increase the planet's temperature in a couple of years.

Open issues

  • What is the required mass of the asteroid?
  • What amount of energy can be brought to the asteroid belt for the initial kick with currently available technology?
  • How long would the journey of the asteroid take?
  • What is the final impact energy?
  • How long would it take for Phobos to crash down on Mars?