Bringing down Phobos
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? Moreover, this moon is an obstacle for building a space elevator. Therefore, a first step to prepare Mars for colonization can be the Bringing 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 would be controlled by maneuvering rockets during the journey. It would ends in a collision with Phobos in an appropriate angle in order to reduce orbital speed of Phobos, which would finally result in Phobos crashing 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.
Results
Once the potential threat is over, the future colonists will no longer have to fear a falling moon and can thrive.
The technology of crashing an asteroid into a potentially dangerous cosmic body may be useful for repelling threats from earth. The knowledge from this Phobos project could be valuable far beyond the Mars colonization.
The crash would have a global impact on Mars' atmosphere and climate. Dust that is evaporated into the atmosphere would temporarily reduce the sunlight on the surface. On the other hand, a huge amount of water steam would cause a greenhouse effect in medium-term, which will increase the planet's temperature in a couple of years.
Alternatives
Millennia will pass until Phobos becomes a real threat. Mankind will possibly develop more powerful technology to be able to pull Phobos up to a higher orbit. Imaginably, this can be done either with a series of small asteroid impacts, huge sunlight-reflecting mirrors, or a gravitational tractor.
Also, a catapult can be built on Phobos with the ability to throw pieces of the rubble pile towards Mars. This produces an impulse to lift Phobos to a higher orbit. The angle can be optimized to both maximize the impulse and avoid smashing facilities on the Martian surface.
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?
