QSO
Because of the moons' low mass and proximity to Mars, actually orbiting them is nearly impossible. However, it is possible to enter a "Quasi-Synchronous Orbit" (QSO) where the craft is actually orbiting Mars in a slightly eccentric orbit in such a way that it circles the moon.
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| Credit: Instituto Superior Técnico |
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| Credit: Instituto Superior Técnico |
QSO is used for mapping and reconnaissance from a safe, stable vantage point. This type of orbit allows for quick mapping of a moon and is stable over long periods, requiring very little stationkeeping. However, in order to remain stable, the craft must remain far enough away from the moon that the moon's gravity does not perturb its orbit. This means a craft in QSO of Phobos must remain about 30 km away. QSOs of Deimos may be slightly closer due to its lower mass.
LPO
Because of the moons' low mass and their proximity to Mars, Phobos's Lagrange point, where the gravity of Mars and Phobos cancel each other out (in this context), is 2-3 km from Phobos's surface. This allows for proximity investigation of Phobos's surface, over a hundred times closer than Earth-based CubeSats ever get to their targets, and over ten times closer than the closest Phobos flyby to date.
A spacecraft at a Lagrange point is like a marble on top of a hill - while it takes relatively little energy to keep it there, without correction it will slowly drift off. A craft in an LPO can take very accurate measurements and extremely high-resolution images of Phobos, but the stationkeeping required to keep it there is rather difficult, especially without the use of ground-based radio telescopes. Most likely the spacecraft will spend a relatively short time in LPO before safely flying out or landing. Thankfully, an LPO can stretch across a large region around the Lagrange point, and can pass over many objects of interest, such as the Phobos Monolith, Stickney Crater, and a number of grooves *.
Artificial LPO
Because of the moons' low mass and their proximity to Mars, Phobos's Lagrange point, where the gravity of Mars and Phobos cancel each other out (in this context), is 2-3 km from Phobos's surface. This allows for proximity investigation of Phobos's surface, over a hundred times closer than Earth-based CubeSats ever get to their targets, and over ten times closer than the closest Phobos flyby to date.
A spacecraft at a Lagrange point is like a marble on top of a hill - while it takes relatively little energy to keep it there, without correction it will slowly drift off. A craft in an LPO can take very accurate measurements and extremely high-resolution images of Phobos, but the stationkeeping required to keep it there is rather difficult, especially without the use of ground-based radio telescopes. Most likely the spacecraft will spend a relatively short time in LPO before safely flying out or landing. Thankfully, an LPO can stretch across a large region around the Lagrange point, and can pass over many objects of interest, such as the Phobos Monolith, Stickney Crater, and a number of grooves *.
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| Credit: Mattia Zamaro |
Artificial LPO
By applying constant thrust in one direction, the stability of an LPO can be significantly increased and orbits can be moved closer to the surface. While propellant-intensive, such an orbit can be used to facilitate a very low-velocity landing*.
Transitions between these orbits will be detailed when I actually understand them.
For more details on orbits of the Martian moons, read Mattia Zamaro's thesis.
-Daniel
*DIFP
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