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A recent question/discussion on this site brought out the interesting fact that most probes around Mars are in Sun synchronous near polar orbit.

The ISRO web-site for the Mars Orbiter Mission writes to say

The inclination of orbit with respect to the equatorial plane of Mars is 150 degree, as intended. In this orbit, the spacecraft takes 72 hours 51 minutes 51 seconds to go round the Mars once.

Wikipedia on Orbital Inclination writes to say

an inclination greater than 90° and less than 180° is a retrograde orbit.

How does MOM benefit from this particular retrograde orbit?

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  • $\begingroup$ It's a good question - I've asked a follow-up question. $\endgroup$ – uhoh Mar 16 '16 at 8:45
  • $\begingroup$ I the main reason are the natural orbit perturbations caused by the gravity field of mars (mainly oblateness) and the influence of the Sun. The Mars oblateness causes the orbit planes to precess in a retrograde direction about the Mars pole, and the Sun's attraction causes a retrograde precession of the orbit planes about the normal to the Mars orbit. But I am not sure, just read a bit of this paper: planetary.brown.edu/planetary/geo287/PhobosDeimos/papers/… $\endgroup$ – ben Mar 16 '16 at 13:58
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According to the abstract Target Martian Orbit Selection for ISRO Mars Orbiter Mission, a number of constraints were taken into account:

The other orbit parameters of Inclination, Right Ascension of Ascending Node and Argument of Perigee were selected after a detailed analysis to meet the following Mission constraints:- maximum eclipse period should be less than 100 min; imaging of Mars full disc from apoapsis region should be possible from Day 1 after MOI; orbit periapsis altitude should be stable at least for one year considering all the perturbative forces. The achievable range of Martian orbit inclination values were found to be from 29 deg to 151 deg in Mars IAU frame of reference, since the declination of the V-infinity of the Mars arrival hyperbola was -29 deg. Seven orbits, two direct, one polar and four retrograde orbits were targeted in the maneuver design and studied with respect to the delta-V cost of achieving the orbit, the sun-orbit geometry after MOI, the eclipse characteristics, orbit stability and imaging opportunities. The star sensor mounting configuration requirement and occult management was also studied for each of the orbits. The retrograde orbit of size 500 x 80000 km, with 151 deg inclination, ascending node of 61 deg and argument of perigee of 206 deg, was found to be the most suitable orbit satisfying all the specified constraints as well as from the viewpoint of star sensor mounting and management of star sensor occults.

This abstract is on the ISSFD website in the 2015 abstracts folder, but I can't find the full paper, nor any evidence on the site that the paper was presented in the 2015 conference. There's another abstract from the ISRO flight dynamics group on the overall mission, again with no full paper that I can find; it's possible that the ISRO FDG planned to attend and present these and then had to cancel.

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A retrograde orbit can help in achieving a sun-synchronous orbit.

The wikipedia page on sun-synchronous orbits shows that a (small) nodal precession (movement of the orbit's point of crossing the equatorial plane) ensures that the satellite is overhead the same location at the same local time every day.

We can use the ellipsoidal nature of the central body to "pull" on the orbit and generate the nodal precession that is required for a sun-synchronous orbit. The rate of nodal precession depends on the inclination of the orbit, and a slightly retrograde orbit yields the desired nodal precession.

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  • $\begingroup$ usually sun-synchronous orbits are circular and have an inclination of 90 degrees plus a bit. MOM has a very different orbit, so is MOM actually sun-synchronous? $\endgroup$ – Hobbes Mar 15 '16 at 20:41
  • $\begingroup$ also, MOM's orbital period does not match up with Mars' rotation, so MOM is not 'overhead the same location at the same local time every day'. $\endgroup$ – Hobbes Mar 16 '16 at 9:25
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From the wikipedia page, it mentions that there are ground imaging sensors, so I could see how a retrograde imaging satellite would allow you to cover more area quickly, but with the highly elliptical orbit that it has, I don't think that's the case.

There is another answer on Quora here:

you are thinking about spacecraft velocity w.r.t Sun (Vs) being pro-grade or retrograde with respect to mars rotation. what we must consider is the spacecraft velocity w.r.t Mars(Vs - Vm =Vsm).

In the case of MOM, the space craft is ahead of Mars and Mars caught up to it (when you see things in solar system reference frame). The relative velocity of spacecraft w.r.t mars (Vsm) is actually in the retrograde direction to the Mars rotation.

So if you analyse from solar system frame, all that happened during the insertion burn is that the Vs of the spacecraft is increased so that when Mars arrives near spacecraft the magnitude of relative velocity of spacecraft w.r.t Mars (|Vsm|) is reduced enough for mars to capture the space craft into its orbit. Note: In this case |Vm| > |Vs|

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    $\begingroup$ There's potentially an answer here, but you're not really applying it to the question as it was asked here, nor actually stating it. Are you saying that MOM is benefiting from a retrograde orbit because it was the only orbit it could afford following Mars Orbit Insertion? $\endgroup$ – TildalWave Nov 5 '14 at 13:47

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