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Posted by: Doug M. Jan 30 2014, 01:52 PM

I've noticed that almost every report on Mars Odyssey that mentions its orbit points out that it is "nearly sun-synchronous", so that it passes over the same spot on Mars' surface at the same local time. (It's a slightly retrograde polar orbit, with an inclination of 93 degrees.) According to NASA, successive ground tracks are separated in longitude by approximately 29.5 degrees and the entire ground track nearly repeats every two sols. This is handy from a science POV, since it means that THEMIS can take pictures of the same features with the same lighting over and over.

That "nearly" makes me thoughtful, though. Mars Odyssey has been orbiting Mars for over 12 years now -- more than 50,000 orbits. If that "nearly" means it's off sun-synchronicity by just a fraction of a second per orbit, its ground track would shift quite noticeably over time.

Probably not a big deal, especially given that Odyssey has adjusted its orbit a bunch of times, most recently in the summer of 2012 to catch the MSL landing. Still, it left me curious. Does anyone know just how close to sun-synchronicity Odyssey is? Brief googling has not turned up a clear answer.

thanks in advance,


Doug M.

Posted by: mcaplinger Jan 30 2014, 02:16 PM

See page 16 of http://books.google.com/books?isbn=1402016964

This is complicated by the eccentricity of Mars' orbit and the higher-order terms of the planet's gravity field (since sun-sync orbits are driven by the orbital precession caused by the planet's oblateness.) And other factors like atmospheric drag, radiation pressure, etc.

And because of regular adjustment it's not a big deal. There is a huge body of literature going back to Mars Observer discussing these orbit designs; try the JPL tech report server if you want to be inundated in this stuff: e.g., http://trs-new.jpl.nasa.gov/dspace/bitstream/2014/6427/1/03-0272.pdf for MRO.

Posted by: Doug M. Jan 30 2014, 04:21 PM

A-ha. Thank you. That's very interesting. -- So, apparently they deliberately had a not-quite-sun-synchronous orbit for the first 22 months of operation, then locked it.

This document also solved a minor mystery I'd been wondering about for years -- the long lag time between Odyssey's arrival at Mars (October '01) and the proper start of its science mission (February '02). The answer is, they had 75 days of aerobraking orbits. I'm used to thinking of aerobraking as something done quickly and dramatically in a pass or two; Odyssey's gentler approach simply hadn't occurred to me. THEMIS wasn't fully activated until Odyssey had done walk-out and settled down in its long-term science orbit.




Wow. Okay, that should keep me busy for a while.

Thank you very much!


Doug M.

Posted by: djellison Jan 30 2014, 04:36 PM

You're probably confusing aerobraking and aerocapture. Aerobraking has always been a very very gentle, protracted effort.

MGS did aerobraking in Sept & Oct '97, suspended briefly because of concerns for pressure on a solar array gimble, restarted in Nov '97 and continued until May '98 where it was suspended to allow the orbit to drift into the location they wanted, and then continued from Nov '98 to Mars '99 - a total of approx 10 months.

Mars Odyssey's aerobraking took 4 months, MRO's took 6 months. Magellan did it for more than 3 months at Venus in the mid '90s.

Posted by: Doug M. Feb 3 2014, 09:54 AM

Ha, you're right. And now I've learned something.


Doug M.