Author Topic: Deimos and Martian comm satellites  (Read 4800 times)

Offline redliox

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Deimos and Martian comm satellites
« on: 12/08/2022 04:04 pm »
The proximity of Deimos' orbit to Martian synchronous orbit, the minuscule knowledge of the Martian moons, and the need for a Martian communication network feel intertwined to me.  I've had thought experiments and decided to lay out some of them here.  Specifically, I noted a few orbits with respect to Deimos and the synchronous orbit and wanted thoughts and opinions.

Deimos has a period of about 30.3 hours (~1.23 Martian sols) with 23,400 km versus the synchronous orbit's ~24.65 hour (1 sol) and 20,400.  The duo differ by 5 and 2/3 hours and a smidge over 3,000 km.  Close in a vague sense and barely noteworthy.

Synchronous orbit, like Earth's saturated GEO, would be a sweet spot for communication needs for Mars.  A sticking point is the fact it would be stationary; more specifically it'd be useless to a rover, lander, or base on the other side of planet.  However, Deimos' not-so-synchronous position means it drifts, very slowly, from a Martian perspective.  The entirety of Mars gets a view of Deimos, and a satellite emulating the moon means it could cover the planet while providing decent coverage for several days to specific sites; a compromise when your budget is low.

For brevity, I'll summarize to say I noted 2 orbits just beyond Deimos':

1) 'Synod' Orbit of 31.3 hours (1.27 sols) - allows for distant encounters of Deimos every 30 days.

2) 'Semi-Synchronous' Orbit of 30.8 hours (1.25 sols) - allows for closer encounter of Deimos less regularly while circling Mars regularly every 4 sols.

I am unsure how much Deimos would destabilize any satellite's orbit, or inversely keep it stable.  But I am very curious if the math could allow a probe do double-duty.  When I was able to crunch numbers for the synod orbit, I got an encounter range of ~520 km; it was difficult to be sure (things like rechecking if I was using radius to surface versus core), but I suspected with sol periods of 1.27, 1.25, and Deimos' own of 1.23 it and a probe could get fairly close frequently; in fact this was the point of the synod orbit, to allow regular encounters at a predictably safe, yet close, distance.

As a further, final example of my thinking, I pictured a scenario of a carrier probe toting 1 to 4 smaller clones via slow, electric propulsion.  As it descends, it drops off the clones into one of the outer orbits.  It'd cross Deimos, probably do one or two very close encounters, and proceed to a synchronous spot....or perhaps even to Phobos and low Mars orbit with the right options (and more specifically larger fuel budget).  Said probes would prioritize communications with a Mars-Deimos camera with the carrier having the more robust fuel tanks and perhaps additional instruments for either the red planet or gray moonlet.

Largely just a thought, but I was serious inquiring if these quasi-Deimos orbits had merit.
« Last Edit: 12/08/2022 04:08 pm by redliox »
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Offline Airlocks

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Re: Deimos and Martian comm satellites
« Reply #1 on: 12/09/2022 10:47 am »
Quote
Deimos has a period of about 30.3 hours (~1.23 Martian sols) with 23,400 km versus the synchronous orbit's ~24.65 hour (1 sol) and 20,400.  The duo differ by 5 and 2/3 hours and a smidge over 3,000 km.  Close in a vague sense and barely noteworthy.

Very interesting. As with libration points (Sun-Mars), the smaller the planet, the closest the "GEO" orbit.
Venus and Earth are not that much different in size & mass, but Mars in comparison is like a small potato.

Never realized before Deimos was that close from Mars own GEO. Nor that the said martian GEO was so "low".

In passing, this also means Venus own GEO must be pretty similar to Earth's - just a bit less than 22 000 miles / 36000 km. At least there is no enormous Moon to gravitationnaly screw Venus GEO.
« Last Edit: 12/09/2022 10:51 am by Airlocks »

Offline redliox

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Re: Deimos and Martian comm satellites
« Reply #2 on: 12/09/2022 11:23 am »
Never realized before Deimos was that close from Mars own GEO. Nor that the said martian GEO was so "low".

It feels like an overlooked factoid.  While speedy Phobos circles Mars three times daily, Deimos appears to move like a turtle; the fact it keeps pace fairly close to the stationary point means it doesn't set for 2 and a half days straight, then likewise vanishing from view for the same again before rising.  Compared to Phobos or a would-be low Mars orbit satellite, this is more constant.

In passing, this also means Venus own GEO must be pretty similar to Earth's - just a bit less than 22 000 miles / 36000 km. At least there is no enormous Moon to gravitationnaly screw Venus GEO.
  Not remotely.  If you want a Venus synchronous orbit, you're asking for something that rotates backward from rest of solar system and circles Venus once every 243 Earth days...because Venus spins retrograde very, very slowly.  I wouldn't even be sure such an orbit is within Venus' Hill (i.e. gravity) sphere.  Mars is just lucky to have a similar spin rate to Earth.
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Offline redliox

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Re: Deimos and Martian comm satellites
« Reply #3 on: 12/09/2022 12:03 pm »
Trying to double-check a few figures, done with Hop David's spreadsheet...

Further numbers I got were these for orbital radii (above the Martian surface):

1) Synod Orbit  20, 580 km with 31.304 hour (1.27 sol) period

2) Semi-Synchronous Orbit   20, 316 km with 30.788 hours (1.25 sol) period

If I checked the numbers relative to Deimos correctly, the 1st orbit encounters Deimos from 518 km away every 30 days while the second encounters at 254 km very close to every 50 sols/51.3 days.  The math seemed to imply a satellite could retain a 1.25 sol orbit and encounter Deimos every 51 days safely.

Verification (with a side of thoughts) appreciated.
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Offline Airlocks

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Re: Deimos and Martian comm satellites
« Reply #4 on: 12/09/2022 12:08 pm »
Never realized before Deimos was that close from Mars own GEO. Nor that the said martian GEO was so "low".

It feels like an overlooked factoid.  While speedy Phobos circles Mars three times daily, Deimos appears to move like a turtle; the fact it keeps pace fairly close to the stationary point means it doesn't set for 2 and a half days straight, then likewise vanishing from view for the same again before rising.  Compared to Phobos or a would-be low Mars orbit satellite, this is more constant.

In passing, this also means Venus own GEO must be pretty similar to Earth's - just a bit less than 22 000 miles / 36000 km. At least there is no enormous Moon to gravitationnaly screw Venus GEO.
  Not remotely.  If you want a Venus synchronous orbit, you're asking for something that rotates backward from rest of solar system and circles Venus once every 243 Earth days...because Venus spins retrograde very, very slowly.  I wouldn't even be sure such an orbit is within Venus' Hill (i.e. gravity) sphere.  Mars is just lucky to have a similar spin rate to Earth.

Good point, I thought about that Venus weirdness too. After posting of course - D'OOOH !!!  My 7 year old kid love the fact that a Venus DAY is longer than a Venus YEAR (243 earth days vs 225).
« Last Edit: 12/09/2022 12:09 pm by Airlocks »

Offline tbellman

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Re: Deimos and Martian comm satellites
« Reply #5 on: 12/09/2022 03:25 pm »
Not remotely.  If you want a Venus synchronous orbit, you're asking for something that rotates backward from rest of solar system and circles Venus once every 243 Earth days...because Venus spins retrograde very, very slowly.  I wouldn't even be sure such an orbit is within Venus' Hill (i.e. gravity) sphere.  Mars is just lucky to have a similar spin rate to Earth.

A Venus-stationary orbit would have a radius of a bit above 1.5 million kilometers.  But the Hill sphere of Venus is just slightly above 1.0 million kilometers.  So your suspicion is correct, there is no such thing as a Venus-stationary orbit.

Offline CuddlyRocket

Re: Deimos and Martian comm satellites
« Reply #6 on: 12/09/2022 08:11 pm »
My 7 year old kid love the fact that a Venus DAY is longer than a Venus YEAR (243 earth days vs 225).

Although often quoted that's actually not true. Although Venus rotates once every 243 Earth days, its solar day (which is what we mean by 'day'; that is the time between successive noons) is only 116 Earth days. This is due to its orbital motion.

Offline Barley

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Re: Deimos and Martian comm satellites
« Reply #7 on: 12/10/2022 01:57 am »

Synchronous orbit, like Earth's saturated GEO, would be a sweet spot for communication needs for Mars.  A sticking point is the fact it would be stationary; more specifically it'd be useless to a rover, lander, or base on the other side of planet.  However, Deimos' not-so-synchronous position means it drifts, very slowly, from a Martian perspective.  The entirety of Mars gets a view of Deimos, and a satellite emulating the moon means it could cover the planet while providing decent coverage for several days to specific sites; a compromise when your budget is low.

I'm having a hard time seeing any mission where this would be a useful compromise for a comm sat.

Maybe if your controller is on the comm sat, but that's not going to be a low budget.

If you're communicating from Earth a five minute window every couple of hours is more useful.  The round trip is not exactly real time so you have to schedule blocks of work anyway.  Being to do this multiple times on any day rather than on half of them is going to be a win.  Unless you have really short autonomous work blocks, but that would hardly be state of the art.

If you're communicating from a Mars base you either want the continuous coverage at the base, or at least a short period so you get lots of store and forward opportunities.

Offline redliox

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Re: Deimos and Martian comm satellites
« Reply #8 on: 12/12/2022 12:38 pm »

Synchronous orbit, like Earth's saturated GEO, would be a sweet spot for communication needs for Mars.  A sticking point is the fact it would be stationary; more specifically it'd be useless to a rover, lander, or base on the other side of planet.  However, Deimos' not-so-synchronous position means it drifts, very slowly, from a Martian perspective.  The entirety of Mars gets a view of Deimos, and a satellite emulating the moon means it could cover the planet while providing decent coverage for several days to specific sites; a compromise when your budget is low.

I'm having a hard time seeing any mission where this would be a useful compromise for a comm sat.

For optical communication higher orbits make it easier to differentiate the target, and likewise the slower orbits makes it easier to track the satellites amongst each other or from the Martian surface.


If you're communicating from Earth a five minute window every couple of hours is more useful.  The round trip is not exactly real time so you have to schedule blocks of work anyway.  Being to do this multiple times on any day rather than on half of them is going to be a win.  Unless you have really short autonomous work blocks, but that would hardly be state of the art.

If you're communicating from a Mars base you either want the continuous coverage at the base, or at least a short period so you get lots of store and forward opportunities.

A synchronous satellite definitely should be over the base ideally, although depending on its location features like Tharsis can tug sats down like the mascons do on Luna.  A distant, mobile satellite would be less effected by gravity anomalies.  Regarding synchronous satellites, a paper about future communication needs and plans cites using one or two synchronous satellites and a mothership in an elliptical 24-hour orbit.
« Last Edit: 12/12/2022 10:02 pm by redliox »
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Offline Barley

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Re: Deimos and Martian comm satellites
« Reply #9 on: 12/13/2022 01:28 pm »

A synchronous satellite definitely should be over the base ideally, although depending on its location features like Tharsis can tug sats down like the mascons do on Luna.  A distant, mobile satellite would be less effected by gravity anomalies.  Regarding synchronous satellites, a paper about future communication needs and plans cites using one or two synchronous satellites and a mothership in an elliptical 24-hour orbit.

On second thought I think synchronous satellites may never appear over Mars.  The use of physics to avoid math, and fixed geometry to avoid computation, is so mid-20th century.  Starlink is the current pinnacle, but there are many related developments in cell phones, GPS and other fields.  I think Moore's law will win the race over areostationary orbits.

Offline Welsh Dragon

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Re: Deimos and Martian comm satellites
« Reply #10 on: 12/13/2022 03:02 pm »

A synchronous satellite definitely should be over the base ideally, although depending on its location features like Tharsis can tug sats down like the mascons do on Luna.  A distant, mobile satellite would be less effected by gravity anomalies.  Regarding synchronous satellites, a paper about future communication needs and plans cites using one or two synchronous satellites and a mothership in an elliptical 24-hour orbit.

On second thought I think synchronous satellites may never appear over Mars.  The use of physics to avoid math, and fixed geometry to avoid computation, is so mid-20th century.  Starlink is the current pinnacle, but there are many related developments in cell phones, GPS and other fields.  I think Moore's law will win the race over areostationary orbits.
Moores law has to be paid for. Launching to Mars is a lot more expensive than launching to LEO (yes even with a Starship that magically hits all design and cost goals). As it will take a long time before you'd need global coverage, single targeted stationary satellites covering a few landing sites will be more economical than low orbit constellations for a long time as well.

Offline Barley

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Re: Deimos and Martian comm satellites
« Reply #11 on: 12/14/2022 06:11 am »

A synchronous satellite definitely should be over the base ideally, although depending on its location features like Tharsis can tug sats down like the mascons do on Luna.  A distant, mobile satellite would be less effected by gravity anomalies.  Regarding synchronous satellites, a paper about future communication needs and plans cites using one or two synchronous satellites and a mothership in an elliptical 24-hour orbit.

On second thought I think synchronous satellites may never appear over Mars.  The use of physics to avoid math, and fixed geometry to avoid computation, is so mid-20th century.  Starlink is the current pinnacle, but there are many related developments in cell phones, GPS and other fields.  I think Moore's law will win the race over areostationary orbits.
Moores law has to be paid for. Launching to Mars is a lot more expensive than launching to LEO (yes even with a Starship that magically hits all design and cost goals). As it will take a long time before you'd need global coverage, single targeted stationary satellites covering a few landing sites will be more economical than low orbit constellations for a long time as well.

Who said a low orbit constellation?  GPS provides quadruple coverage worldwide with 24 satellites.  A constellation of three satellites in say a 25 hour circular equatorial orbits provides the same coverage as the same satellites in synchronous orbits.

Stationary orbits do not provide an advantage to the satellite.  The advantage of a stationary orbit is for the ground station, you can align once and don't have to track.  With a phased array that auto aligns that is not an advantage.

Non-stationary constellations degrade better.  The first few failures result in intermittent coverage rather than complete loss of communications at selected locations.  Not having a strongly preferred orbit also allows relay duties to be shared with satellites put in orbit for other purposes such as sensing.

 

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