Author Topic: Station On Phobos  (Read 55332 times)

Offline TakeOff

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Re: Station On Phobos
« Reply #80 on: 07/08/2016 01:27 pm »
I think that Deimos and Phobos are important targets for scientific reasons, not just as a stepping stone to Mars' surface. How were they formed or captured? What are low density objects like? We could learn more from them than from Mars itself. We have to go there sooner or later anyway, it is unstoppable, so why not start out with them since it is easier and actually demonstrate how we can get out there during two years. They are the perfect combination of lunar, martian and asteroid exploration, and kind of microgravity space station too. It should unify everyone regardless of favorite target to visit. Phobos and Deimos have something for everyone, and certainly surprises.

Offline mikelepage

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Re: Station On Phobos
« Reply #81 on: 03/05/2017 04:26 am »
I think that like everyone else, I have always assumed that any manned station in orbit around Mars will be at/around Phobos or Deimos, but I just remembered that Mars' equator (and Phobos and Deimos orbits) is at ~25 degrees to the ecliptic.

So any craft going to Phobos or Deimos is coming in on a hyperbolic trajectory at 1.85 degrees to the ecliptic (Mars' inclination), and has to aerobrake into an orbit 25 degrees to the ecliptic.

I presume that this is possible if your approach to the limb of Mars occurs during the time point when the Mars equator is co-planar with your trajectory.  BUT (someone please correct me if I'm wrong), doesn't this mean this can only occur twice per Martian year?  Even if you assume you can do some plane-change adjustment burns of (5?) degrees, that puts some serious time constraints on launching to Phobos and Deimos, doesn't it?

It would probably be far easier to have manned stations in orbit around Mars at 1.85 degrees to the ecliptic? Or is this the main reason that people prefer direct-to-Mars-surface missions?

Offline Dalhousie

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Re: Station On Phobos
« Reply #82 on: 03/05/2017 05:01 am »
Phobos has it's points of interest. However it has a surface area of only 1500 km2.  For comparison the 100 km radius exploration zone for the first crewed missions has a surface area of over 30,000 km2, more than 20 times that of Phobos. Plus Phobos by it's nature will be a much simpler and less diverse body than the surface of Mars.  It's won't take long for it to be fairly exhaustively explored.  A month at most.
Apologies in advance for any lack of civility - it's unintended

Offline TakeOff

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Re: Station On Phobos
« Reply #83 on: 03/13/2017 07:51 pm »
I think that like everyone else, I have always assumed that any manned station in orbit around Mars will be at/around Phobos or Deimos, but I just remembered that Mars' equator (and Phobos and Deimos orbits) is at ~25 degrees to the ecliptic.

So any craft going to Phobos or Deimos is coming in on a hyperbolic trajectory at 1.85 degrees to the ecliptic (Mars' inclination), and has to aerobrake into an orbit 25 degrees to the ecliptic.

I presume that this is possible if your approach to the limb of Mars occurs during the time point when the Mars equator is co-planar with your trajectory.  BUT (someone please correct me if I'm wrong), doesn't this mean this can only occur twice per Martian year?  Even if you assume you can do some plane-change adjustment burns of (5?) degrees, that puts some serious time constraints on launching to Phobos and Deimos, doesn't it?

It would probably be far easier to have manned stations in orbit around Mars at 1.85 degrees to the ecliptic? Or is this the main reason that people prefer direct-to-Mars-surface missions?
I don't think there's any problem with using gravity assist at Mars in order to achieve 25 degrees inclination for an orbital insertion. 25 degrees is still small relative to 45 (half) or 90 (zero) gravity assist in the direction of Mars' orbit. And with aerobraking you have a much greater freedom to choose inclination. And add the Oberth effect to that. One doesn't need to match Phobos plane around Mars with the ecliptic in order to enter orbit around it. Mars' gravity and atmosphere is well enough to take care of that adjustment.
« Last Edit: 03/13/2017 07:52 pm by TakeOff »

Offline TakeOff

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Re: Station On Phobos
« Reply #84 on: 03/13/2017 07:56 pm »
Phobos has it's points of interest. However it has a surface area of only 1500 km2.  For comparison the 100 km radius exploration zone for the first crewed missions has a surface area of over 30,000 km2, more than 20 times that of Phobos. Plus Phobos by it's nature will be a much simpler and less diverse body than the surface of Mars.  It's won't take long for it to be fairly exhaustively explored.  A month at most.
Astronauts would spend half a year going to, and another half a year coming home from, Mars' moons. So there remains one year to 14 months or so of a conjuncture period to spend there. 1,500 km^2 for 4 astronauts to explore in milligravity EVA's at a world of a kind thus far completely unknown, is plenty of work, plenty, trust me! And two moons at that.
« Last Edit: 03/13/2017 07:57 pm by TakeOff »

Offline Dalhousie

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Re: Station On Phobos
« Reply #85 on: 03/13/2017 10:13 pm »
Phobos has it's points of interest. However it has a surface area of only 1500 km2.  For comparison the 100 km radius exploration zone for the first crewed missions has a surface area of over 30,000 km2, more than 20 times that of Phobos. Plus Phobos by it's nature will be a much simpler and less diverse body than the surface of Mars.  It's won't take long for it to be fairly exhaustively explored.  A month at most.
Astronauts would spend half a year going to, and another half a year coming home from, Mars' moons. So there remains one year to 14 months or so of a conjuncture period to spend there. 1,500 km^2 for 4 astronauts to explore in milligravity EVA's at a world of a kind thus far completely unknown, is plenty of work, plenty, trust me! And two moons at that.

Phobos is worth a visit, but is hardly worth spending an entire long stay mission there.  There is far more of interest on the surface.
Apologies in advance for any lack of civility - it's unintended

Offline redliox

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Re: Station On Phobos
« Reply #86 on: 03/14/2017 06:19 am »
Astronauts would spend half a year going to, and another half a year coming home from, Mars' moons. So there remains one year to 14 months or so of a conjuncture period to spend there. 1,500 km^2 for 4 astronauts to explore in milligravity EVA's at a world of a kind thus far completely unknown, is plenty of work, plenty, trust me! And two moons at that.

Phobos is worth a visit, but is hardly worth spending an entire long stay mission there.  There is far more of interest on the surface.

Ahem...I spoke of this a while ago:

I love the Martian moons, and certainly would support visiting them.  Personally, I favor Deimos over Phobos because it is closer to synchronous orbit as well as the gravity well edge; both of which would be a boon to orbiting craft; Phobos of course is more scientifically interesting and easier to reach Mars.  The odds of visiting them after seeing Mars are good, but setting up a permanent habitat is more difficult to figure.

The Flexible Plan NASA's currently following favors orbital vehicles.  Because of weak gravity, the same vehicles can double as asteroid/Martian moon landers with minimal tinkering.  Currently the NASA idea to orbit Mars include a Phobos habitat to stay at.  However, that could easily change with politics, and if Red Dragon proves equipment (not crews, but definitely habs) can be directly landed on Mars, NASA might switch funds for a Mars camp instead of a Phobos station.

If a Phobos station is cobbled together, I'd assume it'd be built first in orbit and then fixed to the moon; dust in micro-gravity would be a titanic pain.  Taking the Bigelow ideas for a Lunar station, which likewise would be assembled in orbit before landing it in once piece, could easily be implemented for Phobos (and Deimos).  There could be surface science for the moon, remote observations on Mars with perhaps telerobotics, and even the return vehicles could dock to the station.

Pros: Easily compatible with orbital missions;unique 'asteroid' science with some Mars science (including telerobotics); potentially useful staging point (at either moon)

Cons: Less desirable than Mars camp; micro-gravity and radiation effects; redundant rather than essential v.s. Mars

I believe in any case all that's genuinely needed is an orbital vehicle to visit Phobos.  A habitat is basically the same thing pinned to the moon; you only really need it if the visit lasts more than 30 days (and, especially if the crew are otw home, shorter visits are more likely).  IMO a dedicated habitat is unnecessary, but ultimately it will depend on how NASA's plans get revised in the near future, especially in light of a Red Dragon landing bypassing the orbital route.

I agree with Dal in that we should check out (hopefully both) moons, especially so we can find out if they're useful.

I wouldn't go so far as to suggest establishing stations though.  On top of that, odds are there's going to be an agenda shift for NASA again once Trump puts his full attention on its for a fleeting moment.
« Last Edit: 03/14/2017 06:23 am by redliox »
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Offline mikelepage

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Re: Station On Phobos
« Reply #87 on: 03/15/2017 07:05 am »
Phobos is worth a visit, but is hardly worth spending an entire long stay mission there.  There is far more of interest on the surface.

Not meaning to be too facetious, but replace the word "Phobos" with the word "Space", and you have the opinion of the majority of Earth's population.

We don't know what we don't know.  I suggest we make that call ("long stay missions") after we visit there the first time.  As to making planning decisions based on what we do know, at least from a logistical perspective, it could turn out to be the easiest "asteroid" to study up close in a HSF mission.

Offline Rei

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Re: Station On Phobos
« Reply #88 on: 03/15/2017 11:15 am »
Phobos is worth a visit, but is hardly worth spending an entire long stay mission there.  There is far more of interest on the surface.

Surface access dramatically increases the mission cost.

I agree that the surface is far more interesting.  But if the choice was between a simple LMO mission, or a mission that involved a stay on Phobos or Deimos, which would you choose?

And I don't think the local propellant options should be played down. Both Phobos and Deimos have absorption spectra indicative of unmodified volatile-rich carbonaceous bodies, similar to carbonaceous chondrites (regardless of how they actually formed / ended up as moons of Mars). I've seen estimates suggesting that they're up to 20% water.  And IMHO, if they do actually have carbon similar to carbonaceous chondrites, that's really fascinating and potentially more useful for industry than Mars's plain, low pressure CO2. Carbonaceous chondrites contain a wide range of organics, including aliphatic and aromatic hydrocarbons, polycyclics like naphthalene and PAHs, carboxylic acids, alcohols, aldehydes, and tons of other things, including nitrogen-bearing compounds like ammonia, amino acids, urea, etc. I guess the closest earth analogy to the mixture would be something like bitumen, but with more nitrogen.  Sounds like a great feedstock for varying combinations of hydrocracking and distillation, you could get a full petrochemical industry going based on just that without having to take the sabatier + partial oxidation, or alternatively, SOFC -> syngas ->  liquids -> combinations of cyclization and pyrolysis route.

Of course, any offworld "mining" process at all has serious TRL issues to overcome. Even just water production.
« Last Edit: 03/15/2017 11:39 am by Rei »

Offline Dalhousie

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Re: Station On Phobos
« Reply #89 on: 03/15/2017 08:23 pm »

I agree that the surface is far more interesting.  But if the choice was between a simple LMO mission, or a mission that involved a stay on Phobos or Deimos, which would you choose?

If you are going to go to Mars orbit then you should visit at least one of the moons.  But neither probably justify the cost, except as a stepping stone to surface missions.  You could also visit one of the moons as a part of a surface mission as well.

Quote
And I don't think the local propellant options should be played down. Both Phobos and Deimos have absorption spectra indicative of unmodified volatile-rich carbonaceous bodies, similar to carbonaceous chondrites (regardless of how they actually formed / ended up as moons of Mars). I've seen estimates suggesting that they're up to 20% water.  And IMHO, if they do actually have carbon similar to carbonaceous chondrites, that's really fascinating and potentially more useful for industry than Mars's plain, low pressure CO2. Carbonaceous chondrites contain a wide range of organics, including aliphatic and aromatic hydrocarbons, polycyclics like naphthalene and PAHs, carboxylic acids, alcohols, aldehydes, and tons of other things, including nitrogen-bearing compounds like ammonia, amino acids, urea, etc. I guess the closest earth analogy to the mixture would be something like bitumen, but with more nitrogen.  Sounds like a great feedstock for varying combinations of hydrocracking and distillation, you could get a full petrochemical industry going based on just that without having to take the sabatier + partial oxidation, or alternatively, SOFC -> syngas ->  liquids -> combinations of cyclization and pyrolysis route.

But there's the rub.  Early studies suggested they were carbonaceous chondrite-like in composition, either C or D-type.  More recent work has questioned it,  suggesting that are represent material related directly to Mars, formed
as left over co-accretion debris (which means they are possibly more silicate rich than carbonaceous) or re-accretion of Mars impact debris (in which case they are probably largely anhydrous, like our moon).  The problem is the spectra of Phobos and Deimos are quite nondescript.
 
Of course, any offworld "mining" process at all has serious TRL issues to overcome. Even just water production.
[/quote]

Indeed it does.  It's going to be a lot easier to manufacture propellant on the martian surface.
Apologies in advance for any lack of civility - it's unintended

Offline Rei

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Re: Station On Phobos
« Reply #90 on: 03/16/2017 10:12 am »
If you are going to go to Mars orbit then you should visit at least one of the moons.  But neither probably justify the cost, except as a stepping stone to surface missions.  You could also visit one of the moons as a part of a surface mission as well.

Except that NASA (and other agencies) have repeatedly done studies Mars orbital missions with no surface landing to save cost. So it's worth considering, since that's a type of mission that's gotten significant consideration - whether you like that kind of mission or not.

Quote
But there's the rub.  Early studies suggested they were carbonaceous chondrite-like in composition, either C or D-type.  More recent work has questioned it,  suggesting that are represent material related directly to Mars, formed
as left over co-accretion debris (which means they are possibly more silicate rich than carbonaceous) or re-accretion of Mars impact debris (in which case they are probably largely anhydrous, like our moon).  The problem is the spectra of Phobos and Deimos are quite nondescript.

http://www.sciencedirect.com/science/article/pii/S0019103513004934

Quote
Studies using visible to near infrared spectroscopy show that the moons’ surfaces resemble D- or T-type asteroids or carbonaceous chondrite meteorites (e.g. Murchie and Erard, 1996, Rivkin et al., 2002 and Fraeman et al., 2012), although their specific mineralogy is difficult to determine because they lack strong diagnostic absorption features.

...

Comparison to asteroid spectra

Features similar to both the 0.65 μm and 2.8 μm absorptions are observed on dark asteroids interpreted to have primitive compositions (C-, G-, P-, and D-class asteroids). A search through the Vilas asteroid spectral catalog (Vilas et al., 1998) revealed several asteroids with 0.65 μm absorptions that are similar in shape and wavelength to the corresponding features observed on Phobos and Deimos (Fig. 6). Asteroids that exhibit these features sometimes have an additional absorption near 0.43 μm or 0.9 μm, but all of them are dark and red sloped. Absorptions near 0.7 μm on low albedo asteroids have been ascribed to Fe-bearing phyllosilicates, and almost always are accompanied by additional absorptions associated with hydration or hydroxylation near 3 μm (Vilas and Gaffey, 1989, Vilas et al., 1993, Vilas, 1994 and Rivkin et al., 2002).

...

5.2. Spectral feature at 2.8 μm

The position and asymmetric shape of the 2.8 μm feature is uniquely diagnostic of a fundamental vibration caused by a M–O–H (hydroxyl) stretch (Clark et al., 1990). The specific position of this absorption can vary depending on the cation attached to the hydroxyl, although the lack of reliable CRISM data around 2.7 μm makes it difficult to assign a band center with enough precision to provide a constraint for phase identification. Because this feature is generally stronger in pixels with stronger 0.65 μm bands and the 0.65 μm band is consistent with desiccated clays, the 2.8 μm band could result from an M–OH in a desiccated clay. Alternatively, this feature be caused by solar-wind induced hydroxylation because of the exposure of Phobos’ and Deimos’ surfaces to the space environment.

There is no water visible on their surfaces, which is expected because the surface will quickly lose water to space at those distances. However, even if their is no water beneath the surface - something that is suggested - there are at a bare minimum significant levels of surface minerals with hydroxyl groups, aka, hydrogen-bearing. As for carbon, regardless of how Phobos and Deimos formed, their spectra are similar to that of carbonaceous chondrites.

No, we certainly can't say at this point that Phobos and Deimos are good places for ISRU. But the data is suggestive that they might be.

Of course, microgravity mining suffers from significant challenges concerning anchoring.  On the other hand, removing overburden is much simpler (surfaces are generally only loosely bound, and you can throw large amounts of material significant distances with little energy). Given that we have no experience with either microgravity mining or offworld surface mining, it's quite a bit of speculation as to which would be "easier" overall.
 
Quote
Quote
Of course, any offworld "mining" process at all has serious TRL issues to overcome. Even just water production.

Indeed it does.  It's going to be a lot easier to manufacture propellant on the martian surface.

I don't follow.

1) Mars, too, is offworld.

2) I had just argued in my previous post that there are factors that argue for Phobos/Deimos propellant production vs. on the surface.

So I'm not getting how your comment follows from what I had written.
« Last Edit: 03/16/2017 10:14 am by Rei »

Offline Dalhousie

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Re: Station On Phobos
« Reply #91 on: 03/16/2017 09:23 pm »
Except that NASA (and other agencies) have repeatedly done studies Mars orbital missions with no surface landing to save cost. So it's worth considering, since that's a type of mission that's gotten significant consideration - whether you like that kind of mission or not.

I have read most of them.  What those studies show is that they don't offer a lot of return given the cost and risk.  They might have a place as a precursor mission.  Going to Mars and only landing on Phobos makes as much sense as crossing an ocean to a new continent and then only landing on a small offshore rock.

Quote
http://www.sciencedirect.com/science/article/pii/S0019103513004934

Quote
Studies using visible to near infrared spectroscopy show that the moons’ surfaces resemble D- or T-type asteroids or carbonaceous chondrite meteorites (e.g. Murchie and Erard, 1996, Rivkin et al., 2002 and Fraeman et al., 2012), although their specific mineralogy is difficult to determine because they lack strong diagnostic absorption features.

Comparison to asteroid spectra

Features similar to both the 0.65 μm and 2.8 μm absorptions are observed on dark asteroids interpreted to have primitive compositions (C-, G-, P-, and D-class asteroids). A search through the Vilas asteroid spectral catalog (Vilas et al., 1998) revealed several asteroids with 0.65 μm absorptions that are similar in shape and wavelength to the corresponding features observed on Phobos and Deimos (Fig. 6). Asteroids that exhibit these features sometimes have an additional absorption near 0.43 μm or 0.9 μm, but all of them are dark and red sloped. Absorptions near 0.7 μm on low albedo asteroids have been ascribed to Fe-bearing phyllosilicates, and almost always are accompanied by additional absorptions associated with hydration or hydroxylation near 3 μm (Vilas and Gaffey, 1989, Vilas et al., 1993, Vilas, 1994 and Rivkin et al., 2002).

5.2. Spectral feature at 2.8 μm

The position and asymmetric shape of the 2.8 μm feature is uniquely diagnostic of a fundamental vibration caused by a M–O–H (hydroxyl) stretch (Clark et al., 1990). The specific position of this absorption can vary depending on the cation attached to the hydroxyl, although the lack of reliable CRISM data around 2.7 μm makes it difficult to assign a band center with enough precision to provide a constraint for phase identification. Because this feature is generally stronger in pixels with stronger 0.65 μm bands and the 0.65 μm band is consistent with desiccated clays, the 2.8 μm band could result from an M–OH in a desiccated clay. Alternatively, this feature be caused by solar-wind induced hydroxylation because of the exposure of Phobos’ and Deimos’ surfaces to the space environment.

There is no water visible on their surfaces, which is expected because the surface will quickly lose water to space at those distances. However, even if their is no water beneath the surface - something that is suggested - there are at a bare minimum significant levels of surface minerals with hydroxyl groups, aka, hydrogen-bearing. As for carbon, regardless of how Phobos and Deimos formed, their spectra are similar to that of carbonaceous chondrites.

No, we certainly can't say at this point that Phobos and Deimos are good places for ISRU. But the data is suggestive that they might be.

Given the tenuousness of the evidence, it would be unwise to make this a priority of destination.  Here's another paper of the same vintage.

http://www.planetary.brown.edu/pdfs/5030.pdf

The cumulative data available for compositional analyses across the surface of Phobos and Deimos, however, remain incomplete in scope and character and ambiguous in interpretation. Consequently the composition of the moons of Mars remains uncertain


No conclusive meteorite analogues for either of the two units were identified, and two quite different models for
the composition of Phobos (and Deimos) were formulated and continue to be discussed. (see review by Rosenblatt, 2011 and references therein). (1) The most common concept involves some form of primitive material comparable to that thought to compose low-albedo asteroids currently found in the outer portion of the main belt (e.g., asteroid type D). Such a composition would require a mechanism to capture this material into Mars orbit. (2) The alternate concept involves material related directly to Mars, formed by co-accretion with Mars or re-accretion of Mars impact debris. Such scenarios would require that the surface of the moons have been extensively processed in orbit and/or in the space environment so that the original composition is beyond recognition.


Quote
Of course, microgravity mining suffers from significant challenges concerning anchoring.  On the other hand, removing overburden is much simpler (surfaces are generally only loosely bound, and you can throw large amounts of material significant distances with little energy). Given that we have no experience with either microgravity mining or offworld surface mining, it's quite a bit of speculation as to which would be "easier" overall.

I think we can be very confident that mining on the surface of Mars is going to be a lot easier.  Gravity is an asset in mining.  Techniques can be adapted from terrestrial methods rather than invented from scratch.  Plus we know a lot more about the physical properties, chemistry, and mineralogy of the martian surface and shallow subsurface.  And we know there are multiple possible water resources. by contrast even the possibility of resources on Phobos is speculative at best.

What you see as advantages of mining on Phobos are actually problems.  The low gravity means that material will be easy remobilised to cause hazards.   Gravity cannot be used to aid processing and stockpiling. Conventional mining technologies with won't work or will have severe limits. 

Quote
I don't follow.

1) Mars, too, is offworld.

2) I had just argued in my previous post that there are factors that argue for Phobos/Deimos propellant production vs. on the surface.

So I'm not getting how your comment follows from what I had written.

1) Don't focus so much on the off world character, but the context, conditions under which you are mining, what you are mining and what is known.  Mars is where the interest is, where the action will be, where the demand will be. Phobos is at best a stepping stone, at worst a digression.  We understand martian conditions much better than those of Phobos, we have experience of excavation, drilling, and abrading the surface (on a small scale).  We know the physical, chemical, and mineral properties of the surface. We know the possible resources with some degree of accuracy, we know that terrestrial experience can be readly applied, and can be readily simulated on Earth.  We have no such knowledge for Phobos.

2) However the counter arguments I think are much stronger. ;)  Phobos is worth a visit, certainly.  Spending a lot of time there, based on what we know now?  No.

3) Hopefully you understand better now.

Apologies in advance for any lack of civility - it's unintended

Offline alexterrell

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Re: Station On Phobos
« Reply #92 on: 03/21/2017 02:54 pm »

There is no water visible on their surfaces, which is expected because the surface will quickly lose water to space at those distances. However, even if their is no water beneath the surface - something that is suggested - there are at a bare minimum significant levels of surface minerals with hydroxyl groups, aka, hydrogen-bearing. As for carbon, regardless of how Phobos and Deimos formed, their spectra are similar to that of carbonaceous chondrites.

No, we certainly can't say at this point that Phobos and Deimos are good places for ISRU. But the data is suggestive that they might be.

The logical course of action is to send a probe, complete with drilling mechanism to evaluate some 10s of metres below the surface.

If there is indeed 20% water or Kerogen bearing materials, then from an exploration point of view, Phobos (or Deimos) becomes the most interesting place in the solar system.

If there isn't, then it's not particularly interesting - unless we want to build space habitats with mega-tonnage of radiation shielding.

Offline Rei

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Re: Station On Phobos
« Reply #93 on: 03/21/2017 05:51 pm »
My sentiments exactly. Mars's moons have great potential (or not) for ISRU, and there's some significant questions about them that need answering (which have implications for Mars as well). At least a Discovery class mission seems warranted.
« Last Edit: 03/21/2017 05:55 pm by Rei »

Offline Dalhousie

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Re: Station On Phobos
« Reply #94 on: 03/22/2017 09:57 pm »

There is no water visible on their surfaces, which is expected because the surface will quickly lose water to space at those distances. However, even if their is no water beneath the surface - something that is suggested - there are at a bare minimum significant levels of surface minerals with hydroxyl groups, aka, hydrogen-bearing. As for carbon, regardless of how Phobos and Deimos formed, their spectra are similar to that of carbonaceous chondrites.

No, we certainly can't say at this point that Phobos and Deimos are good places for ISRU. But the data is suggestive that they might be.

The logical course of action is to send a probe, complete with drilling mechanism to evaluate some 10s of metres below the surface.

If there is indeed 20% water or Kerogen bearing materials, then from an exploration point of view, Phobos (or Deimos) becomes the most interesting place in the solar system.

If there isn't, then it's not particularly interesting - unless we want to build space habitats with mega-tonnage of radiation shielding.

Drilling autonomously is hard, drilling tens of m in microgravity is even harder.

There is no need for it.  Impact gardening on Phobos will have exposed free material for analysis.  Neutron instruments should detect any shallow ice of hydrated minerals, radar would tell us about any a depth, as will as inform us on the structure (are these moons rubble piles for example).

This can all be done with an orbiter mission plus or minus a surface hopper.  Let's do things the easy way.
Apologies in advance for any lack of civility - it's unintended

Offline Hop_David

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Re: Station On Phobos
« Reply #95 on: 03/24/2017 08:24 pm »
So any craft going to Phobos or Deimos is coming in on a hyperbolic trajectory at 1.85 degrees to the ecliptic (Mars' inclination), and has to aerobrake into an orbit 25 degrees to the ecliptic.

for an incoming craft, the velocity vector when it enters Mars' sphere of influence is pretty much co-linear with the hyperbola's asymptote. And the hyperbola's focus is the center of mars. This asymptote and focal point set the plane of the hyperbolic orbit.

Any craft incoming from an earth to Mars Hohmann will have a velocity vector pointing the same direction as Mars wrt sun. So no matter what latitude the ship enters the sphere of influence, the velocity vectors would still be parallel to Mars velocity vector.

The inclination is set by what latitude of the Sphere of Influence the ship enters. No big periapsis burn is needed to match inclination with Phobos or Deimos. It is more a question of timing and how precisely we can set the approach path.

I've attached a rough pic indicating different hyperbolic orbits entering the SOI at different latitudes.

Offline mikelepage

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Re: Station On Phobos
« Reply #96 on: 03/25/2017 11:41 am »
So any craft going to Phobos or Deimos is coming in on a hyperbolic trajectory at 1.85 degrees to the ecliptic (Mars' inclination), and has to aerobrake into an orbit 25 degrees to the ecliptic.

for an incoming craft, the velocity vector when it enters Mars' sphere of influence is pretty much co-linear with the hyperbola's asymptote. And the hyperbola's focus is the center of mars. This asymptote and focal point set the plane of the hyperbolic orbit.

Any craft incoming from an earth to Mars Hohmann will have a velocity vector pointing the same direction as Mars wrt sun. So no matter what latitude the ship enters the sphere of influence, the velocity vectors would still be parallel to Mars velocity vector.

The inclination is set by what latitude of the Sphere of Influence the ship enters. No big periapsis burn is needed to match inclination with Phobos or Deimos. It is more a question of timing and how precisely we can set the approach path.

I've attached a rough pic indicating different hyperbolic orbits entering the SOI at different latitudes.

Thanks for responding David, but my question was not just about matching inclination (which I now see can be done easily), but also matching argument of Phobos' ascending node.  As you say, the vector of any approaching ship is parallel with Mars', so does that not mean that a direct approach to Phobos would only be possible twice per year?

Presumably there are other transfer orbits that can be used to precess your starting orbit around quickly, but I'm not sure what the best way to do that is.

Offline redliox

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Re: Station On Phobos
« Reply #97 on: 03/25/2017 02:48 pm »
So any craft going to Phobos or Deimos is coming in on a hyperbolic trajectory at 1.85 degrees to the ecliptic (Mars' inclination), and has to aerobrake into an orbit 25 degrees to the ecliptic.

for an incoming craft, the velocity vector when it enters Mars' sphere of influence is pretty much co-linear with the hyperbola's asymptote. And the hyperbola's focus is the center of mars. This asymptote and focal point set the plane of the hyperbolic orbit.

Any craft incoming from an earth to Mars Hohmann will have a velocity vector pointing the same direction as Mars wrt sun. So no matter what latitude the ship enters the sphere of influence, the velocity vectors would still be parallel to Mars velocity vector.

The inclination is set by what latitude of the Sphere of Influence the ship enters. No big periapsis burn is needed to match inclination with Phobos or Deimos. It is more a question of timing and how precisely we can set the approach path.

I've attached a rough pic indicating different hyperbolic orbits entering the SOI at different latitudes.

Thanks for responding David, but my question was not just about matching inclination (which I now see can be done easily), but also matching argument of Phobos' ascending node.  As you say, the vector of any approaching ship is parallel with Mars', so does that not mean that a direct approach to Phobos would only be possible twice per year?

Presumably there are other transfer orbits that can be used to precess your starting orbit around quickly, but I'm not sure what the best way to do that is.

What about the opposite?  A spacecraft coming from Mars to visit these moons before finally leaving for Earth.  The main disadvantage I know lies with circularizing the orbit to match the moon's.  If this route is chosen, which moon would also be more efficient to visit...more so if the priority is to head to Earth immediately afterwards?
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Offline Rei

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Re: Station On Phobos
« Reply #98 on: 03/25/2017 04:22 pm »
It depends - are you trying to save energy for the ascent stage or the Earth transfer stage?  Also, I suspect (although I haven't run the math) that the optimal trajectories are going to be different for the two bodies. From Phobos the optimal is probably a direct transfer burn, while from Deimos it's probably a burn to lower the periapsis, followed by the transfer burn at periapsis, in order to maximize the Oberth effect. 

Let's at least do the basics... if  you go down to a 150km periapsis from Deimos then I calculate a velocity at periapsis of 4730m/s, versus 1351m/s at Deimos (and 2138 m/s at Phobos). Calculating the same periapsis orbit from Phobos I get... 4431m/s, hmm, not as much difference as I would have expected. But of course you have to spend a lot more delta-V to lower the periapsis from Phobos.

It makes a difference whether you're doing one burn or two for the transfer because inclination changes are easier at lower orbital velocities (aka Deimos)

Offline Hop_David

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Re: Station On Phobos
« Reply #99 on: 03/26/2017 04:49 pm »
Thanks for responding David, but my question was not just about matching inclination (which I now see can be done easily), but also matching argument of Phobos' ascending node.  As you say, the vector of any approaching ship is parallel with Mars', so does that not mean that a direct approach to Phobos would only be possible twice per year?

Am drawing some pictures to help me think about this.

Deimos and Phobos aren't exactly coplanar with Mars' equatorial plane but close. I'll call them equatorial because it makes it easier to visualize and I can use some well known words.

Coming in from a Hohmann transfer, the Vinf velocity vector is perpendicular to the heliocentric position vector.

This Vinf vector needs to lie in the equatorial plane to have the ship enter on a coplanar orbit. Over a complete circuit of the moon's orbit, the moon's velocity vectors will point in every direction in that plane. The ship's Vinf velocity vector must be parallel to one of the moon's velocity vectors.

The only time a moon's velocity vector is perpendicular to the heliocentric position vector is when the moon's high in the sky at Martian noon or midnight.

Also the moon's high noon or midnight velocity vectors must occur at a time when Mars equatorial plane forms a 23.5º angle with the sun's position vector. When does this happen? At Mars' summer and winter solstice.

You might be right. A vexing observation I can't ignore.

If you just do a small braking burn to park into a large capture orbit, plane change expense is minor in the neighborhood of apoapsis. But a large capture orbit can last month to two months. Less of an option when humans are aboard but possibly a way to get less time sensitive supplies and infrastructure on the Martian moons.
« Last Edit: 03/26/2017 05:42 pm by Hop_David »

Tags: Mars Phobos Deimos cubesats 
 

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