Author Topic: Station On Phobos  (Read 15047 times)

Offline mikelepage

Re: Station On Phobos
« Reply #100 on: 03/27/2017 04:35 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 23.5 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.

Add to the cons: from Phobos/Deimos you also have to perform additional plane changes to land anywhere on Mars other than the martian equator.  Likewise any surface ascent craft from elsewhere than the equator would be detouring (more plane changes) to go to Phobos/Deimos on the way back to Earth.

Having said that, there are obviously huge advantages in being able to utilise the moons for resources, and surface colonies will likely be somewhat close to equatorial anyway.

Question: does the plane change maneuver have to occur after capture?  If approaching at a non-optimal time, you could surely shortcut the whole procedure by making a small burn some 30-60 days out from capture, so as to approach Mars from above or below the plane of Mars' orbit.  That way you can perform a simple capture maneuver into the equatorial plane no matter what time of year your approach is.

Offline JasonAW3

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Re: Station On Phobos
« Reply #101 on: 03/27/2017 05:02 PM »
AIUI Phobos may be a dust coated rubble pile with tidal issues. Not sure that's a stable platform for a base.

And because of this uncertainty I find it un-be-lie-vab-le that we still have not found time, money & interest to send even a modest lander / orbiter to Phobos or Deimos...! Those moons are near Mars, are two interesting targets on their own, are also asteroids, give possibility to do Mars observations at the same time, give knowledge for future manned mission... Looking at all this, it seems so weird NASA has no interest at all for those Martian moons.

OK, Russians have tried, but...

Strange thought here;

      Anybody ever thought to send a number of cube sats as probes to Phobos?  They wouldn't require near as much fuel to get there as a much larger probe, and, in theory, they could use the currently established network of orbital sats around Mars to piggyback a data signal back to Earth.  By using the available communications gear in orbit already, you reduce the power requirements for data transmissions significantly.

      Using cold gas propelled tethered penetrator probes, not only could you get an idea of the composition of Phobos, (In the dust ball/rubble pile/hollow rock realm) but you could anchor the probes, say around 5 or so, to the surface to gather more data.  (By using more than one probe, especially self tethering ones with the penetrating sensor probe, one would get a better idea of the actual approximate makeup of Phobos, than if only one location was utilized.  As low a gravity as Phobos has, it would be unlikely that it would have a consistent makeup throughout its structure, as evidenced by some of the cometary and asteroid encounters that have been made thus far).

      Large probes are great for long term exploration and observation, but for something like an initial probe of something like Phobos, "shotgunning" a number of smaller, cheaper and shorter duration probes at it, is likely to produce at least some results, rather than a major failure.  The same would, of course, apply to Deimos as well.

      Once we have a better idea of the ACTUAL makeup of Phobos, rather than what we speculate that it is, we could then make better and more accurate plans for building a station on Phobos, or even if it is practical in the first place.
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Offline redliox

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Re: Station On Phobos
« Reply #102 on: 03/27/2017 05:20 PM »

Strange thought here;

      Anybody ever thought to send a number of cube sats as probes to Phobos?

Yes.  Look: http://www.lpi.usra.edu/meetings/lpsc2017/pdf/1707.pdf
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Offline JasonAW3

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Re: Station On Phobos
« Reply #103 on: 03/27/2017 05:33 PM »

Strange thought here;

      Anybody ever thought to send a number of cube sats as probes to Phobos?

Yes.  Look: http://www.lpi.usra.edu/meetings/lpsc2017/pdf/1707.pdf

Ok, I stand corrected.

      So, why hasn't anyone done anything with this idea?
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Offline redliox

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Re: Station On Phobos
« Reply #104 on: 03/27/2017 06:32 PM »

Strange thought here;

      Anybody ever thought to send a number of cube sats as probes to Phobos?

Yes.  Look: http://www.lpi.usra.edu/meetings/lpsc2017/pdf/1707.pdf

Ok, I stand corrected.

      So, why hasn't anyone done anything with this idea?

Short answer: Mars is the overshadowing celebrity and NASA has limited funds.
Slightly longer answer: Cubesats are a relative new concept, haven't been employed beyond Earth orbit just yet, and you'll get scientists and engineers that'll favor more robust satellites to basically obtain "more-bang-for-the-buck" science.

You may wish to talk more about cubesats in the science section's thread about Deimos and Phobos spacecraft here: https://forum.nasaspaceflight.com/index.php?topic=36977.0
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Offline Hop_David

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Re: Station On Phobos
« Reply #105 on: 03/28/2017 12:44 PM »
I've attached a rough pic indicating different hyperbolic orbits entering the SOI at different latitudes.

My earlier pic of incoming hyperbolic orbits was somewhat inaccurate.

I asked a astrogator friend of mine if we needed to come in at Mars winter or summer solstice if we wanted an easy slide into a near equatorial orbit.

He replied with the attached illustration from Bates Mueller and White. It's a pic of envelope of outgoing hyperbolas but it could just as well be incoming hyperbolas. In his words "You can pick any inclination available by rotating around the incoming asymptote."

Offline mikelepage

Re: Station On Phobos
« Reply #106 on: 03/29/2017 07:02 AM »
I asked a astrogator friend of mine if we needed to come in at Mars winter or summer solstice if we wanted an easy slide into a near equatorial orbit.

He replied with the attached illustration from Bates Mueller and White. It's a pic of envelope of outgoing hyperbolas but it could just as well be incoming hyperbolas. In his words "You can pick any inclination available by rotating around the incoming asymptote."

Great pic, although that doesn't actually answer the question, does it? Your friend appears to have latched onto the inclination part of the question as you did upthread.  We already know any inclination is possible.

One actually has to change the angle of the incoming motion vector relative to Mars motion vector if one wants to influence the final orbit's argument of ascending node, right? and I think you can achieve this with a small burn perpendicular to spacecraft direction of motion/Mars orbital plane some weeks before capture... thus skipping the need to enter large elliptical initial orbit, followed by plane change maneuver at apoapsis.

Tags: Mars Phobos Deimos cubesats