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#1 2005-01-23 17:52:31

Timeslicer
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From: Arizona
Registered: 2004-06-19
Posts: 27

Re: Mars Dogleg - Phobos refueling station

If Phobos turns out to have ice as a high fraction of it's composition, might it be beneficial to use an automated ice-mining/fuel production system there to fill a tank for Mars return?

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#2 2005-01-23 20:59:39

Commodore
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From: Upstate NY, USA
Registered: 2004-07-25
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Re: Mars Dogleg - Phobos refueling station

Theres no reason why it can't, although I think by the time the investment is put into it the requirement will have evaporated.

Phobos is destined to be a space station/elevator anchor.


"Yes, I was going to give this astronaut selection my best shot, I was determined when the NASA proctologist looked up my ass, he would see pipes so dazzling he would ask the nurse to get his sunglasses."
---Shuttle Astronaut Mike Mullane

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#3 2005-01-23 22:31:18

RobS
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From: South Bend, IN
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Re: Mars Dogleg - Phobos refueling station

I think it's unlikely Phobos has ice. The poles both get sunlight, so it has no permanently shadowed areas. The interior is probably shattered carbonaceous chondrite. That substance will have water bound in it, though, maybe up to 10% by mass, though a few percent is more likely.

Some sort of mining machine that roasts the chondrite to drive off the water might be possible. A drill that heats the interior directly and captures the vapor escaping up the shaft would also work.

       -- RobS

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#4 2005-01-23 22:59:17

Timeslicer
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From: Arizona
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Re: Mars Dogleg - Phobos refueling station

Actually, it appears to be pretty commonly assumed that Phobos has a lot of ice. 

For example, read this information on Phobos' density.

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#5 2005-01-24 21:35:32

RobS
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Re: Mars Dogleg - Phobos refueling station

I don't think it's "commonly assumed." It is a fact that Phobos and Deimos are low in density. But we don't know what percentage of the interior is void space caused by impacts and fracturing.

       -- RobS

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#6 2005-03-02 20:23:50

TwinBeam
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From: Chandler, AZ
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Posts: 144

Re: Mars Dogleg - Phobos refueling station

It certainly seems worth at least dropping a probe on Phobos to check if it has ice or at least hydrogen, if perhaps only deep inside.  I'm not aware of any more convenient (potential) source of hydrogen, carbon and oxygen. 

The main issue appears to be that mining Phobos would be difficult, if there isn't ice - certainly more complex and energy intensive than sucking CO2 out of Mars' atmosphere and using a supply of hydrogen brought from Earth.  But if there is ice, it doesn't seem unreasonably complex to send a drilling rig and insert a nuclear-heated "tap" to collect water - the overall difficulty seems considerably smaller than making a Mars aerobraking craft that will work right.

That's a significant point, because with a Phobos refueling station, the Mars Direct plan might be changed to eliminate the risky aerobraking maneuver.  A smallish lander could take on fuel to break orbit and land - leaving most of the Mars transit ship at Phobos. 

Keep the Mars Direct idea of making fuel on Mars, so the lander goes down knowing that in addition to one or two Habs, there's a fueled ship waiting for return to orbit.  By not hauling fuel down for the re-launch, the lander can be kept smaller and safer.   Eventually launching from Mars back to Phobos, the crew finds a refueled Mars transit ship waiting to take them home - no need to launch with enough fuel for that, either.

For Earth re-entry, there are two options.  If the Mars lander can be designed to be safe for Mars and Earth landings, it would be re-fueled at Phobos and brought along.  If not, it could be mothballed and left at Phobos for possible future emergency or other use, and an Earth landing ship would be waiting - recently refueled from Phobos.  In either case, the lander will use its rockets to shed most orbital velocity, for a safer re-entry.

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#7 2005-03-02 22:05:01

RobS
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Re: Mars Dogleg - Phobos refueling station

Phobos and Deimos are useful, but not as useful as one might think:

Mars surface to low Mars orbit (LMO): 4.1 km/sec
LMO to Phobos: 1.4 km/sec
LMO to Mars escape: 1.4 km/sec
LMO to Hohmann trajectory to Earth: 2.3 km/sec
LMO to Earth, 6-month transit: 2.9 km/sec
Phobos to Hohmann: 1.9 km/sec
Phobos to Earth, 6-month trajectory: 2.5 km/sec

So you see, a ship going to the Earth from the Martian surface via Phobos has to accomplish a delta-v of 5.5 km/sec to get to Phobos, then 2.5 km/sec more to get to Earth; 8.0 km/sec versus 7.0 km/sec if going direct. Phobos does help, but there is a penalty.

The reason is because when you get into a fairly high elliptical orbit around a planet, it takes a lot of energy to circularize the orbit. Think of it this way; let's say you have a probe into an orbit that almost reaches escape, then falls almost back to the planet, then back to the edge of the gravitational well. When the probe is out there at the edge of the well, a small push could cause the probe to escape, but a push in another direction could circularize the orbit instead.

A more efficient system is to make fuel on Phobos or Deimos, then move it into a Phobos (or Deimos) transfer orbit and refuel the Earth-bound ship there. If you do that, your Earth bound ship moves from low Mars orbit into an elliptical orbit, refuels, then heads for Earth; it avoids the circularization burn to rendezvous with Phobos and the "decircularization burn" to move back into an elliptical orbit, the best one to head to Earth.

For Phobos, circularization and decircularization both take 0.5 km/sec. For Deimos, they take 0.7 km/sec. These figures are according to a Caltech web page that is no longer up.

        -- RobS

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#8 2005-03-02 23:16:44

Commodore
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From: Upstate NY, USA
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Re: Mars Dogleg - Phobos refueling station

Of course with time and if significant resources are found, it would be worth it to change the orbit.

Actually one of them, I forgot which, has a decaying orbit already, so we more or less have to eventually.


"Yes, I was going to give this astronaut selection my best shot, I was determined when the NASA proctologist looked up my ass, he would see pipes so dazzling he would ask the nurse to get his sunglasses."
---Shuttle Astronaut Mike Mullane

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#9 2005-03-03 12:18:02

RobS
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Re: Mars Dogleg - Phobos refueling station

Change their orbits? Who would be so crazy as to do that? These are not little rock piles, they are miles across and have huge masses. Phobos won't crash into Mars for something like 50 million years, so I doubt anyone will worry about its robit any time soon.

          -- RobS

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#10 2005-03-03 12:50:40

Palomar
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Re: Mars Dogleg - Phobos refueling station

Change their orbits? Who would be so crazy as to do that? These are not little rock piles, they are miles across and have huge masses. Phobos won't crash into Mars for something like 50 million years, so I doubt anyone will worry about its robit any time soon.

          -- RobS

*Thank you, Rob.  :up:  The voice of reason.

--Cindy


We all know those Venusians: Doing their hair in shock waves, smoking electrical coronas, wearing Van Allen belts and resting their tiny elbows on a Geiger counter...

--John Sladek (The New Apocrypha)

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#11 2005-03-03 12:55:01

John Creighton
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Re: Mars Dogleg - Phobos refueling station

Change their orbits? Who would be so crazy as to do that? These are not little rock piles, they are miles across and have huge masses. Phobos won't crash into Mars for something like 50 million years, so I doubt anyone will worry about its robit any time soon.

I wonder what the delta V is between a highly elliptical orbit and Martian synchronous orbit. If phoebes is to be used as a space elevator I would think this is where you would want to put it. I am sure the higher up in Mars orbit phoebes is the more useful it will be for refueling and the less it will get in the way of a space elevator. Obviously you would need a farley high ISP engine to do this or there wouldn’t be much of phoebes left by the time you got it there. As for high ISP engines they could take a long time but each year the phoebes dogleg stop will get that much better. I wonder what other options there might be like using the atmosphere to circularize or an ion tub to hull the fuel up from phoebes to a transfer orbit.

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#12 2005-03-03 13:31:10

Commodore
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From: Upstate NY, USA
Registered: 2004-07-25
Posts: 1,021

Re: Mars Dogleg - Phobos refueling station

Change their orbits? Who would be so crazy as to do that? These are not little rock piles, they are miles across and have huge masses. Phobos won't crash into Mars for something like 50 million years, so I doubt anyone will worry about its robit any time soon.

          -- RobS

Without knowing its make up theres no real way to know how long its got. If its a loose collection of rocks, tidal forces in theory could tear it to sheds tommorrow, making any orbital intertion very dangerous.


"Yes, I was going to give this astronaut selection my best shot, I was determined when the NASA proctologist looked up my ass, he would see pipes so dazzling he would ask the nurse to get his sunglasses."
---Shuttle Astronaut Mike Mullane

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#13 2005-03-04 00:16:46

Timeslicer
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From: Arizona
Registered: 2004-06-19
Posts: 27

Re: Mars Dogleg - Phobos refueling station

Phobos and Deimos are useful, but not as useful as one might think

There's a LOT of difference between 5.5km/sec (Mars to Phobos) and 7km/sec (Mars to Earth) in terms of fuel.  You'd have to have roughly 4.5 times as much fuel in the Mars ascent vehicle to go direct from Mars surface to Earth, vs to just get to Phobos!

The difference coming from Earth is also pretty significant.  With Mars-direct style aerobraking,  2.9km/sec to LMO vs 2.5km/sec to Phobos means about a 50% increase in starting fuel mass - and that's fuel mass that has to be lugged up from Earth if you're not sending fuel from Phobos, and it ignores any increase in fuel needed to bring along the aerobrake.  And at least for the manned landings on Mars, using rockets instead of aerobraking seems more sensible.

The difference gets even more significant if your suggestion for using a Phobos transit orbit can be used on the way to Mars.  Guessing 2km/s from Earth to such an orbit, the fuel for Earth to LMO is about 2.5x greater.  That big a change could make the difference between needing a new heavy lift launch system and using an existing launch system, even assuming the fuel is brought up from Earth in both cases.

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#14 2005-03-04 09:27:29

GCNRevenger
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From: Earth
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Posts: 6,056

Re: Mars Dogleg - Phobos refueling station

"The difference coming from Earth is also pretty significant.  With Mars-direct style aerobraking,  2.9km/sec to LMO vs 2.5km/sec to Phobos means about a 50% increase in starting fuel mass - and that's fuel mass that has to be lugged up from Earth if you're not sending fuel from Phobos, and it ignores any increase in fuel needed to bring along the aerobrake.  And at least for the manned landings on Mars, using rockets instead of aerobraking seems more sensible.

The difference gets even more significant if your suggestion for using a Phobos transit orbit can be used on the way to Mars.  Guessing 2km/s from Earth to such an orbit, the fuel for Earth to LMO is about 2.5x greater.  That big a change could make the difference between needing a new heavy lift launch system and using an existing launch system, even assuming the fuel is brought up from Earth in both cases"

Huh? Why would it cost any less fuel to break Earth orbit and reach Phobos as it would to break orbit and reach LMO? I don't buy your figures. Aerobraking does not require much fuel at all as far as OMS either.


"The power of accurate observation is often called cynicism by those that do not have it." - George Bernard Shaw

The glass is at 50% of capacity

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#15 2005-03-04 17:54:27

TwinBeam
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From: Chandler, AZ
Registered: 2004-01-14
Posts: 144

Re: Mars Dogleg - Phobos refueling station

Huh? Why would it cost any less fuel to break Earth orbit and reach Phobos as it would to break orbit and reach LMO? I don't buy your figures. Aerobraking does not require much fuel at all as far as OMS either.

I just used RobS' deltaV figures for Mars to Phobos/Earth/etc to figure rough relative fuel requirement ratios, and then assumed that the same deltaV's could be used as good approximations for the opposite direction (Earth to Mars) as well.   I presume the difference is due to different orbital velocities and velocity that would be lost escaping Mars' gravity field or gained going into it. 

If we don't try to get into LMO prior to aerobraking, that changes my assumptions - but we'd need to shed even more velocity via aerobraking, making it that much riskier.  Perhaps it's a reasonable risk for dropping unmanned components like the Habs, where a catastrophic failure is bad but probably doesn't shut the Mars program down to hold Congressional hearings, as it certainly would if a human crew died.

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#16 2005-03-04 17:55:55

TwinBeam
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From: Chandler, AZ
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Re: Mars Dogleg - Phobos refueling station

PS - I should clarify - TwinBeam = TimeSlicer, just on different machines.  I really should get it synchronized to just one name...

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#17 2005-03-04 23:28:14

RobS
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From: South Bend, IN
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Re: Mars Dogleg - Phobos refueling station

I wrote a message about twelve hours ago and lost it somehow, so now i'll try again. . .

Regarding the internal strength of Phobos, it is probably irrelevant in this case. The moon will hold itself together because of its own gravity until it reaches Roche's limit, when Mars's gravity will rip it apart. A body of material 10+ miles across cannot hold itself together any way other than gravity; the chemical bonds within and between crystals are too weak for something so large.

At any rate, I can't imagine anyone really moving the moons. Even if the technology existed, the effort would not be economic. If you are worried that they are in the way of a space elevator, do what Kim Stanley Robinson did: vibrate the elevator cable so that it moves out of the way of Phobos and Deimos. That's as plausible an idea as any of the others.

Regarding areosynchronous orbit, it is not far from Deimos (which circles Mars once every 30 hours). Deimos requires a delta-v of 5.3 km/sec to enter a Deimos transfer orbit from the Martian surface (this is an orbit with a periapsis just above the atmosphere and an apoapsis at the height of Deimos) and a delta-v of 0.7km/sec for circularization of the orbit at apoapsis. I suspect areosynchronous is about the same.

Note that escape from Mars, according to this website, is 5.5 km/sec. Deimos is most of the way out of Mars's gravity well.

To reach Phobos from Earth, you'd do this:

1. Enter a Phobos transfer orbit, which would require a delta-v of 1.4 km/sec if one approached Mars on a Hohmann trajectory or about 2 km/sec if approaching Mars on a six-month trajectory. If one used aerobraking, the entire delta-v could be done without fuel.

2. Circularize your orbit at apoapsis (the altitude of Phobos) which requires a delta-v of 0.5 km/sec.

Deimos requires a smaller delta-v to enter a transfer orbit, but a larger circularization delta-v (0.7 km/sec).

         -- RobS

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#18 2005-03-29 19:25:17

Hop
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Re: Mars Dogleg - Phobos refueling station

1. Enter a Phobos transfer orbit, which would require a delta-v of 1.4 km/sec if one approached Mars on a Hohmann trajectory or about 2 km/sec if approaching Mars on a six-month trajectory. If one used aerobraking, the entire delta-v could be done without fuel.

2. Circularize your orbit at apoapsis (the altitude of Phobos) which requires a delta-v of 0.5 km/sec.

Let's see, if periapsis is 300 km above Mars surface and apoapsis 5981 km, I get 1.42 km/sec to shed enough velocity for Phobos transfer and about .52 km/sec to circularize orbit at Phobos (pretty close to your figures). A total of 1.94 km/sec.

However if I set periapsis at 5981 km, a single burn to circularize takes about 1.88 km/sec.

I think the only advantage of entering an elliptical rather than a circular orbit in this case is delta vee savings from aerobraking.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#19 2005-03-29 19:35:57

Hop
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Re: Mars Dogleg - Phobos refueling station

Deimos requires a smaller delta-v to enter a transfer orbit, but a larger circularization delta-v (0.7 km/sec).

Hmmm with a 300 km periapsis and a 20092 apoapsis, I get 1.02 km/sec to enter Deimos transfer and .65 km/sec to circuralize at apoapsis, for a total of 1.67 km/sec.

Entering circular orbit directly at periapsis takes 1.92 km/sec. For Deimos, an intermediate elliptical transfer saves delta-v even without aerobraking.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#20 2005-03-29 21:37:46

RobS
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From: South Bend, IN
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Re: Mars Dogleg - Phobos refueling station

I was quoting figures from a Caltech web page that no longer works. Fortunately I had printed it out. Are you calculating the delta-vs directly? Very useful. I don't have the formulas for that.

        -- RobS

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#21 2005-03-30 15:08:35

Hop
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From: Ajo
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Re: Mars Dogleg - Phobos refueling station

Rob,

The Delta-Vs come from my Hohmann Excel spreadsheet
http://www.clowder.net/hop/railroad/Hohmann.xls
The periapsis and apoapsis cells are treated as distance above planet's surface. So if you get Phobos or Deimos distance from Mars center (for example) be sure to subtract Mars' radius to get altitude.

This version doesn't include elliptical apoapsis velocity or circular orbit velocity at apoapsis. I hope to upload an updated, improved sheet soon.

The spreadsheet assumes coplanar, circular orbits which isn't too bad an approximation except for Pluto, which is why Pluto's excluded.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#22 2005-03-30 18:05:06

RobS
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From: South Bend, IN
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Re: Mars Dogleg - Phobos refueling station

By the way, I LOVE that spreadsheet of yours! I saved it on my computer a month or so ago and refer to it every week.

I wonder whether it can do more things than I have used it for. This version does not allow you to change parameters; for example, you have to use Hohmann trajectories. What I'd like to be able to do is plug in a launch date from Earth and a travel time to Mars and get a delta-v. Is this possible?

         -- RobS

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#23 2005-03-31 12:58:48

Hop
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From: Ajo
Registered: 2004-04-19
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Re: Mars Dogleg - Phobos refueling station

There is a section in Prussing and Conway's "Orbital Mechanics" (An excellent book!) on Lambert space triangles.

I am hoping it will enable me to find delta vees if I have space-time coordinates of departure and destination points from two orbits.

Sadly, I don't yet grok this chapter.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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#24 2005-03-31 17:44:08

RobS
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From: South Bend, IN
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Re: Mars Dogleg - Phobos refueling station

Well, when you do, let me know! If you'd like to experiment with some times and dates, I can offer some. I am writing a big science fiction novel series (The Mars Frontier) about the exploration, then settlement of Mars. It is easy to figure out the earliest flights between the planets because you pretty much have to go with the lowest delta-v and use conjunction-class trajectories. But as Mars gets a bigger outpost population, as it begins to develop cryogenic fuel resources on Phobos and Deimos, and as it can handle larger numbers of immigrants, then more options present themselves. I have no way to figure out, for example, when a flight would leave Mars, fly to Earth, stay there a short time (say a month) then return to Mars. My guess is that you leave Mars about five months before opposition, fly to Earth in 3 to 3 1/2 months, stay 2-4 weeks, then leave Earth 1 month before opposition and fly back to Mars in 3-6 months. One can do the same from Earth to Mars for a tourist flight, but the Earth to Mars and back flight requires one to depart from the low-energy Hohmann trajectory much more, so it probably requires higher delta-vs and therefore is more expensive (too bad for tourists).

(The conjunction-class lowest energy flights, according to Zubrin's *Case for Mars,* are as follows: leave Mars for Earth 5 months before opposition, arriving at Mars 1 month after; leave Earth for Mars 2 months before opposition, arriving at Mars 4 months after. As you can see, it is easier to speed up the Mars to Earth leg to make it fit the Earth to Mars leg, than vice-versa.)

It also appears possible to depart Mars (or Earth) about 3-4 months after opposition, fly to Mercury (or at least to its orbit, if the planet is not conveniently placed) and then on to the other planet in about 270 days (100 days between Earth and Mercury, 170 between Mercury and Mars). This is nice because it gets a passenger spacecraft back to the other planet for reuse well ahead of the next opposition and essentially increases by fifty percent the number of passenger flights between planets every twenty-six months. One vehicle (or pair of vehicles, for safety) would fly Earth to Mars to Earth, then back to Mars via Mercury; another vehicle or pair would fly Mars to Earth and back to Mars, then to Earth via Mercury. Unfortunately when you arrive at Earth (or Mars) via Mercury, the other planet is not favorably placed for a flight for close to year, at which point you are approaching opposition anyway. For small vehicles a Mercury-leg is not particularly safe because of the increased exposure to solar radiation; but a larger passenger ship (more than 100 passengers) has much more shielding inherent in its structure and so solar radiation is not so much of a problem, and the sun's magnetic field deflects cosmic radiation more when you are close than when you are far away, so cosmic ray exposure actually decreases on such a trajectory.

Anyway, that's some food for thought for you, if it is of any interest.

                -- RobS

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#25 2005-03-31 18:23:20

Hop
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From: Ajo
Registered: 2004-04-19
Posts: 146
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Re: Mars Dogleg - Phobos refueling station

Well, when you do, let me know! If you'd like to experiment with some times and dates, I can offer some.

I'd like to take a shot at it (am not guaranteeing delivery though). Offer away.


Hop's Orbital Mechanics Coloring Book - For kids from kindergarten to college.

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