10.0 Phobos and Deimos |
Phobos and Deimos are the two satellites of Mars. Phobos is the larger of the two and its orbit is closer to Mars than Deimos' orbit. What do we know about them? Not much. Both are believed to be captured asteroids. The following data were obtained from various sources as indicated. * Phobos (the closer and larger)
Datum Value Units Source
Mass 1.08e+16 kg [3, p.290]
Mean density 1.95 g/cu.cm. (1)
Equivalent radius 10.975 km author
Surface gravity 5.98e-3 m/sec/sec author
Escape velocity 11.46 m/sec author
Mean orbital velocity 2.14 km/sec author
Sidereal rotation period 7.65 hours [17, p.408]
Sidereal orbital period 7.65 hours "
Distance from surface 5984 km "
Mean distance from Mars 9380 km [17, p.95]
Inclination to equator 0.01 degrees "
Size 20x21x18 km "
Orbital eccentricity 0.015 "
(1) - Ad Astra, May '90, p.5.
Deimos Datum Value Units Source Mass 1.8e+15 kg [3, p.290] Mean density 1.7 g/cu.cm. " Equivalent radius 6.323 km author Surface gravity 3.00e-3 m/sec/sec author Escape velocity 6.163 m/sec author Mean orbital velocity 1.35 km/sec author Sidereal rotation period 30.3 hours [17, p.408] Sidereal orbital period 30.3 hours " Distance from surface 20060 km " Mean distance from Mars 23460 km [17, p.95] Inclination to equator 0.92 degrees " Size 16x12x10 km " Orbital eccentricity <0.001 " . Although Mariner 4 (US) was the first spacecraft to fly by Mars, Mariner 9 (US) was the first spacecraft to photograph Phobos and Deimos late in 1971. The US sent two spacecraft called Viking 1 and Viking 2 to Mars late in 1975. A very good account of the Viking missions is given in "Solar System Log" by Andrew Wilson [Ref 21, p.88-93]. Phobos and Deimos are in nearly circular orbits above Mars' equator. Both satellites are locked in orbit so that the same side of each constantly faces Mars. This makes their rotational period equal to their orbital period. Both are very dark. They reflect only 3-5% of the sun's light [40, p.167]. Spectroscopic studies by the Viking spacecraft revealed that their composition is similar to carbonaceous chondrites [31, p.149]. Perhaps the most important piece of information we have is that their densities are less than 2 grams per cubic centimeter. This is very low and strongly indicates that water is present in large quantities. It is difficult to imagine what they could contain if not water - unless perhaps they are filled with pockets of gas. The following table shows the required percentage of ice necessary to produce the observed densities of Phobos (1.95) and Deimos (1.7) assuming various given densities for the remainder of the body. * Table 10.0-1 Percent of ice on Phobos and Deimos
Non-water density Percent ice
(gm/cc) Phobos Deimos
2.0 4 27
2.25 22 40
2.5 34 49
2.75 43 56
3.0 49 61
.
Large amounts of ice are also believed to be present on Ganymede
and Callisto, two satellites of Jupiter. They both have densities
which are less than 2 grams per cubic centimeter and both are
estimated to consist of more than 60% water ice by John
Lewis and Mark Lupo [3, p.172].
The Soviet Union sent two probes to Mars in 1988 called
Phobos 1 and Phobos 2. Sadly, contact with Phobos 1 was lost within
a month of launch. Phobos 2 continued on to Mars arriving in
February of 1989. It orbited Mars from February 8 to March 25,1989
[AA 5, p.5]. A French made near-infrared imaging
spectrometer (IMS) studied both Mars and Phobos. "Phobos was found
to have nearly no water and low surface hydration [chemically bound
water], reported Y. Langevin of the Institut d'Astrophysique
Spaciale, even in the interiors of the craters" [AA 5, p.5].
Contact with Phobos 2 was lost too.
This lack of evidence of water should not be taken as too
discouraging because it doesn't change the density. In fact when
you consider that the surface of the moon is devoid
of water, you might expect the surfaces of Phobos and Deimos to
show no water. The conclusion is that we will have to dig into
Phobos to find the water.
10.1 Asteroids Asteroids are known primarily as a group of planetoids
or planetesimals which orbit the sun between Mars and Jupiter,
but there are also other groups of asteroids. There are three
groups of near earth asteroids. They are called the Atens, the
Apollos, and the Amors. The Atens orbit inside the earth's
orbit most of the time and thus take less than a year for each
revolution around the sun. The Apollos have orbits which cross
the earth's orbit and the Amors have orbits which vary from
1.017 AU to 1.3 AU, which is basically the region between the
orbits of earth and Mars.
The number of such asteroids is not well known but the
following estimates were developed by Gene Shoemaker of the USGS.
* Table 10.1-1 Near-Earth Asteroids
Delta V < 6 km/sec
Class Known Est. number Est. number Est. number Est. number
10/84 dia > 1km dia > .1km dia > 1km dia > .1km
Aten 5 100+/-40 30,000 20 6,000
Apollo 37 700+/-300 200,000 140 40,000
Amor 22 500+/-200 150,000 100 30,000
totals 64 1300+/-540 380,000 260 76,000
Source: E.M. Shoemaker, Ann. Rev. Earth Planet. Sci. 1983, 11:461.
as cited in [14, p.246].
.
Since the Apollo asteroids cross the earth's orbit there is
always the potential of a collision with the earth. On April 10,
1972 a house sized asteroid hit the atmosphere over Idaho but
skipped off and back into space [38, p.105]. Had it hit the
ground, perhaps somewhere in Alberta, it would have caused a great
deal of damage. For a more detailed account see "A Meteorite that
Missed the Earth", by L. Jacchia in Sky and Telescope 48(1974), p.4.
Some of these asteroids can be reached more easily than can
the moon. Lewis and Lewis give the following delta velocities
to a few selected Apollo asteroids. (It takes a delta velocity
of about 6000 m/s to go from LEO to the surface of the moon.)
* Table 10.1-2 Delta velocities of Apollo asteroids (from LEO) Asteroid dv out flight time dv back flight time
m/s days m/s days
1982 DB 4450 210 60 480
1982 XB 5300 220 220 470
1982 HR 5300 180 260 320
1980 AA 5400 690 360 450
Anteros 5270 390 390 290
Source: Lewis and Lewis [14, p.169].
.
There are other groups of asteroids one of which is
called the Trojans. These are groups of asteroids which orbit
the sun in the same orbit as the planet Jupiter, but at the L4
and L5 Lagrangian points of the sun-Jupiter system. Astronomers
Dunbar and Helin have searched for earth Trojans from the
Palomar observatory and have determined that no asteroids
larger than about 25 kilometers exist at the L4 or L5 points
of the sun-earth system [48, p.76].
In 1977 Charles Kowal discovered an asteroid between
Saturn and Uranus which was named Chiron [36, p.133]. It is
likely that this is merely the first of many to be discovered
in this region of the solar system. They are so distant
and so small and so dark that they are almost impossible
to detect from here on earth.
It seems likely that asteroids are present in significant
numbers beyond the orbit of Jupiter and that space travelers
will need to be very observant so as to avoid any collisions.
In order to determine the composition of asteroids, their
spectra are compared to the spectra of known groups of meteorites.
Meteorites are divided into various groups by their composition.
The most common ones are called ordinary chondrites. Other groups
are carbonaceous chondrites, achondrites, irons, and stoney irons.
The breakdown of these groups by frequency percentage is as
follows [38, p.97].
* Table 10.1-3 Meteorite class distribution
Meteorite class Percentage (approx)
carbonaceous chondrite 5
ordinary chondrite 81
achondrite 9
stoney iron 1
iron 4
.
Perhaps the most important of these groups is the carbonaceous
chondrites. The reason is that they contain up to 22% water
[38, p.102] although they average about 10% water [14, p.229].
Water is extremely important because of its ability to sustain
life and its potential is a rocket propellant. These asteroids
also contain carbon - as their name indicates. Carbonaceous
chondrite meteorites contain 3-5% carbon [114, p.31].
"The chemical composition of chondritic meteorites closely
matches that of the sun, suggesting that such meteorites represent
primitive materials that have survived without significant change
since the formation of the solar system [34, p.42]."
The class called ordinary chondrites is further subdivided
into three subgroups depending upon the concentration of iron
in the meteorite: H - high iron, L - low iron, and LL - very low
iron. These meteorites and corresponding asteroids are of
special interest because of their high concentrations of the
platinum group metals. Lewis and Lewis give the following table
of concentrations of metals in ordinary chondrites [14, p.257].
* Table 10.1-4 Concentrations of Platinum group metals in ppm
chondrite sub-class
----------------------
Element Symbol LL L H
Ruthenium Ru 12 8 5.7
Rhenium Re 1 0.6 0.5
Osmium Os 10 6 4.7
Iridium Ir 10 5 4.8
Platinum Pt 21 13 11
total 54 32.6 26.7
.
Consider a one kilometer diameter asteroid. At a density of
four grams per cubic centimeter it would weigh about 2 billion
metric tons. The metal percentages in ordinary chondrites
are: H = 16+/-3; L = 9+/-2; LL = 4+/-1 [14, p.257]. Thus this
sample asteroid would have about 200,000,000 tons of metal. The
platinum group metals would weigh about 6520 tons. Since these
metals are worth at least $10,000 per kilogram or $10 million
per metric ton, the total value would be about $65 billion.
10.2 Visiting Phobos Why go to Phobos? There are at least two very good answers.
The first is to establish a refueling station there. If we can
produce fuel on Phobos, we won't need to carry fuel for the return
trip. (If the spaceship uses our momentum exchange propulsion system,
then the fuel produced on Phobos will be used to land on and take
off from Mars itself.)
The second answer is that Phobos is a fantastic platform from
which to observe Mars. Many people will want to do a detailed survey
of Mars in order to select optimal landing sites. From Phobos we
should be able to see from the equator up to nearly 69 degrees North
or South latitude. That should be sufficient to pick out good
landing sites.
The velocity change required to go from low earth orbit
(LEO) to Phobos is about 4.37 kilometers per second [LB1, p.812].
The velocity change to return to LEO is about 3.54 kilometers
per second [LB1, p.812]. Thus the velocity change of the
whole trip from LEO to Phobos and back to LEO is about 7.91
kilometers per second. To calculate the mass ratio we use
equation 6.1-1 again. First, we will use a propulsion system with
a specific impulse of 475 seconds (LOX-LH2). This yields:
* M = m * exp( dv/g*Isp ) 6.1-1
M = m * exp( 7910/9.8*475 ) or
M = m * exp( 1.69925 ) or
M = m * 5.46983
.
The mass ratio is 5.4698. Now we consider using a
nuclear thermal rocket with a specific impulse of 950 seconds
to do the same job. Plugging the numbers in we get:
* M = m * exp( 7910/9.8*950 ) or
M = m * exp( 0.849624 ) or
M = m * 2.33877
.
The mass ratio is the square root of 5.4698 or 2.33877. Now
consider how much fuel we could save if we could refuel at Phobos.
The following table shows the mass ratios and the fuel requirements
for outbound, inbound, and round trip journeys. The mass ratios
were calculated using equation 6.1-1 and the fuel ratios were
calculated using equation 6.1-5 with the tanks and engines estimated
to be 8% of the weight of the fuel.
* Table 10.2-1 Phobos mass ratios and fuel requirements
Delta velocity Mass ratios Fuel ratio
(m/s) Isp=475 Isp=950 Isp=475 Isp=950
4370 (outbound) 2.55685 1.59901 1.778 0.629
3540 (inbound) 2.13929 1.46263 1.254 0.480
7910 (round trip) 5.46983 2.33877 6.958 1.499
.
The fuel savings for the LOX-LH2 propulsion system would
be 6.958 - 1.778 = 5.18 tons per payload ton or 74.4% because
the fuel for the return portion of the trip would come from
Phobos. Actually it probably wouldn't be quite that good because
a multi-stage rocket would probably be used which would need
less than 6.958 tons of propellant per payload ton.
The savings for the NTP (Isp=950) system would be 1.499 - 0.629 =
0.87 tons per payload ton or 58%. It can also be seen
from this table that the NTP rocket only needs 35% of the fuel
that the LOX-LH2 rocket needs for the outbound leg of the journey.
Thus the fuel cost for the NTP rocket would be about 35% of the
cost for the LOX-LH2 rocket.
10.3 The first spaceship to Phobos As in the first lunar base, the most important question is whether there will be humans on board. And again the answer should be no! Why not? There are many good reasons such as: * 1. Humans need food, water, air, protection from radiation,
waste facilities, etc. - but androids don't.
2. In order to sell high priced seats for the first trip to
Mars, it must actually be the first manned trip there;
therefore the Phobos trip cannot be manned.
3. Checkout of the momentum exchange propulsion system
could be very dangerous and is better done with no
people on board.
4. Deployment of the fuel production facility on Phobos
can be done by the androids - humans are not needed.
5. Deployment of Mars observation equipment on Phobos or
the spaceship can be done by androids as well.
.
The main components of the Phobos spaceship will probably be:
* 1. Electromagnetic projectile launcher
2. Projectiles and projectile racks
3. Nuclear power systems
4. Hydroponic food production facilities (unassembled)
5. Crew's quarters (unassembled)
6. Androids
7. Fuel production facilities
8. Mars landing systems (rocket engines, fuel tanks, etc.)
9. Mars observation equipment
.
The following paragraphs will briefly review each of the
main components.
10.3.1 The EMPL This EMPL will be very much the same as the others mentioned
previously. The internal diameter will be about one meter and the
length will be about 6 kilometers. As mentioned before, the longer
the EMPL the lower the power necessary to drive it because the
projectile takes longer to pass through the launcher. This also
means less acceleration and less stress on the structure.
The following table shows the expected velocity change in the
spaceship due to each projectile (whose mass is one metric ton)
for several different projectile velocities and several different
spaceship masses.
* Table 10.3.1-1 Spaceship velocity change per projectile
Projectile Spaceship velocity change ( m/s )
velocity ship mass =4000 4500 5000 5500 6000(MT)
(km/s) ----------------------------------
10 2.50 2.22 2.00 1.82 1.67
20 5.00 4.44 4.00 3.64 3.33
30 7.50 6.67 6.00 5.45 5.00
40 10.00 8.89 8.00 7.27 6.67
50 12.50 11.11 10.00 9.09 8.33
.
If the mass of the projectile were two metric tons, then all
the entries in the table would simply double, and so on.
10.3.2 Projectiles and projectile racks The heaviest part of the cargo of the Phobos spaceship
will be its projectiles. They will be used not only to maneuver
this spaceship, but also to stop and start the manned spaceship
arriving a few months later. As mentioned in section 6.4.2,
stopping and starting the manned spaceship will require several
thousand projectiles.
In order to limit the structural elements needed to store
the projectiles, they will simply be stored in tubes along the
length of the EMPL. The tubes will be open to the vacuum of space
because the projectiles are build to survive in space. All of
the handling of the projectiles will be automatic.
10.3.3 Nuclear power systems There will be several separate nuclear power systems - perhaps
as many as 15. During the arrival and positioning at Mars, they
will all work together to fire the projectiles with as high a
velocity as possible to reduce the number of projectiles expended
for that operation. The purpose is to save projectiles for
the arrival of the manned spaceship.
Each of the Mars landing sites will require its own nuclear
power supply. Thus if we plan to have 10 sites, we will need 10
power systems, but they can small SP-100 systems. Most of the
power must remain with the spaceship to drive the EMPL.
It is safe to assume that the power systems which drive the
EMPL can also run the fuel production facilities which will be
deployed on Phobos (or Deimos), but the initial exploration of
Phobos (and/or Deimos) must have an independent power supply.
The number of additional power systems needed will depend on
how many backups are deemed required and how many probes we wish
to deploy.
10.3.4 Hydroponic food production facilities (unassembled) The hydroponic food production facilities will be carried on board the spaceship, disassembled in containers. When sites on Mars have been selected and rocket propellant is available, these units will be deployed to the surface along with crew's quarters, a power system, and at least one android to set them up. 10.3.5 Crew's quarters (unassembled) The crew's quarters will also be carried on board the spaceship disassembled in containers. When sites on Mars have been selected and rocket propellant is available, these units will be deployed to the surface along with the hydroponic food production facilities, a power system, and at least one android to set them up. 10.3.6 Androids The androids will be our eyes and brains on board the Phobos spaceship. This requires a level of artificial intelligence quite a bit advanced from the current technology. The development of that technology will provide thousands of jobs over the next few years. Many groups may view the androids as threats to their jobs and indeed if they are in manufacturing - they are! However, just as the computer revolution created more jobs than it destroyed, so will the androids. 10.3.7 Fuel production facilities The number of fuel production facilities we need really depends
on how widely the landing sites are distributed. If they are more
than a few miles apart, we will need fuel production facilities
at all the landing sites. The type of rocket fuel produced by the
facilities on Mars may be different than that produced by the
facilities on Phobos. In the latter case we are hoping to use
water as the primary fuel. On Mars it may be easier to use carbon
dioxide from the air to produce fuel. Water may also be available
in sufficient quantity on Mars either in permafrost under the
surface or in the water of hydration in the rocks and soil. If
a polar site is chosen (which is very unlikely), then water ice may
be available very easily at that site.
Since the failure to produce fuel on Phobos (or Deimos) means
we can't establish bases on Mars, there should be at least one backup
fuel production facility for use on Phobos. In addition, both these
facilities must have the capability to produce some different
type of fuel if water isn't available. This fallback position
could be eliminated if we were prepared to send a separate small
mission to Phobos to provide this capability if it were needed.
10.3.8 Mars landing systems (rocket engines, fuel tanks, etc.) Since aerobrakes or heat shields are useless to get off Mars,
there is no reason to have them at all. We must use ordinary
rockets to get off Mars and those same rockets can be used to
land there in the first place. This implies bringing them down
from Phobos together with their fuel tanks, guidance systems, and
so on.
Obviously the minimum number of such systems is one, but this
doesn't seem prudent. Suppose instead that we have one landing
system for each landing site or 10 systems. We expect that each
system (without fuel) will weigh 3-5 tons. 50 tons may be less
than 1% of the mass of the Phobos spaceship when it leaves earth.
We also need landing systems for the human crew who will come
later. One possibility is to have each Martian fuel production
facility produce fuel and use that fuel to send the rockets back
to Phobos. They could then wait there and be used again to take
the crew down to the sites. They could then be used a fourth time
to return the crews to Phobos. Another option is to have a second
set of landing systems for the crews.
Faced with choosing between the two, I would pick the first
choice. The reason is that for each landing system that makes
it back to Phobos, we could be sure that the entire fuel cycle
was working without endangering any crew members. In addition
we would save the weight and cost of the second set of landing
systems.
10.3.9 Mars observation equipment By the time our spaceship reaches Mars, other satellites such as the Mars Observer may have already completed much of the work necessary to select appropriate landing sites. However, we expect that the androids will deploy some equipment to observe Mars and to help in site selection. Equipment needed to communicate with the various landing sites may need to be deployed too. 10.4 Outline of the Phobos mission The primary purpose of the Phobos mission is to set up all the infrastructure required to support a major manned mission to Mars. The mission will consist of the following milestones. 1. Assembly of the spaceship in high earth orbit.
2. Departure from earth's orbit on a Hohmann trajectory
3. Checkout of projectile catching during coast phase
4. Insertion into Martian orbit
5. Closure on Phobos
6. Deployment of Phobos investigator
7. Deployment of Deimos investigator if needed
8. Closure on Deimos if needed
9. Landing on Phobos (Deimos)
10. Deployment of Mars observation equipment
11. Deployment of fuel production facilities
12. Assembly of Martian landing units
13. Selection of landing sites
14. Deployment of landing units to surface of Mars
15. Deployment of science experiments and mini-rovers
16. Deployment of Mars bases' fuel production facilities
17. Assembly of bases' hydroponic food production facilities
18. Activation of food production facilities as appropriate
19. Assembly of bases' crew's quarters
20. Return of rockets to mother ship
10.5 Timeline Design and development of the components of the spaceship
could begin at any time, but actual assembly cannot begin until
the lunar polar EMPL is operational. This will be about 15 years
after the beginning of the project. If the earth-to-moon EMPL has
been built, it could contribute to the assembly of the Phobos
ship, perhaps at an earlier time.
Construction of the Phobos spaceship is expected to take two
to three years. Launch windows to Mars occur about every 778.7
days, so you can not just leave anytime you want to. As was
mentioned in section 6.6, the Hohmann transfer time to Mars is
about 257 days. So, that will be the approximate travel time of
the Phobos ship.
There is no need to rush to select landing sites. We could
allow, perhaps, as much as 3 years to pick them out. Once they
have been selected, the fuel can be produced and loaded into the
tanks of the landers. Deployment of the crew support modules (see
steps 14 - 19 above) will probably take only a couple of months.
10.6 Financing
Estimating the cost of the Phobos spaceship is complicated by
several factors which we are not accustomed to here on earth: (1)
lunar resources, including electric power, will be free, (2) the
exact ratio of human labor hours to lunar android labor hours is
not known, (3) the cumulative effects of artificial intelligence
will hopefully greatly reduce the number of human labor hours
needed, (4) the difficulty of assembling the ship in the
microgravity environment of L4 will surely increase the labor
required and hence the cost, and (5) the amount of nuclear fuel
(and perhaps other components) which will have to be sent from
earth is difficult to estimate.
Since lunar resources are free, the cost of each component of
lunar origin will be in direct proportion to the number of human
labor hours needed to manufacture it. Therefore, the more
intelligent our androids are, the less everything will cost.
By this time in the project, we expect to be selling helium-3
to customers on earth. Part of those funds will be used to finance
the cost of the Phobos spaceship. In the eventuality that helium-3
is not providing any income, we will rely on the profits of the
hydroponics and android industries to finance the Phobos ship.
|