Appendix: The Science Behind the Story
The mission described in the
story tried to convey a quirk of spaceflight that isnt widely appreciated:
some things are catastrophically hard to do, but others are surprisingly easy. The
story hopes to show that if we defer whats very hard, we can still
realistically plan some amazing missions.
First lets talk about whats
easy. Space is a unique environment, and building spacecraft entails meeting
design challenges that occur in no other industry. But there are also
commonalities, most notably with aircraft and submarine design. One striking
similarity with commercial aircraft design is as follows: both kinds of vehicle
spend much of their service life in a surprisingly benign environment. For
commercial aircraft its high-altitude cruise, and for spacecraft its orbital
cruise. The reason airliners can stay in the air for so much of their lifetime
is cruising at high altitude induces very few loads on the airframe. The engine
thrust exactly matches the drag produced by the oncoming air stream, and the
net stresses on the vehicle are not significantly different from those acting
on an aircraft supported by its undercarriage in a hangar on the ground. The
same is true for spacecraft. Once in orbit, regardless of the speed it travels,
the stresses on the craft are not that significant.
When closest to Earth, Mars is
140 times further away than the moon. Thats a long way. But it doesnt follow
that its 140 times technically harder to get there. It certainly would
take 140 times longer to get there (assuming you didnt upgrade your
moon rocket engines). If there were humans on board youd need 140 times as
many sandwiches as the moon missions, but you dont need 140 times as much fuel
because once at speed, theres no friction slowing you down.
The point is that the difference
between a journey to Mars and the Moon is principally one of duration. Its no
co-incidence that NASA and Russia have spent the last several decades learning
about long duration spaceflight, most recently aboard the International Space
Station alongside European, Canadian, and Japanese colleagues. Several humans
have spent the equivalent of a Mars mission journey aboard space stations, so
the technical challenges here are well understood.
So, building a Mars Orbital
Laboratory and sending it all the way to Mars, as described in the story is not
that big a challenge. Today, a typical mission to the Space Station lasts for
six months, which just happens to be how long it takes current rocket
technology to get to Mars. It would make no difference to an astronaut if that
time were spent endlessly circling the Earth, or on a journey that concluded in
orbit around Mars.
But landing a human outpost on
Mars is very probably 140 times technically harder than landing
on the Moon for reasons I explore briefly in the story. If you ignore the
problems with landing large masses, then a mission with a surface research
outpost would require the development of Mars-capable spacesuits, tools, and
vehicles, etc. Importantly, such an outpost would need to land with a fueled
vehicle that can take the crew back to Mars orbit where it would dock with an
Earth-bound sister vehicle, so thats two additional spacecraft. Advocates of
this plan suggest the Mars research equipment would be first tested and refined
on the Moon, so thats an additional mission (or missions).
In the story, Sashas dad is
aware that in 2025 none of the infrastructure for a landing has been developed.
The MOL plan was approved in 2011 because it was achievable without the
high risk and cost of developing the Mars lander, Mars ascent stage, Lunar test
missions, and the surface operations hardware. This is why he doesnt want
Sashas to get her hopes up about a Mars landing...
But the number one challenge for
a Mars surface outpost is one that also plagues the orbital mission concept in
the story, and in fact plagues every other space project: Energy.
With unlimited energy you could
land large masses on Mars, and could build robust extra-safe hardware with
countless backup systems. You could also get to and from Mars more quickly.
But to date, every space mission
has been starved for energy because the cost of getting hardware and fuel into
orbit is so high. Once in orbit, the energy available for a mission is limited
by either the mass of rocket fuel brought along, or the size of solar panels,
or the capacity of RTGs (radioisotope thermoelectric generators). But in every
case its a limiting factor.
While landing an outpost on Mars
requires phenomenal energy (which translates into numerous launches, and cost),
the energy needed to orbit Mars is LESS than that needed to land an equivalent
mass for a Moon mission. My goal for the story was to keep the realism high, so
while Sci-Fi writers can assume plentiful energy, I did not. Im confident
well land on Mars some time in the distant future, but in 2025 a surface
outpost is not realistic, but an orbital mission is.
There are two other key
technical considerations, and both are related to astronaut health.
The first is the muscle and bone
loss that occurs in weightlessness. This can only be partially alleviated when
astronauts follow a vigorous exercise program while in space. Understanding and
mitigating this problem drives much of the research onboard the International
Space Station today. Critics of missions to Mars have suggested that after six
months (or more) of weightless flight-time astronauts would arrive incapable of
standing, or would at least have trouble completing the mission. The gravity on
Mars is roughly one third as strong as here on Earth, so its not clear that
such a sudden increase in gravity would be debilitating. On the other hand,
since astronauts could spend up to two years on the surface, its also unclear
if thats sufficient gravity to prevent further deterioration. If not,
explorers will find they need to spend much of their time on Mars at the gym.
While the International Space
Station was designed for zero-G research, a Mars Orbital Laboratory would not
have that requirement. As far back as Gemini XI in 1966 NASA experimented with
inducing artificial gravity by spinning two craft around a central point, one
acting as a counter-weight. Without going into details, we might reasonably
expect this technique could be used to fly the MOL astronauts in a 1 G
environment – just like here on Earth – for the majority of their mission.
(Sadly, if the spin were maintained while at Mars, the view out of the window
could make the astronauts pretty dizzy. But in 2025 its a good guess that the
display technology, including virtual reality, will give them a spectacular
view of their surroundings.)
The second consideration is
potentially more grave. Here on Earth there are several mechanisms that shield
us from Solar and cosmic radiation. Astronauts that head for the Moon or Mars
will fly beyond that protection. There are several technologies that could
mitigate this problem which are on the cusp of becoming science-fact rather
than science-fiction, but for now the health of the crew is best served by
simply making the mission as short as possible. Shielding the crew behind
water, or polyethylene provides some protection, but because this is unknown
medical territory, the first Mars explorers will once again be chosen from
volunteers with the right stuff.
And were back to energy again.
If we have energy to spare we can reduce the journey time by accelerating and
then braking as we near Mars. Similarly, if we have the energy we can build a
spacecraft with lots of (heavy) radiation shielding.
Most of the design studies for
Mars missions assume traditional chemical rocket technology will be used. And
sending the required mass to Mars needs plenty of rockets and lots of fuel. The
most recent credible plan requires launching about 12 times as much payload as
the Apollo lunar missions. To loft this much mass with current launchers like
the Space Shuttle would take 57 separate flights. These numbers are for a
surface mission, so an orbital mission will need far fewer, but the scale is
There is no doubt that at some
point in the future space exploration will switch to nuclear propulsion and
nuclear power; the options opened up by the availability of energy are just too
enticing to ignore. The difference will be similar to the shift that occurred
in commercial air transportation from propeller driven craft to jets. We could
have stuck with propeller aircraft and learned to live with their limitations,
but the considerable cost of developing efficient jet engines has proven to be
a winning investment. So nuclear propulsion will happen, its just a question
of when, who will do it first, and precisely what technology will be used.
One of the key findings of the
Augustine Commission is NASA has been unable to drive technology development
forward while busy operating the Shuttle and building the Space Station. As a
result, new plans for exploration are to a large extent picking up from where
we left off in the 1970s. I wholeheartedly agree with the commission;
developing new technology is the right role for NASA.
The story doesnt require it,
but in one possible variant we can imagine that an aggressive nuclear space
propulsion technology program begins in 2010. The technology has not been worked
on much since the 1970s, but if it were funded in parallel to the other
elements of the mission in the story, it might be ready by the 2020 timeframe,
and the story timeline would still add up. It would be a gamble, but the payoff
would be significant.
One related technology that
looks promising is a plasma rocket called VASIMR. Its far enough along that a
test version will be used on the Space Station in 2012. (This is actually
happening; its not part of my story!) Its a small rocket, but the designers
claim that if it were scaled up and driven by a 200 megawatt nuclear powerplant
it could send a 60 metric tonne spacecraft to Mars in just 39 days.
Progressively smaller powerplants give more modest transit times, but still
beat chemical rockets by a wide margin.
The current NASA Mars Design
Reference Architecture requires a 540 day stay on the surface of Mars and a 30
month mission duration. This is driven by energy limitations; once you have set
up the outpost you need to wait for Mars and Earth to become aligned properly
in order to get home.
With a nuclear propulsion system
driving a Mars Orbital Lab, we can conceive of a 120 day shakedown mission (one
month stay), returning to Earth for a refit, and then longer duration missions.
If I were Sashas dad, I know
which one Id choose.
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| Contributed by: <!g>Adrian Wyard