Indeed, the difference between 100 metric tonnes and 140 metric tonnes is a lot, when it comes to Mars missions. The standard Mars mission baseline, Mars Direct (Robert Zubrin, David Baker, 1990), assumes a heavy lift vehicle like a Saturn V will launch a crew of 4 explorers to Mars. The same launch would lift over ten tonnes of perishable supplies and over two tonnes of rover equipment. A total mass of 25.2 tonnes would be delivered to the surface of Mars for the piloted mission (see The Case for Mars, Touchstone, 1997).
Fifteen years later, Mars Direct is still the best way to get a crew of four explorers to Mars and return them safely back to Earth. But what happens if you don't have a heavy lift vehicle capable with the same capabilities as the mighty Saturn V? A 30% reduction in rocket capacity (140 to 100) translates to about 7 less tonnes of mass delivered to the surface of Mars.
Obviously, some changes to the mission are required. The NASA solution is to upgrade everything... send a crew of 6 explorers to Mars with even more equipment and perishables. To get around the launch problem, current NASA plans rely upon on-orbit assembly and two launches of a 100-tonne lift vehicle. Some would view this as cheating, and rightly so. By using two launches of a weaker vehicle, NASA gets a 200-tonne spacecraft heading toward Mars.
There's no such thing as a free lunch, however, and cheaters rarely prosper. In this case, the penalty is increased risk of total failure before the mission even leaves low Earth orbit (LEO). NASA would require two successful launches in short succession, and the crewed and fueled launches both would have critical on-orbit time limits. The only previous space project using the additional, critical step, on-orbit assembly, is the ultra complex (expensive) International Space Station project. Besides adding complexity, any serious assembly issue would doom the mission. Even if all goes well in Earth orbit, NASA would attempt to land a far more massive vehicle on Mars - another complication in one of the already-riskiest parts of the mission.
NASA and private industry appear to be fixated upon the creation of a 100-tonne-to-LEO heavy lift vehicle. Mike Griffin, the new NASA administrator, has stressed the need for this vehicle during testimony to Congress in 2003. Morton Thiokol has a plan to upgrade the space shuttle stack so it can lift 100 metric tonnes to LEO. Boeing, Lockheed Martin, and SpaceX all have plans to upgrade their own rockets to the same capacity.
Fortunately, a 100-tonne-to-LEO capability IS good enough to send human explorers to Mars, with substantially less risk than the NASA plan. All that's needed is a different place to rendezvous and a smarter mission strategy. Shadows of Medusa presents one such strategy... or perhaps two?
The Shadows of Medusa approach(s) is presented within the safe realm of science fiction, but the mission profile(s) is rock-solid. The centerpiece of the plan relies upon a Mars-surface rendezvous, rather than the LEO rendezvous. This requirement is not new, since the NASA plan and Mars Direct both require a limited surface rendezvous capability anyway.
Unfortunately, this web page can't present any more details without spoiling the plot of the novel. After reading the book, anyone interested in debating the logistics of either Shadows of Medusa approach is most welcome to contact the author directly through e-mail. Please be prepared to defend your position, however, because the author certainly is.
Note that much of the talk of heavy lift vehicles is premature because we currently don't have on-the-shelf technology to land 20 tonne payloads on Mars. The Entry, Descent, and Landing (EDL) issues might be solved by equipping the landing craft with a hypercone, but the technology requires more research and has never been tested on a live spacecraft. Without a hypercone, we limit current EDL mass to about 5 tonnes... so what's the point in launching 100, 140, or 200 tonnes from Earth? Existing launch vehicles can already send 5 tonnes to Mars, and the Shadows of Medusa mission plan can be adapted accordingly (but at far greater risk to the crew). Again, details will need to wait until you've read the book.
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"That's one small step for a man, one giant leap for mankind. "
Neil Armstrong