Buzz Aldrin's roadmap to Mars

[phongn]

In 1961, NASA was mulling over two possible flight plans to put a man on the moon. While agency officials argued the merits of Earth Orbit Rendezvous versus Direct Ascent, John C. Houbolt, a little-known engineer at the Langley Research Center in Hampton, Va., came up with a daring and ingenious alternative: Lunar Orbit Rendezvous. LOR, which would require two spacecraft to link up a quarter-million miles from Earth, initially struck many people--me included--as dangerously complex, even bizarre. But Houbolt stubbornly kept pushing his plan, and the elegant logic of LOR eventually won over the skeptics. On July 20, 1969, thanks to Houbolt's persistence, Neil Armstrong and I walked on the moon.

More than three decades later, as NASA debates how to send humans to Mars, it's time once again to invoke the outside-the-box spirit of John Houbolt. NASA's latest thinking for a manned Mars mission is basically the Apollo program writ large: a massive disposable spacecraft that must be boosted from Earth to interplanetary velocity, and then slowed back down to alight on Mars. This flight plan has a huge energy requirement that translates directly into size, complexity and cost. Because each mission would be so extremely expensive, it's all too likely that such a program will lead to the kind of short-term "footprints and flagpoles" thinking that eventually killed Apollo.

We can do better this time. My blueprint for manned travel to Mars, based on reusable spacecraft that continuously cycle between Earth and Mars in permanent orbits, requires much less energy over the long term. Once in place, a system of cycling spacecraft, with its dependable schedule and low sustaining cost, would open the door for routine travel to Mars and a permanent human presence on the red planet. Its long-term economic advantages make it less susceptible to cancellation by congressional or presidential whim. In effect, this system would go a long way toward politician-proofing the Mars program.

FORWARD MOTION
The key advantage of a permanently orbiting spacecraft, or Cycler, is that it must be accelerated only once. After its initial boost into a solar orbit swinging by both Mars and Earth, the Cycler coasts along through space on its own momentum, with only occasional nudges of thrust needed to stay on track. This dramatically reduces the total energy required for a Mars mission. Because conventional chemical rockets are so thirsty--the mass of the Apollo 11 craft that carried us to the moon was more than 90 percent fuel on takeoff--every pound saved pays a huge dividend in the form of less propellant and smaller, cheaper boosters.

Once established in orbit with the long-term human survival systems, radiation shield and artificial gravity mechanism necessary for a lengthy space journey, the Cycler swings by Earth and Mars on a predictable schedule. Astronauts piloting "taxi" spacecraft, such as NASA's planned Crew Exploration Vehicle (CEV), rendezvous and dock with the Cycler as it passes Earth, using only the propellant necessary to accelerate the smaller craft. As the Cycler swings by Mars, the taxi casts off and brakes into Mars orbit, like a commuter stepping off a train. The Cycler, meanwhile, speeds on beyond Mars and eventually loops back toward Earth, ready for another passenger pickup.

The idea of a Mars-Earth Cycler has been around since the 1960s. In one early scenario, space habitats called CASTLEs circled the sun in eccentric orbits that passed by both Earth and Mars. However, a Cycler using those orbits would take as long as 7-1/2 years to complete a round trip between the two planets, and the planetary encounters would be irregular. A reasonable Mars mission schedule would have required up to six such Cyclers in staggered orbits.

It seemed to me there must be a more efficient way. Using techniques of orbital mechanics I'd developed at MIT during my Ph.D. studies, as well as firsthand insight gained by my flights on Gemini 12 and Apollo 11, I calculated that the time could be significantly reduced by using gravity assist from Earth to slingshot the Cycler into a better orbit.

"Gravity assist" is a well-proven technique for interplanetary flight, routinely used on unmanned probes like Voyagers 1 and 2, Cassini-Huygens and Galileo. If a spacecraft flies close enough to a planet, its orbit will be bent by the planet's gravitational field. The process can be likened to a ball (the spacecraft) bouncing off a wall (the planet). If the wall is moving toward the ball, the rebound speed will be higher than the speed prior to impact. Similarly, if the wall is angled, the ball will change direction. In either case, a great deal of energy can be added to the spacecraft with no expenditure of propellant.

By taking advantage of gravity assist from Earth, and to a lesser extent from Mars, I was able to plot a Cycler orbit with a round-trip period of just 26 months. The Cycler would take only five months to reach Mars, comparable to the fastest transit times that NASA is now considering.

A downside of the gravity-assisted Cycler concept, however, is that the vehicle flies by Mars at quite a high speed, up to 27,000 mph. This velocity is not a showstopper on the outbound leg, where the CEV taxi craft would aerobrake, relying on the friction of the Martian atmosphere to slow down without using any propellant. But departing Mars for the leg back to Earth, the craft would need a large amount of propellant to catch up with the speeding Cycler.

To circumvent this problem, I envision a hybrid craft called a Semi-Cycler for the return leg. Like the Cycler, the Semi-Cycler would shuttle between Earth and Mars in a gravity-assisted orbit. But it would use aerobraking in the Martian atmosphere to slow down, interrupt its cycle and loiter for four months in a wide, lazy orbit around Mars, waiting to pick up the next Earthbound taxi. With a flyby velocity as low as 5000 mph, the Semi-Cycler would be an easy target for a low-propellant taxi rendezvous. Once it discharged the spacecraft to aerobrake into the Earth's atmosphere, the Semi-Cycler would be slingshot on a circuitous 14-month route back to Mars for another run.

One drawback of the Semi-Cycler is its need for propellant to accelerate out of Martian orbit back toward Earth. But compared to a direct flight in a conventional rocket, the overall savings are still substantial. A second drawback is a longer transit time back to Earth, about eight months. But with the help of top engineers at NASA's Jet Propulsion Laboratory, Purdue University and the University of Texas, I am continuing to refine Semi-Cycler orbits to achieve optimum transit times, orbital periods and flyby velocities.

TECH SUPPORT
The Cycler itself is only the capstone of a long process of space development. NASA's proposal to revisit the moon using a CEV is a first step in the right direction. A second step would include exploratory flights to Mars's moon Phobos, which would serve as an early launchpad to the planet's surface. Creating a sustainable Mars transportation system, though, would require a huge support infrastructure.

A permanent base on the moon would use lunar ice to produce liquid oxygen and hydrogen fuel for the taxi's sprint to catch the Cycler. NASA's Clementine and Lunar Prospector missions in the 1990s discovered tantalizing hints that ice might exist deep inside craters near the lunar poles.

Liquid oxygen and methane fuel for the outbound taxis, Semi-Cycler and a Mars lander/ascender would be manufactured at a permanent base on Mars. The propellant plant would combine a feedstock of liquid hydrogen with carbon dioxide from the Martian atmosphere. If frozen water can be mined from under the poles, where recent Mars rover missions have detected it, hydrogen could also be produced.

A fleet of unmanned freighters would resupply the Cyclers and surface bases on Mars and the moon. Because they can be launched years in advance, instead of chemical rockets the freighters could use the efficient, low-thrust ion-drive engines, too slow for manned travel, that were tested on NASA's Deep Space 1 probe in 1998.

OUTBOUND JOURNEY
How would the Mars Cycler System work on a practical level? Fast-forward to the year 2040, and climb aboard for a five-year hitch in the Red Planet Corps.

You and your fellow astronauts (I envision a crew of about eight) launch from Earth in a CEV-type taxi spacecraft fueled by a high-performance hydrogen booster. While in low Earth orbit, your CEV docks with a Mars lander and a propulsion module previously launched from Earth. Linked up in this Apollo-style triple unit, you burn into a highly elliptical six-day "marshalling" orbit around the Earth that takes you roughly halfway out to the moon. There, you join up with a resupply ship carrying a load of liquid oxygen and hydrogen fuel manufactured on the moon. You top off the tanks of your propulsion module so that you can catch up with the Cycler, which is now fast approaching Earth.

The Trans-Mars Injection burn lasts about 7 minutes at an acceleration of about 2 g's. If you've done it right, you rendezvous with the Cycler about 10 days later, a million miles out from Earth. The CEV and Mars lander separate from each other and dock at the hub of the Cycler (see lead illustration), which is spinning lazily to simulate Mars's gravity--38 percent that of Earth's. You transfer from the CEV into the habitation module, which is stocked with food, water, a radiation shield and all the necessities for a long-term journey. Here's your chance to finish War and Peace; there's not much to do for the next five months.

As you approach Mars, it's back into the CEV for the descent to Mars orbit. Wave goodbye to the Cycler and, with lander still attached, enter the Martian atmosphere for a few minutes of aerobraking before you skip back out into a low orbit. Here, you transfer into the lander--just like Neil Armstrong and I did on Apollo 11--undock from your faithful CEV and fire the lander's retrorockets for the descent to the surface. Using aerobraking, a parachute and precision rocket braking, you touch down at the main base.

Expect a champagne welcome from the crew that's still there from the previous mission, which landed 26 months earlier. They're already looking forward to using your Mars lander/ascender to head for home 18 months from now. You, however, have to wait substantially longer than that for your own rotation back to Earth.

RETURN TRIP
For the next 44 months, you explore the Martian surface, monitor a number of research projects and manage the all-important fuel-making plant. In Month 18, you send off your compatriots. Month 26 brings the arrival of the next crew and the lander/ascender you'll be using to start your eventual journey home. You then launch a refueling rocket to top off the tanks of the CEV the arriving crew left in orbit. Around Month 38, the Semi-Cycler arrives and aerobrakes into its four-month Mars orbit; you might see the bright streak as it hurtles through the upper atmosphere. As departure time draws near, the Semi-Cycler drops down into low orbit to link up with the still-orbiting CEV. You send up an unmanned rocket with fuel for the Semi-Cycler.

When it's time to go, your crew fuels up the lander/ascender and lifts off into Martian orbit to rendezvous and dock with the Semi-Cycler, now joined with the CEV. After a modest return-to-Earth burn, your Semi-Cycler departs Mars orbit for the eight-month trip home.

Once on the proper trajectory, you float in zero-g; the Semi-Cycler doesn't spin. I believe that artificial gravity won't be necessary on the homebound leg because the effects of long-term weightlessness (see "The Challenges of Interplanetary Travel," page 6) aren't as problematic upon returning to Earth's full gravity. Restorative exercises, in fact, will provide a fine opportunity to reflect upon your epochal journey.

As Earth closes in, the CEV detaches from the Semi-Cycler and aerobrakes into the Earth's atmosphere. The recovery chute deploys as you descend to a final touchdown, either into the ocean next to a waiting recovery ship or on land. The Semi-Cycler, meanwhile, whizzes on by Earth and gets slingshot back onto its return trajectory.

LOOKING AHEAD
The Cycler system alters not only the economics of a Mars program, but also the philosophy behind it. It makes possible the dream of regular flights to Mars and a permanent human presence there. Instead of a wasteful, short-term, "let's just get there as soon as possible" approach, the Cycler sets the stage for long-term thinking, planning and commitment. That's the only way we'll ever succeed in taking mankind's next giant leap: a subway-in-the-sky between our planet and our future second home.

[dragon]

Ehh its an ok plan but there are so many new designs on the drawing book that by the time frame he lists projects such as Promethesus and ICAN II will hopefully be built.

[GrandMasterTerwynn]

Ehh its an ok plan but there are so many new designs on the drawing book that by the time frame he lists projects such as Promethesus and ICAN II will hopefully be built.

You're missing the point. We can do this using propulsion schemes and technologies that exist now. For ICAN II, you need to develop a whole slew of new technologies, not to mention a method of generating and storing the requisite amounts of antimatter. Prometheus proposes to deliver a robotic space-probe, which has nowhere near the mass of a crewed vehicle, and scaling up such a system is a non-trivial task, given the fiendish complexity of a space propulsion system, and the safeguards that have to be designed into the system for use around humans (and the titanic amounts of bitching you'd have to put up with from the ZOMG NUKES R T3H EVIL!!!111 crowd.)

To start an earnest effort at manned exploration of Mars, we have to start with what's feasible in the near-term, because that's what's going to be funded. Programs relying exclusively on exotic technologies tend to find themselves repeatedly scaled-back, and sometimes terminated outright. Especially since the point of a mission with a new, highly-complex, exotic technology . . . tends to be the showcasing of the aforementioned new, highly-complex, exotic technology. This is the "politician-proofing" Aldrin refers to, since interplanetary exploration is going to remain the province of governments and the politicians holding their leashes for the first half of this century, at least. (One is welcome to point out Virgin Galactic, the groundbreaking on the private spaceport in New Mexico, and others . . . but these are sub-orbital efforts, with the first tentative orbital efforts being perhaps a decade off. And even then, there's very little profitable market outside of low-Earth orbit until costs drop enough to make commercial exploitation of just the Moon feasible.)

There's no point to waiting for "miracle" technologies to solve the problem of going back and forth between Mars and Earth. Such technologies will have their place when they've become proven and not-so-miraculous. Then you gradually retire those dusty old chemically-driven Aldrin cyclers and introduce more direct-to-Mars super-rockets.

[Guardsman Bass]

There's no point to waiting for "miracle" technologies to solve the problem of going back and forth between Mars and Earth. Such technologies will have their place when they've become proven and not-so-miraculous. Then you gradually retire those dusty old chemically-driven Aldrin cyclers and introduce more direct-to-Mars super-rockets.

Even if the continously cycling system can be set up with present technology, it appears that you would be trading a lot of time to set it up, in order to build the multiple craft with sufficiently long human habitations. It'd be like setting up a series of space stations tough enough to last years with or without human habitation, and that's without the expense and political capital necessary to get a real moon base built.

[CmdrWilkens]

The difference then becomes the long-term cost. Since politicians love to talk about saving money long term or the long term effects of policies one could start looking at the economics of building a half dozen cyclers, another half dozen semi-cyclers and a half dozen CEVs (which apparently are on the board anyway) then servicing them over fifty years with the number of trips they can make versus the cost to make an equivalent number of trips using conventional direct-to-Mars systems. I'd wager if you parse it out over a quarter century the Aldrin cycler system (with its 18 vehicles supporting one trip every 18 months) versus a brand new direct flight rocket for each out and back trip or any of the other direct flight solutions the Aldrin system would win out for how much it can deliver.

[drachefly]

I'd agree, but as much as politicians like to TALK long-term, they almost always vote short-term.

[Guardsman Bass]

The difference then becomes the long-term cost. Since politicians love to talk about saving money long term or the long term effects of policies one could start looking at the economics of building a half dozen cyclers, another half dozen semi-cyclers and a half dozen CEVs (which apparently are on the board anyway) then servicing them over fifty years with the number of trips they can make versus the cost to make an equivalent number of trips using conventional direct-to-Mars systems. I'd wager if you parse it out over a quarter century the Aldrin cycler system (with its 18 vehicles supporting one trip every 18 months) versus a brand new direct flight rocket for each out and back trip or any of the other direct flight solutions the Aldrin system would win out for how much it can deliver.

Even with the long-term savings, that is still a long time for development, particularly if the country doing this is the United States, where short-term development is favored unless there is a very pressing reason. It would be hard to guarantee the political stability of the project.

[GrandMasterTerwynn]

The difference then becomes the long-term cost. Since politicians love to talk about saving money long term or the long term effects of policies one could start looking at the economics of building a half dozen cyclers, another half dozen semi-cyclers and a half dozen CEVs (which apparently are on the board anyway) then servicing them over fifty years with the number of trips they can make versus the cost to make an equivalent number of trips using conventional direct-to-Mars systems. I'd wager if you parse it out over a quarter century the Aldrin cycler system (with its 18 vehicles supporting one trip every 18 months) versus a brand new direct flight rocket for each out and back trip or any of the other direct flight solutions the Aldrin system would win out for how much it can deliver.

Even with the long-term savings, that is still a long time for development, particularly if the country doing this is the United States, where short-term development is favored unless there is a very pressing reason. It would be hard to guarantee the political stability of the project.

It's much more politically stable than any number of direct-to-Mars rockets, or direct-to-Mars ships using expensive, exotic drive tech. You only have to build a cycler (or semi-cycler) once, regardless of how many decades it stays in service. It doesn't need big, expensive engines that must be ready to deliver full thrust over and over again, since it only needs to be boosted twice. First time to insert it into its permanent Earth-to-Mars transfer orbit, and the second time to bring it home when you want to decommission it (plus whatever accel/decel cycles you'll need to take the ship offline for significant repair/upgrade/servicing . . . but it will still need less propellant and less beefy engines than partly reusable direct-to-Mars ship, and significantly less propellant and materials than any number of non-reusable direct-to-Mars ships.) Unlike a direct-to-Mars ship, the cycler doesn't need a means of getting through an atmosphere safely, nor does it need means of climbing out of the bottom of a planetary gravity well. While the short-range ships in Aldrin's plan do need to get in and out of atmosphere, these ships will be cheaper than direct-to-Mars rockets, since they don't have to carry the sorts of systems a long-duration interplanetary craft would need to cary.

[drachefly]

The main cost savings, as I understand, is that it doesn't need to accelerate the heaviest objects you need to get from one end to the other at all quickly.

Accelerating something slowly over a long period is something we can do cheaply. Accelerating something quickly is expensive.