Space travel is one of the most potent forces fueling the popular imagination. We've all seen the exotic spaceships of science fiction zooming through the void, visiting one fanciful locale after another.
Yet the reality of future space travel may turn out to be far stranger than anything we've previously imagined. Many concepts are being actively researched that may seem like the fanciful notions of a science fiction author, but are nonetheless grounded in hard science and practical engineering.
This article will take a look into some of these projects, and where they may fit into the scheme of space exploration in the decades to come.
Lightcraft are an unusual but undeniably workable idea that use beamed light from an external source to "push" a specially designed vehicle into orbit by exploding the air under it. Experiments with such vehicles are being actively sponsored by NASA, the US Air Force Research Laboratory, and private interests like the Foundation for International Non-Government Development of Space (FINDS) and Lightcraft Technologies, Inc. (LTI). In 2000, LTI launched a lightcraft weighing 1.8 ounces to a height of 233 feet using a US Army 10-kilowatt pulsed carbon dioxide laser.
The lightcraft vehicles being used in experiments are small, cone-shaped devices with a specially-designed parabolic mirror on their aft ends. The vehicles "ride" along a pulsed infrared laser beam fired from the ground. The reflective surfaces on the underside of the craft focus the energy from the beam into a ring, where it heats air to a temperature of over 50,000 degrees Celsius, or roughly ten times the temperature of the sun's surface. This causes the air under the ring to explode for thrust. The forward motion of the craft feeds new air into the focusing ring for the next pulse. Lightcraft are spun for stabilization, much like a rifle bullet, and are launched at some 10,000 rpms.
Eventually LTI and others researching lightcraft would like to build an actual satellite launcher. While the laser-launch abilities of the concept will probably not be able to haul multiton payloads into orbit like conventional rockets, it does have the capability of getting many smaller payloads (100 kg or less) into orbit significantly faster, making it ideal for supplying space stations and orbital hotels. Getting a lightcraft to orbit would mostly involve ratcheting up the launching laser's power and focusing ability, enabling it to "push" heavier payloads farther by creating far more energetic air bursts.
When a lightcraft runs out of usable air density at about thirty kilometers up, it can switch to an onboard store of liquid hydrogen to use as laser-combusted propellant for the final boost into orbit. A micro-satellite weighing one kilogram would need about one additional kilogram of hydrogen fuel to make orbit.
Researchers envision eventually orbiting payloads with parabolic focusing mirrors of about 1.4 meters in diameter. The output power of the launching laser for such a payload would have to be on the order of 100 megawatts. Alternately, the launching beam need not be from a single laser but many grouped together creating a single combined pulse of equivalent power; this latter arrangement may be more efficient in focusing the appropriate power on the lightcraft at varying altitudes.
Masers (microwave lasers) have been mentioned as an alternative to infrared lasers in launching lightcraft. Masers do not have the energy density of their higher-frequency cousins, and thus lightcraft mirrors would have to be made larger, but they are also considerably easier and less expensive to scale up power-wise.
One interesting bit of trivia from the LTI test in 2000 is that the experimenters had to work closely with NORAD to time their tests so their laser would not accidentally "blind" a satellite passing overhead. Whether this danger could have an influence on the development or proliferation of lightcraft technology remains to be seen.
Leik N. Myrabo, one of the scientists who pioneered the original lightcraft concept at the US Air Force Research Laboratory, has also designed a more sophisticated beamed-energy craft. Instead of a cone-shaped craft powered by ground lasers, Myrabo's version uses a saucer-like reflective lifting body to create electrohydrodynamic thrust from power beamed to it from orbit.
The lightcraft would tap most of the beamed energy to generate wide, powerful electrical fields around the saucer, ionizing the surrounding air. Superconducting magnets around its rim would create strong magnetic fields, which would accelerate the newly-charged air particles past the vehicle. This would create a slipstream around the craft, generating lift and thrust.
Reflective surfaces on the craft would focus some of the incoming beamed energy from orbit one vehicle-diameter ahead of the vehicle. The intense heat at the pinpoint focus creates an explosive "air spike" that diverts oncoming air past the vehicle, decreasing drag and reducing the heating of the craft.
By varying the amount of energy it reflects forward, the lightcraft can control the airflow around the vehicle. Myrabo demonstrated reduction of drag using a controlled airspike in April 1995 in a hypersonic shock tunnel at Rensselaer Polytechnic Institute, using an electrical plasma torch. Tests aimed at generating magnetohydrodynamic thrust, using a 15-cm-diameter mock-up of his advanced lightcraft, are underway. A person-sized lightcraft of this type driven by a 1000 megawatt pulsed laser should be able to accelerate easily to the velocities needed to obtain orbit.
Myrabo's advanced lightcraft would be dependent on orbiting solar-power satellites, basically enormous disks constructed in orbit about one kilometer in diameter and a few millimeters thick. One side would be the solar collector, while the earth-facing side would hold millions of miniature solid-state microwave transmitters. This arrangement could theoretically provide up to 4.3 gigawatts of power to any "lifting" beam aimed at the lightcraft. The vehicle could be fed this power gradually, allowing for a five-minute ground-to-orbit flight with no more than 3 Gs of acceleration, similar to forces the Space Shuttle experiences upon launch. Or the solar power satellite could dump all its power into a single, 54-second burst that could zing the lightcraft into orbit at 20 Gs.
Dr. Leik Myrabo recently launched the Lightcraft Project at the Rensselaer Polytechnic Institute, which envisions using the above concepts to create an even more advanced 12-man lightcraft orbiter that would regularly ferry cargo and passengers between the ground and space facilities. These lightcraft would be supplemented by ion engines to help facilitate takeoff, landing, and in-space maneuvering. The interior would be filled mostly with a helium/oxygen mixture, to make the craft buoyant in the lower atmosphere and to assist in takeoff.
The Solar Boiler drive scheme is known more formally as Solar Thermal Propulsion.
Solar Boilers use large parabolic mirrors to focus sunlight on a fluid fuel, most often cited as water or liquid hydrogen, superheating it and expelling the resulting steam for thrust. The sunlight is collected and focused using large, lightweight parabolic mirrors. These mirrors are either silver-coated inflatable structures or thin sheets of silvered plastic supported by lightweight inflatable trusses. These focus the sunlight on an open area of the engine, either directly on the fuel or into a graphite block which in turn heats the fuel. Graphite has superior heat-absorption properties compared to water or hydrogen, but this would also result in some energy loss due to the indirect heat transfer.
In and around Earth's orbit, the sun provides enough power to impel a specific impulse  of about 800 to 1000 seconds, with medium thrusting power. A boiler's engine power tapers off as the ship moves away from the sun, so solar boilers may not be a very practical method of propulsion in the outer solar system.
Solar Boilers are designed to be lightweight, inexpensive craft, and they are primarily envisioned as obtaining fuel cheaply from in-space sources such as comets. Water is the most likely choice for fuel, as it is readily available on many comets and it can be stored easily as ice or liquid. Also, such a craft would not need the heavy, pressurized tanks necessary to carry liquid hydrogen.
Though experimental solar boiler craft may be launched in the coming decades, they probably will not come into their own until a permanent infrastructure is established in space, where they could be used as cheap "workhorse" vehicles puttering cargo around the inner solar system.
Metastable Helium Rockets
Metastable substances are materials that exist in a long-lived excited molecular state. Metastable Helium is one such substance, comprised of a Helium atom with two electrons, one in the first orbital level and one in the second orbital level. The electrons have parallel spin properties. The molecule is balanced on a knife's edge of quantum forces. The electrons want to enter into the ground state (where both electrons occupy the first orbital level) but are forbidden because of the forces acting upon them from their locked spin states. The atom can remain in this state for a mean time of 2.3 hours, but just a small input of energy (from jostling, molecule formation, electromagnetic forces, etc.) will send it over the edge into its ground state, resulting in a large release of energy, about 114 kilocalories per gram, or roughly twice the energy of the most powerful conventional chemical fuel, atomic hydrogen.
Metastable helium can be manufactured by several methods. Absorption of photons via laser or particle beam can excite the atom to its metastable state.
But the true problem with metastable helium is not in obtaining it, but in storing it. The 2.3 hour limitation only applies to a completely isolated atom; metastable helium packed in with anything else, even other helium atoms, will result in jostling and it losing its metastable state in a fraction of a second.
Research is being conducted to see if metastable helium can be formed into a room-temperature solid if bonded with diatomic helium molecules, made from one ground state atom and one excited state atom. This solid, called Helium-IV-A, can in turn be used as a solid rocket propellant. Simply heating the fuel is enough to release the energy, so an oxidizer is not needed. These rockets would produce a specific impulse of about 2200 seconds—compared to the 450-second specific impulse for modern liquid hydrogen-oxygen rockets—and would have enormous thrust capabilities, on the order of 31,000 meters per second, matching types of proposed plasma and fusion rockets.
Robert Forward, in his fiction novel Saturn Rukh, suggested bonding 64 metastable helium atoms to a single excited nitrogen atom, forming a stable super-molecule called Meta. Whether this is possible remains to be seen. However, as the Meta fuel is much purer than the Helium-IV-A form above, it might allow a metastable helium rocket's specific impulse capabilities to jump to around 3150 seconds.
A cycler is a spacecraft with a complex multibody orbit that takes it close to two or more celestial objects once per cycle. The craft uses the gravity of one planet to bend its trajectory toward its next target, and when it reaches that destination it uses a gravity slingshot there to bend its orbit back toward its origin point, so on and so forth in an endless cycle. A cycler spacecraft may need to carry propellant to occasionally correct its flight path, but once locked onto its trajectory it's pretty much a free trip.
The most common route discussed among proponents is an Earth-Mars cycler, which once set up would basically provide a regular "bus service" between the two planets every eighteen months or so. A smaller subcraft would have to be launched to rendezvous with the cycler and deliver cargo. This same craft would take cargo bound for Earth with it when it left. A similar transfer vehicle would be waiting for the cycler on Mars.
"This is sort of like a bus that doesn't stop," explains team member James Longuski, a professor of aeronautics and astronautics at Purdue University studying the possibility of an Earth-Mars cycler. "When it comes by, you have to run alongside of it and grab on. Then, when you get to Mars, you get in the taxi and de-orbit down to the planet."
The design of the cycler itself would be more akin to current space station designs than to actual spaceships per se, as its primary purpose would be long-term life support for its crew, and whatever engines it carried would be used primarily for simple course corrections. Longuski likens cyclers to space hotels planned by the private sector, as the cycler craft could be rotated slowly to create artificial gravity and prevent the debilitating effects of weightlessness on its passengers. They would also have to be roomy and accommodating enough to make the trip tolerable.
Cycler habitats can begin as small "seed" constructs, not much larger than a few interconnected modules, and slowly grow with each orbit as the transfer shuttles bring it more supplies and building materials the crew can use for add-ons en route. They could over the years grow into true space-liners, holding hundreds of passengers in comfort each trip.
One of the major hurdles foreseen in cycler design is how to precisely calculate the trajectories involved because of the complex orbital relationship between two planets as the planets travel around the sun. For example, while Earth orbits the sun in a nearly circular route, Mars's orbit is much more elliptical. That means the distance between Mars and the Earth varies dramatically depending on Mars's orbital position around the sun, complicating the design of spacecraft trajectories between the two planets.
Once the system is established and proven viable, multiple cyclers could be employed along the same route, staggered a few months or even weeks apart. This system would make interplanetary trade and multiple-planet economies viable beyond the novelty stage and allow the full-scale colonization and exploitation of the greater solar system to become a reality.
Though an Earth-Mars cycler is the most talked about route, cyclers could be established between any two planetary bodies. As complex orbital mechanics become better understood, it may even be possible to create cycler routes among three or more celestial bodies.
As the flights of privately owned spacecraft have shown, we may be on the cusp of a new era in space exploration, one using much more innovative approaches than those engendered by a half-century of conservative NASA culture. The "oddball" ideas discussed in this article may help to forge that future, taking us in directions we are only just beginning to imagine.
Saturn Rukh, by Robert L. Forward
Lightcraft Technologies Inc. Homepage: http://www.lightcrafttechnologies.com/
The Lightcraft Project at Rensselaer Polytechnic Institute: http://www.eng.rpi.edu/mane/lightcraft/Curriculum/TAVD/
Metastable Helium Rockets:
Solar Boiler Rockets: