When Star Trek's U.S.S. Enterprise hit the television screen in 1966, the science fiction series had trouble finding its own space and time slot.

There is no doubt there are worlds out there beyond our own cabal of planets, but even if you've got the heaviest of foot on the accelerator, plotting a speedy route to the stars is not easy.

In case you missed it, the first interstellar probes are already en route. Pioneer and Voyager spacecraft are headed into interstellar space. But they weren't targeted toward any nearby stars. Furthermore, these craft lack the power sources or communications gear that a true interstellar probe would require.

More to the point -- you don't have to be a rocket scientist or an astronomer to appreciate the fact that space is vast, said Steven Howe, co-founder and chief executive officer of Hbar Technologies, LLC, based in West Chicago, Illinois.

As example, Howe points out, NASA's Voyager 1 -- the most distant object produced by humans -- is now some 90 astronomical units (AU) or 8.4 billion miles from the Sun. It was boosted from Earth back in 1977, and is clocking a speed of some 3.6 astronomical units per year. Contrast that to the Kuiper Belt around our Solar system at around 200 AU, the Oort cloud at some 10,000 AU out, and the nearest star at 260,000 AU away.

"Any technology current today will require a 'miracle' of one sort or another to send a probe to the next star in a reasonable time," Howe said. To send one pound of mass to the next star in 40 years means that the energy contained in 100 million pounds (50 kilotons) of high explosive has to be expended to get the "pound" up to speed. Thus, the development of very small payloads and ultra-light propulsion systems is essential, he added.

"The only on-board propulsion technologies that we know of that might have a chance of enabling an interstellar flight are fusion and antimatter. Fusion has remained an elusive animal. Antimatter technology is in its infancy, but is rapidly growing," Howe said. Within the next fifty years, antimatter technology may have the same impact as the laser has had over the past fifty years, he forecasts.

"The ages of human civilization can be defined very simply by observing the intensity of the energy sources they controlled: the Stone Age had wood fire; the Bronze Age had coal; the Industrial Age has gasoline," Howe said. "We are at the threshold of the nuclear age…but only if we have the fortitude to take on the challenge. Otherwise, we fall back."

Howe and Hbar Technologies are evaluating the concept of the Antimatter Driven Sail. The work is being sponsored by the NASA Institute for Advanced Concepts (NIAC), an institute of the Universities Space Research Association.

The ultimate goal of the project is to identify the technological hurdles of dispatching a lightweight instrument package to another stellar system. First phase of work is foreseen as a "less demanding" mission: Sending a probe to the Kuiper Belt in a decade's time.

"Such a mission is still beyond the capability of NASA or any other agency using currently available technology," Howe reported to NIAC. The company's work to date, however, indicates that a small instrument payload could be sent to 250 AU in 10 years using 30 milligrams of anti-hydrogen.

"This amount of antimatter is clearly within the product on potential of the U.S. within the next 40 years using currently accepted accelerator technologies, Howe said. Preliminary calculations also indicate that a similar probe could be sent to the next star, Alpha Centauri, actually a triple star system, in 40 years using grams of antimatter, he explained.

The ongoing work on the Antimatter Driven Sail may in fact "allow humanity to consider sending probes to the stars," Howe said.

If you are hurling vehicles out amongst the stars, road maps are essential. Scooting about aimlessly is not an efficient way to cross the great gulf of time and space.

In many ways, an interstellar version of MapQuest is already being drawn up.

Such telescopic wonders as the venerable Hubble Space Telescope, the recently lofted Space InfraRed Telescope Facility (SIRTF) -- along with ground-based observations -- are helping piece together the true picture of other planetary systems circling distant stars.

An armada of future spaceborne instruments will join in on the search for solar systems likely to contain Earth-sized worlds.

Among these space-based platforms is NASA's Kepler mission, scheduled to launch in 2007. It will determine the frequency of terrestrial and larger planets in or near the habitable zone of a wide variety of spectral types of stars. Kepler is specially geared to continuously fix its gaze at a region of space containing 100,000 stars. The spacecraft will be on the lookout if Earth-sized planets make a transit across any of the stars.

In following years, increasingly powerful, but hard-to-build astronomical hardware will dot the heavens too.

NASA's Space Interferometry Mission (SIM) is headed for launch in 2009, assigned the job of determining the positions and distances of stars several hundred times more accurately than any previous observations. This accuracy will allow SIM to look for the positional (astrometric) wobble of nearby stars induced by orbiting planets. In some cases, planets as small as a few Earth masses should be detectable.

Data from SIM will help establish a catalog of likely targets for another longing-look outward: the NASA Terrestrial Planet Finder, or TPF for short.

With an anticipated launch between 2012-2015, TPF will be capable of detecting and characterizing Earth-like planets around as many as 150 stars up to 45 light-years away. TPF is slated to make 5 years of observations to detect the atmospheric signatures of habitable or even inhabited planets.

The European Space Agency (ESA) has selected the InfraRed Space Interferometer -- better known as Darwin -- as a mission for its Horizons 2000 program. Selection of a launch date, probably in the 2014-2015 time frame, will be made on cost, science and technology grounds sometime before then, according to ESA. As now envisioned, Darwin will use a flotilla of six space telescopes. Working together, the telescopes are to look for signs of life on Earth-like planets.

Real star travel -- as opposed to "interstellar precursor" missions that just get a little way outside the solar system -- is very hard. But it can be done drawing upon physics we know about today. More good news: Warp drives and hyperspace jumps are not required!

That's the view of Jordin Kare, a technical consultant on advanced space systems based in Seattle, Washington. "The basic problem is that getting to the stars in a reasonable time takes a very high velocity, and therefore an enormous amount of energy, no matter how you do it."

"Reasonable time" is of course a matter of opinion, Kare quickly added. "If you're willing to take a thousand years to go a few light years, ordinary nuclear fission power and ion thrusters will get you there. That's the 'generation ship' approach that has appeared in science fiction many times. Given a good enough reason, such as finding out that the Sun will explode on Jan. 1, 2100, we could start building interstellar ships today," he said.

Kare said that antimatter could provide enough energy to make fast interstellar treks possible. But it's incredibly expensive to make and we don't know how to store it or use it efficiently for thrust, he said.

"The best prospects for interstellar travel seem to rely on using resources outside the spacecraft. One approach is to collect propellant as you go. Unfortunately, collecting interstellar hydrogen doesn't seem to work. It's not dense enough, and efforts to design magnetic 'scoops' have shown that it's hard to gather hydrogen in flight. In fact, magnetic scoops work better as drag brakes for slowing down than they do as part of a propulsion system for speeding up," he told SPACE.com .

Kare has designed one interstellar propulsion system that uses a long trail of pellets of fusion fuel, pre-placed along an acceleration path, to supply both power and reaction mass to a vehicle.

The impact fusion part comes from using stationary pellets that slam into similar pellets carried onboard. The impact itself will set off the fusion reaction. Akin to the "buzz bomb" of World War II heritage, the vehicle would be propelled by a string of explosions happening so fast that they'd sound like a buzz. "Except this buzz would be baby hydrogen bombs, going off 30 or so times per second," Kare said.

But the best way to do interstellar propulsion seems to be to use beamed momentum - use the pressure of a laser beam or microwave beam to accelerate a reflective "sail" up to high velocity. In Kare's SailBeam design, a stream of small sails carries the momentum from the laser accelerator to a vehicle. At the far end, a magnetic drag brake can be used to slow down.

"It's still a huge project," Kare admitted. Launching a one-ton probe to another star with SailBeam would take many gigawatts of laser power for several years.

"But it doesn't take any new physics, and the technologies required are only modest extrapolations from what we can do now. My very rough guess is we could start building a SailBeam launcher in 20-30 years, and be launching probes by 2050," he said.

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