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Reaching for Interstellar Flight

Leonard David, Space.com

original source |  fair use notice

Summary: 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. Decades later, a similar visionary zeal to seek new worlds and new civilizations is a factual enterprise for a new generation of galactic explorers. They are taking on spacetime and hoping to boldly go where no spacecraft has gone before -- out to far-flung stars and the planets that circle them.

By Leonard David, Senior Space Writer, Space.com, 17 December 2003

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.

Decades later, a similar visionary zeal to seek new worlds and new civilizations is a factual enterprise for a new generation of galactic explorers. They are taking on spacetime and hoping to boldly go where no spacecraft has gone before -- out to far-flung stars and the planets that circle them.

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.

Vast distances

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.

Technological hurdles

"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.

Sign posts up ahead

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.

Longing looks outward

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.

Not required: warp drives and hyperspace jumps

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.

Buzz bomb to the stars

"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.

Hitting paydirt

"Most people don't realize how ambitious interstellar flight is," said Raymond Halyard, an engineer with United Space Alliance in Houston, Texas. The 4.3 light-years to Alpha Centauri equate to approximately 25 trillion miles. That's about 100 million times the quarter million miles to the Moon," he noted.

Halyard is a rocket propulsion engineer, with years under his belt working at NASA's Johnson Space Center in Houston, Texas on the Apollo/Saturn 5 booster engines, as well as on space shuttle propulsion systems. He is conducting his own independent research on Inertial Confinement Fusion propulsion.

"I've worked on Inertial Confinement Fusion propulsion designs for years," Halyard said. The best Alpha Centauri flyby design has a terminal velocity of 1/10th the speed of light and a flight time of 50 years. The cost of the effort would be approximately $100 billion, roughly the price tag of the Apollo program corrected for inflation, he said.

"There's the rub. The experimental work on the various breakthrough physics propulsion concepts remains to be funded, let alone accomplished," Halyard said.

"But like the Wright Brothers" Halyard added, "there are several people working on designs in their garages with shoestring budgets and somebody may hit paydirt!"

Although Halyard doesn't personally gauge any breakthrough in propulsion physics as probable, he does see it as certainly possible. "Since this would be like winning the lottery, work in this area is certainly worth the pittance it will cost to investigate," he said.

Is there a way to get past the dead-end rocket technology of today?

New physics

Given time and hard work, propellantless space travel, or even more exotic propulsion systems might open up the Universe to human exploration, said Paul March of Lockheed Martin Space Operations in Houston, Texas.

March said that he personally doesn't view human interstellar sojourns as possible using the current stable of chemically fueled rockets. Without the use of antimatter energized vehicles, reasonable trip times to even the closest stars for a crewed vehicle is not feasible, he said.

"Conventional chemical or fission/fusion rockets just won't cut it for crewed interstellar flights! And even antimatter-powered rockets will only be able to take us to the nearest stars, just like our current chemical based rockets can just barely get us to Mars," March said, noting that his views are his own and do not reflect the official positions of his employer.

Given sufficient resources, March said, nuclear rocketry could open up exploration of our own Solar System within 30 years. "But interstellar flight would be a tough sell."

What's needed is rocketry that can recycle its propellant, commonly referred to as propellantless or field-propulsion. "This little trick requires a new understanding of physics that goes beyond standard Newtonian physics," March said. "This new physics requires a new understanding of the Cosmos that will allow us to manipulate spacetime in such a way that field propulsion becomes possible and Star Trek-like 'warp bubbles' become doable."

Most physicists go screaming in the other direction when faced with this problem, March observed. Nevertheless, there are those dabbling in such arenas, "where these types of propulsion concepts won't be science fiction for much longer."

"If we are lucky…very, very lucky, we might have a solution to the propellantless propulsion side of this interstellar propulsion problem within the next ten years," March said.

Boundary breaker

Astronomer and author, Gregory Matloff, said he suspects that the first human starship will be a private venture. He points to the work of Team Encounter in Houston, Texas now developing a solar sail spacecraft that will carry human genetic material.

Team Encounter's premiere solar sail mission, Humanity’s First Starship, is scheduled for launch in 2007 and is designed to leave the Solar System. Team Encounter’s precursor mission, Flight One, will launch in 2005 on a journey in near Earth space.

"NASA could launch an interstellar sail with a science payload to explore the heliopause before 2020," Matloff told SPACE.com . The heliopause is the boundary that separates Earth's solar system from interstellar space. "This will be faster than the Team encounter effort and could reach the nearest star in 7,000 years or so," he said.

Matloff is an Assistant Professor of Astronomy and Physics at the Department of Biological and Physical Sciences, New York City College of Technology, CUNY. He also consults on solar sail research for the In-space-propulsion team within the Advanced Space Transportation Directorate at NASA's Marshall Space Flight Center in Huntsville, Alabama.

Interstellar doorstep

Matloff said that before the beginning of the 22nd century, both nuclear and solar drives should be approaching their interstellar potential. If a craft was launched then using such propulsion, it could reach Centauri within about 1,000 years, he said.

"New forms of interstellar propulsion will hopefully reduce this travel time. Right now we can realistically talk about interstellar transit times approximating the lifetime of a civilization. But it would be very nice if we could reduce this to a human lifetime or less. Even though robots wouldn't necessarily care if their flight time were long, the people back home waiting for data might lose interest. And humans aboard a generation ship might arrive having forgotten their goal," Matloff said.

If we search for and do not discover Earthlike worlds, the goal of interstellar expansion will not be advanced, Matloff reasoned.

Why fly for decades or centuries to colonize a comet, asteroid, or Mars-like world circling Alpha Centauri A when we can travel for a few years at much lower cost to reach a similar world in our own solar system?

"But if we search for and find Earths on our interstellar doorstep, watch out! Even if we must reach them by worldships, that's what we'll do. And the hunt for a true star drive will be well funded," Matloff predicted.

Head for the stars

But advanced space systems designer, Jordin Kare, said that taking a "robots first" approach may be the correct plan of action.

"I suspect the only way we're likely to send humans to the stars is if robots go first and find a planet we can live on," Kare said. "At that point, some group may decide to take the chance and set out on a multi-century trip so their descendants can have their very own world. If we already have people living in space colonies, or even space hotels, with closed ecologies, it's not that big a step to add a nuclear power plant and ion drives…and head for the stars.

Of course, there's another way to send people to the stars, Kare said, not by sending them physically but by sending them as information on a laser beam.

The first stage is downloading your brain into a computer, then have a copy emailed to Alpha Centauri, where pre-positioned nanotech robots build you a new body. "You wouldn't even notice the 4.3 years you spend in transit. Of course, if the email bounces, you're in trouble," Kare cautioned.

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