Once habitable planets are discovered and chemically fingerprinted around nearby stars, there will be a lot of interest in dispatching interstellar probes to check out the best candidates for life.

But dreams of star travel present an awfully steep slope when one calculates the energy needed to make the journey. Though exotic “massless” drives, somehow tapping the “vacuum energy” of space may eventually be realized, let’s assume for this discussion only simple action/reaction engines based on good ol’ Newtonian physics can be used.

ANALYSIS: Harnessing Solar Energy to Sail to the Stars?

Whatever the drive — nuclear fusion, matter-antimatter, and even black hole — enormous reservoirs of fuel are needed to be transported along with the starship. And, that requires even more energy to accelerate and decelerate a fuel-laden massive vessel.

The way around this dilemma is to generate enormous amounts of energy close to home and just beam energy across space to an interstellar probe. Think of propelling a leaf with the power of a garden hose. The leaf is a bare fraction the mass of the hose and water supply.

“It is the only method of interstellar flight that has no physics issues,” writes James Benford of the R&D; firm Microwave Sciences.

In the mid-1980s physicist Robert Forward proposed interstellar sails pushed along by energy beams transmitted from Earth. Forward even described how a laser beam could be used to “reverse-thrust” a probe to decelerate into a star system.

ANALYSIS: Tau Zero Takes Aim at Interstellar Propulsion

Giant microwave transmitters aren’t as precise as the collimated beam of a laser, but they are cheaper to build. The aiming issue is offset by “firehoseing” the probe with so much energy it rapidly accelerates before overheating from the microwave blast.

Benford is carrying out laboratory experiments using microwaves to test the basic features of “beam-riding.” They show that a broad conical sail shape appears to work best. The space sail and probe would need to be made of extremely lightweight materials: carbon nanotubes, microtrusses, graphene, and beryllium.

They would have to withstand seething temperature of 2,000 degrees Fahrenheit from the intense beam energy. This calls for an extremely reflective surface that doesn’t absorb many photons.

The directed-energy beam “launcher” requires a substantial investment into a power-sucking huge transmission antenna. Building an interstellar launch a system to propel a U-Haul truck-sized payload would cost $180 trillion by Bedford’s estimates and each mission would cost $500 billion. As bank-busting as this sounds, it is still probably much cheaper than building a self-propelled starship.

ANALYSIS: Using Fusion to Propel an Interstellar Probe

What’s more, by being kept on (or near) Earth, the launch system can be serviced and maintained. You wouldn’t find any space parts for repairing your star drive at Alpha Centauri, after all. The entire approach is much more fault-tolerant because a failed probe can be replaced by one from the assembly line.

Benford says that microwave beaming could first be tested to rapidly “FedEx” delivery of critical payloads within the solar system for emergencies like replacing serious equipment failures, or medicine for Mars colonists. The payload could reach speeds approaching 1 million miles per hour after just of few hours of being “beamed.”

A decelerating “beamer” or aerobraking would bring it into Mars orbit. Transit time would be less than two weeks.

A precursor interstellar mission out to the Oort cloud of comets, one light-year away, would call for a 24-gigawatt 2-mile diameter antenna costing around $144 billion, Benford estimates. The payload would weigh 150 pounds, half of the weight being the half-mile diameter sail. After five hours of beaming the probe would be accelerated to 140,000 miles per hour.

A true starship, capable of reaching the neighboring Alpha Centauri system within 40 years at 1/10th the speed of light, would likely weigh several tons unless it made extensive use of nanotechnology. Former NASA Administrator Dan Goldin talked of reducing the size of interstellar probes to that of as soup can.

The killer is that 300 terawatts would need to be beamed from a 60-mile diameter antenna to accelerate this sucker! That’s about 20 times the daily electric power consumption of the entire planet.

The probe would need to be accelerated rapidly to avoid vaporizing the sail. The acceleration would therefore be 50 times the force of gravity on Earth’s surface, or 50G’s – so don’t plan on stowing away onboard, you’ll be flattened like a pancake.

To borrow an idea from Carl Sagan’s 1985 novel Contact, imagine a nearby civilization transmitting blueprints to us for building a practical beaming system. It would be used to launch a time-capsule probe to their star where their own beaming system would decelerate and capture it.

Such interstellar parcel packages might be the only practical way to exchange physical samples between neighboring extraterrestrial civilizations. We might send them the DNA of various species on Earth. Imagine replicating the San Diego Zoo on Alpha Centauri.

Editor’s note: This article was originally published on March 20, 2012.

Image credits: NASA