Beyond: Our Future in Space (25 page)

So what does nanotechnology imply for space exploration? The big difference between us and our machine proxies is that robots can be shrunk and we can’t. Nanobots are too small for batteries or normal solar cells, so tiny amounts of radioactive materials would be used to power them. Conversely, nanotechnology is leading to new, efficient designs of solar cells, so nanobots could be used to assemble solar panels at a remote location.

The Curiosity rover is a wonderful machine, but this SUV-size Mars vehicle will seem like a dinosaur when nanobots arrive on the scene. The first wave would be dropped from an orbiting spacecraft—in weak Mars gravity, they would ride the wind like a dust storm of smart motes. Each nanobot would have a processor, an antenna for communicating with other nanobots and the orbiter, and various sensors. The skin of each nanobot would be a shape-shifting polymer to optimize drifting on air currents or being blown along the surface. All these capabilities have been prototyped at millimeter scales; there are no fundamental obstacles to shrinking them further.
¹²

For more complex missions, robots would move independently under their own power. NASA researchers have developed the concept of ANTS, or Autonomous Nanotechnological Swarm—robots a millimeter across made of a tetrahedron of carbon-nanotube struts connected by joints. Each robot would move by shortening or lengthening its struts, altering its center of gravity to tumble in the right direction. Imagine thousands of these tiny rovers, linked by a neural net, fanning across the surface and carrying out geological tests to look for signs of life. NASA recently auctioned off the patent for the robots, hoping to spur innovation.
¹³

Nanobots could also convert carbon dioxide into oxygen; if they were self-replicating, they could greatly accelerate the process of terraforming Mars. And not just Mars. The same NASA research group thinks nanobots made of carbon nanotubes could survive the 900-degree temperature on the surface of Venus. Following on from the billion-dollar, multi-ton Cassini mission, a spacecraft the size of a shoebox could seed remote-sensing nanobots on Titan and Enceladus. With asteroids, they could do an inventory of precious metals and rare earths as the prelude to full-scale mining.

Nanobots can also help humans be safer when they explore space. Constantinos Mavroidis, a professor of engineering at Northeastern University, formed a team to map out concepts that are attainable within forty years. One of their ideas was a strong but lightweight spacesuit that could repair itself with protein-dispensing nanounits built into the layers of fabric. The same suit could also monitor the vital signs of astronauts and carry emergency drugs. When there’s room at the bottom, the sky’s the limit.
¹⁴

Solar Sailing

The cost and difficulty of space travel is rooted in dependence on the way rockets work. Sunlight is used to generate electricity and power on Earth and in space. What if it could be used for propulsion as well?

It can. Johannes Kepler noticed that comet tails point away from the Sun, and in a 1610 letter to Galileo, he suggested: “Provide ships or sails adapt to the heavenly breeze, and there will be some who brave even that void.” Jules Verne was the first to outline the concept of the solar sail in 1865, seizing on James Clerk Maxwell’s theory that light has momentum as well as energy so it can exert pressure on objects. In
From the Earth to the Moon
, Verne wrote: “Light or electricity will probably be the mechanical agent [by which we shall] travel to the moon, the planets, and the stars. . . . ”
¹⁵

The physics is simple.
¹⁶
When photons hit any reflective surface, their momentum changes as they reverse their velocity, imparting a very small unit of force to the object they hit. When the shiny surface of a sail faces the Sun, the sail is pushed away from the Sun. Solar sails can do anything nautical sails can do, including tacking. Changing the angle of the sail relative to the Sun affects the direction in which the sail is propelled. The sail can direct a spacecraft toward the Sun by using photon pressure to slow the spacecraft and lower its orbit. A solar sail can even act like an antigravity machine, using solar pressure to balance the Sun’s gravity and hover anywhere in space.

Light pushes very gently on a solar sail in Earth orbit, so acceleration is feeble when compared with the kick imparted by a rocket. But a rocket will run out of fuel while the Sun keeps shining. This steady stream of photons creates acceleration that adds up to a substantial velocity boost. A solar sail should be big and light, to catch as much sunlight as possible and thus impart maximum possible velocity to the spacecraft. Cosmos 1 was intended to be a prototype for interplanetary travel using a solar sail. It was funded by the Planetary Society and Cosmos Studios, the film studio founded by Ann Druyan, Carl Sagan’s widow. Sagan had died in 1996 and she wanted the 600-square-meter Mylar sail to be his Taj Mahal, a monument to a man who had argued for us to weigh anchor and set sail for the stars.

Sunlight would exert a tiny acceleration of half a millimeter per second per second on the huge sail. After one day, the speed would be just 100 mph, but after 100 days, its speed would be 10,000 mph. After 1,000 days, it would reach a blistering 100,000 mph. Unfortunately, the Volna rocket that was launched from a Russian submarine in June 2005 failed, and Cosmos 1 went to the bottom of the Barents Sea.
¹⁷

Solar-sail development has continued, but the ambitions and the size of the sails have been scaled back. A team from NASA built NanoSail-D, based on the CubeSat specifications. CubeSat is a miniaturized satellite designed to spur space research by using standard components and off-the-shelf electronics. A CubeSat is a bit bigger than a Rubik’s Cube—10 centimeters on a side and weighing less than 1.3 kilograms. Most CubeSat launches have come from academia, but companies such as Boeing have built CubeSats, and amateur satellite builders have gotten their projects off the ground using crowdfunding campaigns on websites such as Kickstarter.

NASA’s NanoSail-D was designed to use three CubeSats to de-
ploy triangular sails totaling 10 square meters. Unfortunately, it too was scuppered by the launch vehicle when its Falcon rocket malfunctioned in 2008. But NASA persisted, and a twin was successfully launched in 2011 (
Figure 43
). NanoSail-D was never intended to be more than a test of solar-sail deployment, and it burned up after 240 days in low Earth orbit. The previous year, the Japan Aerospace Exploration Agency (JAXA) had sent IKAROS toward Venus. It was the first spacecraft to be fully powered by a solar sail. NASA had plans to launch a 1,200 square-meter sail called Sunjammer but cancelled the project in late 2014.
¹⁸
The name comes from an Arthur C. Clarke short story.

CubeSats are at the epicenter of the business plans of commercial space companies. In the next five years, more than a thousand nanosats will be launched, some larger than a CubeSat and some smaller.
¹⁹
In early 2014, a batch of twenty-eight CubeSats was released by a satellite “shooter” aboard the International Space Station to take pictures of the Earth. In 2013, the first PhoneSat went into orbit. This used sensing plug-ins for a Google smartphone to measure magnetic fields, pressure, and more. If the launch cost drops below $1,000 per kilo, anyone will be able to get into the game. Nanosats will be the first choice for remote sensing on moons and planets in the Solar System.

Like space elevators, solar sails are still in their infancy. The challenge is to launch a gossamer film that’s reflective, the size of a football field, and a hundred times thinner than a sheet of paper. The sail has to be launched in a compact configuration and then unfurled in space and held rigid by a frame or inflatable boom. Solar sails are slow and steady in their acceleration, but they face diminishing returns as they reach the outer Solar System, because the amount of “push” from solar photons goes down by the square of the distance from the Sun. The sail doesn’t slow down, but its speed does increase more and more slowly.

As a result, some people are exploring radical ideas. An electric solar sail doesn’t look like a sail; rigid conducting wires extend radially from the spacecraft and a current keeps them charged up to 20,000 volts. Their electric field makes them look 50 meters thick to the solar-wind ions, and that interaction drives the spacecraft. Magnetic solar sails also use the solar wind, but they deflect the ions with a magnetic field made by running current through wire loops. Magnetic sails can use planets and the Sun for thrust, by pushing against their magnetic fields.
²⁰
To venture beyond our Solar System “harbor,” we’ll want to build up as much speed as possible before coasting across the vast interstellar sea.

Finding Alien Technology

We’ve had the ability to leave the Earth for two generations, a tiny fraction of the thousands of generations since we made our epic journey out of Africa. We share intelligence, and perhaps sentience, with species such as chimps, dolphins, orcas, and elephants. But we’re the only species that has bent the material world to our will and fashioned computers and skyscrapers and rockets. Are we the only creatures who have developed the technology to venture beyond their home planet?

The best way to answer this question is to use the fastest thing there is: electromagnetic waves.

Remote sensing has let us diagnose distant planets and look for signs of microbial life. It can also let us vault over uncertainties in biological evolution and look for the hallmarks of intelligence and technology. For decades, science fiction writers have woven stories of aliens who are biologically bizarre or who have eclipsed us with their technology. Scientists play this game too, and it’s known by the acronym of SETI: Search for Extraterrestrial Intelligence.

In an influential paper published in
Nature
in 1959, “Searching for Interstellar Communications,” Giuseppe Cocconi and Philip Morrison argued that a search was warranted even though there was no evidence for life any place other than Earth. They wrote: “The reader may seek to consign these speculations wholly to the domain of science-fiction. We submit, rather, that the foregoing line of argument demonstrates that the presence of interstellar signals is entirely consistent with all we now know, and that if signals are present the means of detecting them is now at hand.”
²¹

They argued that we target nearby Sun-like stars and look for narrow-bandwidth microwave signals. Radio waves aren’t naturally produced by stars, so radio waves appearing from a star could only come from an artificial source next to the star. Radio telescopes and powerful radio transmitters had been developed a decade earlier. Visible light isn’t the best choice for signals because thick planet atmospheres are opaque and billions of stars in the galaxy present a confusing noise source. The radio regime is much quieter because stars don’t emit radio waves. Moreover, there’s a particularly quiet zone in the cosmic environment between 1 GHz and 10 GHz where water vapor doesn’t absorb radio waves, so they travel freely across large distances in the galaxy. It also happens to be the region where hydrogen has a fundamental spectral transition that would be noteworthy to any alien civilization that knew about physics.

Cocconi and Morrison urged astronomers to search this region, and to doubters they noted: “The probability of success is difficult to estimate, but if we never search, the chance of success is zero.”

Soon afterward, a young researcher named Frank Drake pointed the 25-meter dish at the National Radio Astronomy Observatory in Green Bank at two nearby, Sun-like stars: Epsilon Eridani and Tau Ceti. This experiment was named Project Ozma after the ruler of L. Frank Baum’s fictional land of Oz. Even though Tau Ceti is now known to have an orbiting exoplanet in the habitable zone, Drake saw no artificial signals in his brief experiment.

In 1961, Drake hosted a small meeting at the Green Bank radio observatory. He recalled: “I realized a few day[s] ahead of time we needed an agenda. And so I wrote down all the things you needed to know to predict how hard it’s going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number,
N
, which is the number of detectable civilizations in our galaxy.”
²²
In its original formulation,
N
is the product of these factors: the average rate of star formation in the Milky Way galaxy, the fraction of those stars that have planets, the average number of those planets that can support life, the fraction of habitable planets that actually develop life, the fraction of planets with life that evolve intelligent life (i.e., civilizations), the fraction of those civilizations that are detectable from space, and the span of time the civilizations are in a detectable or communicable state.