Space propulsion


It’s been more than 40 years since Yuri Gagarin became the first human to orbit the Earth, and, in the process, traveled faster than any human before.  Today, we continue to send humans into orbit…where they travel at pretty much the same speed.  Oh, sure, unmanned spacecraft have traveled much faster, and so did the Apollo astronauts–30 years ago.

To get humans to other planets in the solar system in a reasonable amount of time, and speed up robotic exploration, too, we need new methods of spacecraft propulsion.  The Marshall Space Flight Center in Huntsville, Alabama, is leading NASA’s new In-Space Transportation Investment Area effort to develop them.  High-priority technologies include aerocapture, electric propulsion, and solar sails.

Aerocapture is the technique of using a planet’s atmosphere to slow a spacecraft enough for it to slip into orbit without firing braking rockets. That means it doesn’t have to carry as much fuel, which saves mass, which makes it cheaper to launch and build.

Electric propulsion includes the ion engine.  In the most advanced ion engine tested so far, molecules of xenon are electrically charged, then pulled through an oppositely charged grid at up to 100,000 kilometres per hour and hurled out the back of the spacecraft.  The ion engine develops less than 10 grams of thrust (equivalent to the weight of a sheet of paper on Earth), but it can run for a very long time, imparting thrust for hours or days instead of the minutes typical of chemical rockets.  As a result, 82 kilograms of xenon in the ion-engined Deep Space 1 spacecraft imparted the same total amount of thrust as 820 kilograms of conventional rocket propellant would have.

Solar sails are one of the most romantic methods of space travel ever conceived.  The sun constantly streams photons into space; craft with massive, extremely lightweight solar sails are pushed along by the tiny pressure exerted by those photons.

There are other, more “far-out” technologies, being examined both inside and outside of NASA.

Robert Winglee of the University of Washington in Seattle envisions a “plasma sail,” a huge magnetic bubble around a spacecraft, which would be pushed along by the high-speed charged particles shed by the sun, cutting travel times to the outer planets in half.

Then there’s tether-based propulsion.  A spinning satellite in low Earth orbit would snare slower-moving objects with a tether and toss them higher at a higher speed–like skaters playing crack the whip.  The technology could be used to throw payloads to the moon or even to Mars.

Lasers hold promise.  Mounted on the ground or on a space station, an intense laser beam could be used in combination with a light sail or to create a kind of piston effect, cycling between a spacecraft-mounted mirror and a mirror fixed to the laser source.

Getting to the outer planets is one thing; getting to even the nearest star is another. Our nearest neighboring star, Proxima Centauri, is about 41.6 trillion kilometers from Earth.  At the speed of light, the universe’s absolute speed limit, that trip would take more than four years…and the speed of light is 17,000 times faster than our fastest spacecraft to date, Voyager, launched in the 1970s and only now just leaving the solar system.

To get to the stars, we need to circumvent the speed-of-light limit.  In science fiction, you do that with a warp drive.  It’s theoretically possible:  all you need to do is expand space-time behind your spacecraft and contract it in front, so that the segment of space-time containing your ship moves, carrying the ship with it.  The ship would never exceed the speed of light within its space-time bubble, but the apparent motion through the universe would be faster than the speed of light.

Another option would be to find a wormhole, a theoretical shortcut between two distant regions of space.

Of course, we don’t know how to find or create wormholes or warp space.  (Calculations show that to create a metre-wide wormhole you would need to convert something with the mass of Jupiter into negative energy–whatever that is!)

Perhaps we’ll figure it out someday.  Even if we don’t, researching the possibility could lead to unexpected and important new discoveries.  And in the meantime, we have an entire solar system of our own to explore–something the new technologies now being studied will enable us to do faster and more efficiently.

First, the planets:  tomorrow, the stars.

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