In the movies, X-wings and TIE fighters dart and weave in dogfights out of a WWII epic. But what if we had to build a maneuverable spaceship that could potentially take evasive actions -- and we had to do it today?

In a hundred years, experts at the Marshall Space Center expect us to be using a process called magnetized target fusion to move around in space.

John Cole, the center's manager of space transportation research, compares magnetized target fusion to a nuclear pulse engine, only using fusion instead of fission bombs. The resulting explosions would generate huge thrust in the form of gamma- and X-ray bursts.

"You could go anywhere in the solar system and bring the crew home in a year's time," Cole said.

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In fact, let's say aliens decided to invade Earth tomorrow. While their gigantic command ship camps in high orbit, their attack fighters cruise above the maximum altitude of our jets, dropping bombs on us.

What do we do?

Earth-shattering kaboom

Bryan Palaszewski, leader of advanced fuels research at NASA's Glenn Research Center in Cleveland, suggests a nuclear pulse propulsion system to power the ship.

"You'd need to be able to avoid spacecraft that could change orbits quickly," he told space.com. "The alien vessel would probably have a really advanced system that could manipulate gravity, and we just can't do that."

The ship would carry many small nuclear bombs, which would be ejected one by one behind the ship and explode.

The explosions would then bounce off a blast shield, called a "pusher plate." Springs and cushioning would support the blast shield to absorb the impact of the explosion and propel the ship forward.

It's kind of like a pogo stick, but instead of bouncing off the ground to propel the stick up, you set off a nuclear explosion under the stick and ride the recoil.

As for moving in other directions, Palaszewski said multiple pulse chambers could theoretically be installed on all sides of the ship, although, he cautioned, "that design would produce some interesting structural problems."

Short of that, he said, such a vehicle could slowly turn itself while explosions propelled it in a different direction .

One of the Advanced Propulsion Technology Group's schemes for a nuclear-powered pulse ship.

Lots of thrust

How strong would those nuclear engines be?

"Not only would it be fast, but it would use very little fuel," Palaszewski noted. "So the spacecraft would be very compact."

Les Johnson, project manager of interstellar propulsion research at Marshall Space Flight Center in Huntsville, Ala., said a nuclear pulse engine would deliver 20 times the thrust of a space shuttle.

Once the ship is built, how would we fight with it? Stressing that this was a purely speculative idea , Palaszewski suggested that a pulse chamber could be put on the front of the ship and used as a weapon.

"I'm sure it'd be OK to just launch a little bomblet at one of those aliens."

Another option might be to simply turn around and aim the nuclear-bomb-launching engine at the target. It's an idea Larry Niven called "the Kzinti lesson" in his Known Space stories, referring to an alien race that learned about it the hard way.

Of course, a nuclear pulse ship would have its limitations, Palaszewski said.

Although each chamber could produce several thousand explosions, each explosion would vaporize part of the blast shield. When the blast shield finally wears out, that chamber would be useless and, in the meantime, radiation from the nuclear blasts could endanger the crew.

To offset this problem, Palaszewski suggested the blast shields be made of lead, which absorbs radiation. On a ship with multiple chambers, the cockpit would be put in the middle of all the blast shields.

How far into science fiction are we? Not too far, actually.

Legendary physicist Freeman Dyson worked on a nuclear pulse propulsion concept with a project called Orion in the late '50s and early '60s, creating several working test vehicles propelled by explosives before the 1963 Nuclear Test Ban Treaty ended his research..

"If aliens were attacking, no one would care about a test ban treaty," Palaszewski said.

Back in the science fiction world, Larry Niven featured a battleship powered by a nuclear pulse engine in his 1985 novel Footfall, written with Jerry Pournelle, and it's still a popular SF concept, appearing as recently as Stephen Baxter's new novel, Manifold: Time.

A sail-powered vehicle.

Earth to Mars in two months

In case the hypothetical battle for the Earth goes badly, how could we get really far away, really quickly?

Les Johnson from the Marshall Space Center says we could travel to Mars in as little as two months using a nuclear pulse engine, and it would take only a few more months to reach Jupiter.

Meanwhile, a nuclear pulse engine is probably the most immediately available of many possibilities, according to John Cole, the Marshall Center's manager of space transportation research.

"[Pulse engines] can be built any time," Cole said, while hastening to note that we could also use a solar sail to travel far into space. In cooperation with the Jet Propulsion Laboratory in Pasadena, Calif., the Marshall Center is currently working to build a solar sail that would power an uncrewed vessel outside the solar system.

The sail will be made of a super-thin carbon fiber that would catch sunlight like a sail catches wind.

"Light photons hit the sail and push it forward like a rubber ball thrown at door would push it open," Cole said.

The unmanned ship would be six times faster than the Voyager probes which toured the planets of the solar system.

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