Clockwise from top left, images courtesy of MSNW, LLC’s website, Y.K. Bae Corp’s website, L’Garde Inc via YouTube, MSNW, LLC’s website.

Soon, using big, expensive rockets won’t be the only way of reaching space.

Here are 5 alternative propulsion technologies currently being developed:

1. Nuclear-powered rocket engines

Photo courtesy of MSNW, LLC’s website.

Current combustible fuel used to launch spacecrafts has a low energy-to-mass ratio, meaning massive quantities of it are required per launch.

Instead, researchers at NASA and companies such as Ad Astra Rockets are hoping to use the high energy density of nuclear power to fuel fast, long-distance flights.

A trip to Mars could be as short (pdf) as 39 days, one way, compared to the roundtrip of 500 days estimated with current tech, according to space.com.

The engines being developed by Ad Astra would use its nuclear power to turn a gas such as argon, xenon, or hydrogen into superheated plasma, which generates thrust.

Other researchers, such as those at MSNWC LLC, dream of employing nuclear fusion to create super-fast, long-lasting craft. Of course, fusion would have to be accomplished on Earth, first.

2. The Grasshopper rocket

Much more conventional, the Grasshopper – currently in development by SpaceX – is essentially a regular rocket, using traditional combustible fuel rocket engines.

However, it separates itself from the pack with one important aspect: it’s reusable.

Other rockets separate themselves from their payload after it’s reached space, burning up on their way down through the atmosphere.

The Grasshopper could drastically reduce the cost of space launches, as the cost of a single flight is $200,000 – only 0.4% of the cost of the rocket itself.

Space-X has successfully launched and landed another, larger reusable rocket called the Falcon 9. The rocket has 9 rocket engines, and stands 15 stories high, costs more to operate than the Grasshopper, but can carry much heavier loads.

3. Water-powered thrusters

Photo courtesy of MSNW, LLC’s website.

Yup. A rocket engine currently being tested by the Texan company MSNW, LLC can use water – or even Martian air (tests were run on simulated Red Planet air) – as a fuel for its plasma beam that has “significantly greater thrust and power densities than any realizable propulsion technology.”

The rocket engine would be highly flexible, allowing refueling of spacecraft in orbit, and missions that could travel to deep space and return.

4. Solar sails

Photo courtesy of L’Garde Inc via YouTube.

Named after sail ships powered by the wind, solar sail spacecraft are propelled by the photons that make up light from the sun.

While this technology obviously can’t be used to launch anything from Earth, it was already used in 2010 to successfully propel a Japanese spacecraft towards Venus.

It’s a lightweight, low-cost, propellant-free method of space travel, and several projects are already underway, including L’Garde Inc’s 13,000 square feet “Sunjammer” project launching in 2015.

5. Laser propulsion

Image courtesy of Y.K. Bae Corp’s website.

Of course, lasers are involved. In October, 2013, the Y.K. Bae Corp received $500,000 in NASA funding to further scale up its laser propulsion project.

The method would essentially function “like a gun,” according to (video below) founder Young K. Bae, using a powerful laser to propel spacecraft outward.

After it reaches the estimated laser range of 1,000km, it would be left on its own, however – but the removal of the energy source and propulsion device could mean much lighter spacecraft carrying larger payloads of scientific (or commercial) equipment.

Watch Dr. Bae explain the concept below:

Updated