NASA wants to lasso an asteroid and tow it close to the moon, and this week it published a picture of the engine that might make that audacious plan possible.

The space agency recently posted a photo of a mysterious blue ring circling what looked like a white-hot thruster flame surrounded by a blue halo. NASA described it as a "cutting-edge solar-electric propulsion thruster."

The technology is in development at NASA's Jet Propulsion Laboratory in Pasadena, Calif. The picture was taken through a porthole looking into the vacuum chamber where the engine is being tested.

"This engine is being considered as part of the Asteroid Initiative, a proposal to robotically capture a small near-Earth asteroid and redirect it safely to a stable orbit in the Earth-moon system where astronauts can visit and explore it," said the caption on the photo posted by NASA.

The thruster uses xenon ions for propulsion and is the latest evolution of an earlier version that is currently guiding NASA's Dawn mission to the asteroid belt.

From sci-fi to reality: How ion propulsion works

Ion propulsion, once a concept that existed largely in the imaginations of science fiction writers, is now a reality for NASA. According to a fact sheet from the space agency, ion propulsion enables spacecraft to travel "further, faster, and cheaper" than any other system.

The system uses an inert gas such as xenon as a propellant, eliminating the risk of explosion associated with more unstable fuels. The xenon gas is then electrically charged, or ionized, in an ion thruster before it’s pushed out the back, generating the force to move the spacecraft forward.

"Modern ion thrusters are capable of propelling a spacecraft up to 90,000 meters per second (over 200,000 miles per hour). To put that into perspective, the space shuttle is capable of a top speed of around 18,000 mph," NASA said.

Downsides to ion propulsion

However, the trade-off for such an efficient engine, much like with early hybrid automobiles, is a lack of power and a painfully slow acceleration rate

"Modern ion thrusters can deliver up to 0.5 Newtons (0.05 kilograms) of thrust, which is equivalent to the force you would feel by holding nine US quarters in your hand," NASA said. “To compensate for low thrust, the ion thruster must be operated for a long time for the spacecraft to reach its top speed."

As part of its efforts to study asteroids, NASA plans to send a robotic spacecraft after a near-flying asteroid. The goal is to capture an asteroid and move it to a stable orbit near the moon, where astronauts could land on the asteroid and study it by 2025.

"We are developing a first-ever mission to identify, capture and relocate an asteroid," NASA chief Charles Bolden said in a statement.

"This mission represents an unprecedented technological feat that will lead to new scientific discoveries and technological capabilities and help protect our home planet."