A Laser-Powered Ion Engine for Deep Space

When we kick around ideas for deep space propulsion, we have to keep in mind that the best solutions may involve hybrid technologies, leveraging the best of several methods. JAXA (Japan Aerospace Exploration Agency) demonstrates this with its ambitious plan to take a sail like IKAROS to Jupiter, for a study of the Jupiter trojan asteroids. Like the original, the upgraded IKAROS will use liquid crystal reflectivity control devices as a means of attitude control.

But operating at the limits of solar sail functionality, the new JAXA sail will also carry a high specific impulse ion engine to facilitate its maneuvers among the trojans. Here we have a mission that couldn’t be flown with just a sail, because the numerous trajectory changes required at destination demand a reliable thruster, one that in this case will be fed by some 30,000 solar panels in the form of thin film solar cells attached to the sail membrane.

What of missions into still deeper space? We’d like to return with robust technologies to the outer planets, not to mention the need to follow up the Voyagers and New Horizons with craft specifically designed to study the interstellar medium beyond the heliosphere.

For that matter, we’re seeing increased interest in exploring the Sun’s gravity lens, whose effects can be examined beginning at 550 AU. That last is a hot and controversial topic, as the spirited debate between Slava Turyshev (JPL) and Geoff Landis (NASA GRC) showed at the recent TVIW conference in Huntsville (keep an eye on the TVIW 2017 video page, where these discussions will soon be available). To resolve the matter, we need to actually go there.

JPL’s John Brophy has been exploring another form of hybrid technology to make such missions possible — I talked about this one when it surfaced last April (see NIAC 2017: Interstellar Implications). Now working under a Phase 1 grant from the NASA Innovative Advanced Concepts (NIAC) program, Brophy proposes using lasers to provide the power source for a spacecraft’s ion engines, beaming to solar panels on the craft. The idea here is to take advantage of a power source separate from the spacecraft in return for major gains in weight and efficiency.

Brophy’s ion engine infrastructure depends upon a 10-kilometer 100 MW laser array capable of beaming power across the Solar System. The beam would be captured by a 70% efficient photovoltaic array tuned to the laser frequency and producing power at 12 kV, according to this precis prepared for the NIAC program. As Brophy has noted, the array output here far surpasses our best solar arrays today, found on the International Space Station, which produce 160 volts. With an areal density of 200 grams per square meter, the 10-kilometer array would be heavier than today’s solar sails but substantially lighter than existing solar arrays.

The laser array in question has its roots in high-power laser concepts of the kind Philip Lubin has advocated for Breakthrough Starshot, only Brophy’s array is in space, avoiding the numerous issues raised by Starshot’s ground-based installation (and creating construction issues of its own). What you achieve with this kind of configuration is the ability to deploy powerful ion engines in the outer Solar System, as Brophy is quick to note in his NIAC precis:

Our innovation is the recognition that such an array increases the power density of photons available to a spacecraft illuminated by the laser beam by two orders of magnitude relative to solar insolation at all the solar system distances beyond 5 AU, and that this enormous power can then be used to great effect by driving a highly-advanced ion propulsion system.

Image: The laser-powered ion thruster concept as developed by John Brophy and colleagues.

The thruster being powered by the laser beam is a lithium-fueled gridded ion propulsion system that does away with what would otherwise be heavy power processing hardware and associated thermal radiators. The 58,000 second specific impulse — compare this to Dawn’s 3,000 seconds — goes well beyond current state of the art in spacecraft systems — about 20 times — and takes advantage of the fact that lithium is both easily ionized and easily stored. The result:

This allows the thruster to be operated with nearly 100% ionization of the propellant which effectively eliminates neutral gas leakage from the thruster and the production of charge-exchange ions that are responsible for thruster erosion and current collection on the photovoltaic arrays. This key benefit enables very long thruster life and facilitates the development of the 12-kV photovoltaic array.

Brophy believes such a system could achieve velocities of 260 kilometers per second, which would make missions to Jupiter feasible within one year of flight time, while reaching the gravity focus would be a matter of 10 to 12 years. If that isn’t tantalizing enough, he also talks about a Pluto orbiter mission with a travel time in the area of 4 years. Thus the hybrid concept ramps up the performance of ion engines in places far enough from the Sun to pose serious power issues, and also taps into a laser infrastructure that could one day drive missions system-wide.