Ion engines could take us to the solar gravitational lens in less than 13 years, suggests paper

Sending an object to another star is still the stuff of science fiction. But some concrete missions could get us at least part way there. These “interstellar precursor missions” include a trip to the solar gravitational lens point at 550 AU from the sun—farther than any artificial object has ever been, including Voyager.

To get there, we’ll need plenty of new technologies, and a recent paper presented at the 75th International Astronautical Congress in Milan this month looks at one of those potential technologies—electric propulsion systems, otherwise known as ion drives.

The paper aimed to assess when any existing ion drive technology could port a large payload on one of several trajectories, including a trip around Jupiter, one visiting Pluto, and even one reaching that fabled solar gravitational lens. To do so, they specified an “ideal” ion drive with characteristics that enabled optimal values for some of the system’s physical characteristics.

First among those characteristics is the power plant. Ion thrusters need a power source and an effective one if they will last more than a decade under thrust. The paper defined an ideal power plan that can output 1 kW per kg of weight.

This is currently well outside the realm of possibility, with the best ion thruster power sources coming at something like 10 W per kg and even nuclear electric propulsion systems outputting 100 W per kg. Some potentially better technologies are on the horizon, but nothing tested in the literature would meet this requirement yet.

Thrust efficiency is another consideration for this idealized mission. The authors, who are writing under the banner of the Initiative for Interstellar Studies, a non-profit group based out of the U.K., suggested that an idealized thrust efficiency is 97%. That would also significantly improve existing technologies, which average closer to 75%–80% efficiency for working models.

Additional improvements could increase this number, such as magnetic containment fields around the thruster’s walls. Still, as it gets closer to that 97% range, finding efficiency improvements becomes harder and harder.

The last characteristic the authors considered was the specific impulse. This one has the most comprehensive variability regarding the theoretical potential of all three systems. Their idealized value of 34,000–76,000 seconds of specific impulse is well within the bounds of the potential values for more speculative technologies.

The paper mentions that specific impulse values twice the suggested upper range could be possible with the proper selection of thruster and propellant. They also point out that development on these technologies is stalled not because we can’t make drives with better specific impulse but because we can’t produce power plants that support them yet. So, solving the power plant issue will enable further development in this area.

Suppose all three characteristics were combined into a complete functional propulsion system. In that case, the authors calculate that it could deliver a payload of almost 18,000 kg to the solar gravitational lens in just 13 years—much faster than any previous mission would be capable of.

But that optimization is still a long way off, and while there are missions planned for deployment to the SGL someday, it is still a long way off before they launch and even longer before they arrive there. In the meantime, engineers have some additional problems to solve if they want to optimize the potential of ion thrusters.