Why Would a Spacecraft With a Gentle Ion Engine Need a Nuclear Reactor?

An Ion Engine Pushes Gently but Wastes Very Little Fuel

A rocket moves forward by pushing something backward. A chemical rocket throws hot gas out in one huge burst. An ion engine does it far more calmly: it gives a gas, usually xenon, an electric charge and then speeds the charged particles out the back using electric forces.

The push is small. NASA says an ion thruster makes little thrust compared with a chemical one, but it uses its fuel so well that it is 10 to 12 times more efficient. The price is time: the agency notes it must run for more than 10,000 hours to speed a spacecraft up enough to reach the asteroid belt or beyond [1].

That is not just a promise. NASA's test engine called NEXT ran for more than 43,000 hours, about five years, and during that time used more than 770 kilograms (about 1,700 pounds) of xenon [1]. Much earlier, NASA's Deep Space 1, launched on October 24, 1998 as a test flight for a dozen new technologies, fired its ion engine nonstop for 14 days in November 1998, and the engine was not finally turned off until December 2001 [2].

So a gentle engine is not a weak idea. It trades a hard shove for a very long, very thrifty one. For a trip to the far solar system, that trade can be worth it. But it comes with a catch.

The Catch: An Ion Engine Needs a Steady Supply of Electricity

An ion engine is an electric machine. If the electricity drops, the push drops with it. Most spacecraft with ion engines so far have used solar panels to make that electricity, which works well near Earth, where sunlight is strong.

Far from the Sun, the trouble begins. A NASA paper on solar arrays says that Jupiter, nearly 800 million kilometers (about 500 million miles) from the Sun, receives only 3.4 to 4.1 percent of the sunlight that reaches Earth [4]. The same paper points to the Juno probe, on its way to Jupiter at the time, as carrying the largest solar array NASA had flown other than on the International Space Station [4]. Even a giant panel gets only a few percent of the light it would get near Earth.

You can check the size of that drop with a simple rule I am adding myself, not quoting from NASA: light spreads out as it travels, so when you are twice as far away, you get one quarter as much on the same surface. Jupiter is about 5.3 times farther from the Sun than Earth (800 million divided by 150 million kilometers). Dividing 1 by 5.3 twice gives about 3.6 percent, which sits inside NASA's range.

Making the panels bigger helps only so much, because bigger panels are heavier and harder to launch. That is where the reactor comes in.

A Small Nuclear Power Plant Makes Electricity Without Sunlight

A fission reactor splits atoms of uranium to make heat, and a machine turns that heat into electricity. It works the same way far from the Sun or in the dark. Using a reactor to power electric engines is called nuclear electric propulsion. The engine itself is still an electric one. Only the source of the electricity changes.

NASA's plan for Space Reactor-1 Freedom, which it calls the first fission-powered interplanetary spacecraft, has these main parts [3]:

• A reactor using a type of uranium fuel called HALEU, producing 20 kilowatts of electricity through a closed-loop heat engine called a Brayton cycle.
• A 12-kilowatt Hall thruster, a kind of electric engine that also pushes out charged gas.
• A launch weight of about 12,000 kilograms (about 26,500 pounds).
• Aim: launch in late 2028 and head for Mars, where it would release SkyFall, a payload of three small Mars helicopters built on the design of Ingenuity.

The mission is not mainly about Mars. NASA lists "operate a nuclear-electric propulsion system in space" as the first of its goals, and calls the flight a pathfinder for later uses such as a power plant on the Moon [3]. A news report on NASA's announcement says the spacecraft is expected to take about a year to reach Mars [5].

NASA also says nuclear electric propulsion moves heavy loads efficiently through deep space and makes high-power missions beyond Jupiter possible, where solar arrays are not effective [5]. In plain words: this flight is a test of the method, and the far planets are the long-term goal.

Is It Safe to Launch a Reactor? The Plan Is to Keep It Switched Off Until Space

It is natural to worry about launching something nuclear. According to a news report that quotes the NASA official who will oversee the mission, the reactor is off on the ground, no radiation comes from it then, and it does not turn on until the spacecraft is up in space [5]. That describes the plan from the mission's own manager. It is not a test result, because the spacecraft has not yet flown. Engineers, regulators and the public will keep examining it, and NASA itself says the mission is meant to help set the rules for future nuclear flights [3].

What Nobody Knows Yet About Flying a Reactor to Mars

Nothing here has been proven in space. The ion engine part is well tested, and the reactor part has not flown on a mission like this. That is exactly why NASA calls it a first. Dates are goals: the mission page says "targeting launch in late 2028," and plans like this can slip [3]. Whether the reactor and the engine keep working together for a full year in space is the question the flight is built to answer.

See How Light Fades With Distance Using a Flashlight

In a dim room, point a small flashlight at a wall from about 1 meter (3 feet) away and look at the bright spot. Then back up to 2 meters (6 feet) and look again. The spot gets dimmer, and it also gets bigger. Do not shine the light at anyone's face or eyes.

This is only a rough picture. A flashlight beam is not the same as sunlight, so you will not measure exactly one quarter. But it shows the idea: the same light, spread over more space, means less for each square inch of a solar panel. To follow the real mission, visit NASA's Space Reactor-1 Freedom page and check whether the launch target has changed since this note was written.

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