NASA lunar probe finds it’s not easy to be green

If you are a space fan, these are very exciting days. There is so much going on at height that it can sometimes be difficult to keep up to date with the latest news. Artemis, I just got back from the Moon, the International Space Station crew is dealing with a broken down Soyuz, SpaceX is making incredible progress with its Starship architecture, CubeSats are being launched all over the solar system, and it looks like every month a new company unveils its own commercially developed launcher.

Lunar flashlight

So with everything going on, we wouldn’t be surprised if you haven’t heard of NASA’s Lunar Flashlight Mission. The briefcase-sized spacecraft launched aboard a special “carpool” flight of SpaceX’s Falcon 9 rocket on December 11 – accompanying two other craft heading to our closest celestial neighbor, the Japanese Hakuto-R lander, and a small rover developed by the United Arab Emirates. There was a time when a launch like that would have been big news, but given that it was only the second of seven launches SpaceX conducted in December alone, it didn’t make a lot of headlines.

But recently that has started to change. There’s a growing buzz around Lunar Flashlight, but sadly, not for the reasons we’d usually hope for. It looks like the little explorer has run into trouble with its state-of-the-art “green” thruster system, and unless the problem is fixed quickly, the promising mission could end before it’s even had a chance to begin.

Shedding light on the lunar ice

After decades of false starts, it looks like NASA is finally seriously considering refreshing the Moon’s supply of American flags. But whereas the Apollo missions have only seen astronauts take short jaunts to the lunar surface, the Artemis program promises to establish a long-term human presence on and around the Moon. To achieve this, the space agency needs as much information as possible about the lunar environment, hence the notable increase in reconnaissance missions recently.

Lunar Flashlight is one such mission. Its objective is to research and quantify water ice deposits on the Moon, particularly in permanently shadowed craters located in the polar regions. An accurate map of these ice deposits, once confirmed by a subsequent robotic mission, will almost certainly influence the decision-making process when it comes time to establish the location of mankind’s first lunar outpost. A local supply of ice means the facility could produce its own oxygen and rocket propellants, not to mention provide vital drinking water for astronauts.

Originally, the lunar flashlight was to be equipped with a large solar sail, which would have served to reflect 50 kW of sunlight back to the Moon’s surface to aid in visual and spectral observations. At an altitude of 20 kilometers (12 miles), it was estimated that the spacecraft could have illuminated a spot 400 meters (1,300 ft) in diameter.

As expected, the final version of the material turned out to be much less fancy. It will still descend to an altitude of 20 km, but the solar sail has been replaced by four lasers operating at different infrared wavelengths: 1.064, 1.495, 1.85 and 1.99 µm.

The beams will illuminate a 35 m (115 ft) point on the lunar surface, with the reflected light being collected by a mirror and directed to an indium gallium arsenide (InGaAs) photodetector. The basic principle is that the ice should absorb some of the laser light, while the highly reflective regolith will bounce more of it back to the spacecraft. The hope is that these reflectivity data, when combined with existing observations, will finally confirm the location and density of surface water ice.

Of course, all of this is moot if the Lunar Flashlight spacecraft never makes it to the Moon.

In Push We Trust

Since last NASA update, ground controllers noted that three of the spacecraft’s four thrusters are underperforming. This would seem to hint at some sort of systemic issue rather than an issue with the thruster design or control hardware, which is where things start to get interesting.

Lunar Flashlight is the first spacecraft to use a new “green” thruster known as AF-M315E outside of low Earth orbit. Not only is this new propellant less toxic than the hydrazine traditionally used, it is more efficient: it offers 12% higher specific impulse (ISP) and is approximately 50% denser, allowing you to accumulate more pushed into a tank of the same size. For performance reasons alone, the new propellant was worth looking into – the lower toxicity, which will make handling ground gear easier and safer, is really just an added bonus.

Lunar Flashlight uses four tiny thrusters to maneuver.

Now there is no doubt that the AF-M315E works. The Green Propellant Infusion Mission (GPIM) spent more than a year in orbit testing the new thruster and quantifying its performance, and the formula had been studied for years by the Air Force Research Laboratory (AFRL) on the ground. While it’s true that it’s never been used this far from Earth before, there’s no obvious reason that should impact its behavior. Yet the fact remains that if three out of four thrusters experience the same problem, it indicates a problem with the thruster itself.

Or at least, the system that delivers the propellant. It should be noted that the Lunar Flashlight’s propulsion system, which was co-developed by NASA’s Marshall Spaceflight Center and the Georgia Institute of Technology’s Space Systems Design Lab, used 3D printing to produce a one-piece thruster manifold with integrated fluid channels. While this is undoubtedly a more efficient way to mass-produce the component, it seems likely that inspecting it for debris or foreign objects would be more difficult compared to traditional plumbing.

Although the reason remains to be determined, NASA says ground controllers are currently operating on the assumption that something is clogging the spacecraft’s thruster lines, and that running the thrusters longer may clear the blockage. As it happens, in early February, the spacecraft will need to start making larger course corrections as it approaches the Moon, so ground controllers will have a chance to open the throttle very soon. We cross fingers.

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