When the moon falls into Earth's shadow Thursday night into Friday (March 13-14), observers will be treated to views of a "blood moon" total lunar eclipse. But given that lunar spacecraft are solar-powered, how do they survive when they're cut off from the sun?
For NASA's Lunar Reconnaissance Orbiter (LRO), which has been orbiting the moon since 2009, it will be time to batten down the hatches.
"LRO's science instruments and some components not needed by the spacecraft will be powered off during the total lunar eclipse," Noah Petro, project scientist for the LRO mission at NASA's Goddard Space Flight Center, said in an interview with Space.com.
The danger is to LRO's battery. The spacecraft is solar-powered and spends the daylight portion of its roughly two-hour orbit around the moon using its solar arrays to charge its onboard battery, which it then relies on while on the nightside of the moon. However, NASA revealed that by 2018, the efficiency of LRO's battery had already degraded to 70% as a result of continual charging.
During the March 13-14 total lunar eclipse, LRO will be in darkness — either on the moon's nightside or in Earth's umbral shadow — for 5 hours, 48 minutes. That's a relatively long time, but Petro is confident that LRO's battery can keep the spacecraft alive.
"Before entering the eclipse, we will have charged the battery to its maximum capacity," he said. To prevent too great a drain on its power, every scientific instrument on the spacecraft is being shut down. During an eclipse, the temperature inside the spacecraft can drop to 23 degrees Fahrenheit (minus 5 degrees Celsius), but for the spacecraft's wide-angle camera, which is less protected, it can feel as cold as minus 22 F (minus 30 C). So steps will be taken beforehand to make sure everything is as rosy as possible.
"Because being in darkness also cools the satellite, we will heat the instruments and spacecraft prior to the eclipse," Petro said. "This ensures minimal added power draw from heaters during the eclipse while maintaining spacecraft components within normal operating temperatures and the instruments well above survival limits."
The precautions taken for a lunar eclipse haven't always been quite this drastic. During previous eclipses, LRO was able to keep one instrument switched on. Called Diviner, it is a radiometer that detects thermal emissions from the lunar surface to study how the surface warms and cools over the course of a lunar day.
The idea behind keeping it switched on during a lunar eclipse was to study how the lunar surface cools while in Earth's shadow, which provides information about the composition and size distribution of material in the lunar regolith, which is made up of rocks, small particles and dust. The size and composition of the rocks affect how quickly they cool.
"Diviner's measurements during prior eclipses, particularly of targeted regions, allows for insight into the uppermost surface of the moon," Petro said.
During the total lunar eclipse on Oct. 8, 2014, Diviner specifically observed a crater called Kepler and a lunar nighttime "cold spot." In particular, cold spots are areas around some craters where Diviner had found unusually cool nighttime temperatures.
In addition to determining the size distribution of the material ejected from the crater during the impact, Diviner found that the "cold spot" was 18 F (10 C) warmer than the surrounding material. This suggests that the vertical layering of material within the lunar regolith may be more complex than previously thought, to allow it to radiate heat away at a different rate from its surroundings.
What will happen to Blue Ghost?
LRO isn't the only active lunar mission. Firefly Aerospace's Blue Ghost, the second private mission to soft-land on the moon, touched down in Mare Crisium on March 2 as part of NASA's Commercial Lunar Payload Services program.
Related: Private Blue Ghost lander spotted on the moon by NASA lunar orbiter (photo)
Blue Ghost is carrying 10 NASA experiments designed to investigate the surface of the moon. Like LRO, it is solar powered. Blue Ghost is designed to last only during the daytime, which is about two weeks on the slowly rotating moon. Indeed, Blue Ghost landed in time to see the sunrise. Once the sun has set, the lander's batteries will power down, its heaters will switch off, and the cold of lunar night will probably kill it by mid-March.
But will Blue Ghost survive that long? Sunset from Blue Ghost's landing site comes on March 16, but the total lunar eclipse is two days before that and will plunge the lander into shadow.
The mission can survive several hours into lunar night on battery power alone. The lunar eclipse will last 6 hours, 3 minutes overall, but only 65 minutes of that will be in totality, under what's called the dark umbral shadow. The rest of the eclipse will take place in the penumbral shadow, where some sunlight does reach the surface.
Indeed, the Firefly Aerospace team is so confident that Blue Ghost will survive the eclipse that they intend to take high-resolution images of the event from the lunar surface while the lander is in Earth's shadow. In other words, on the moon, Blue Ghost will see a total solar eclipse!
So, as you look up at the moon while Earth's shadow moves over it on the night of March 13-14, spare a thought for those lonely, vulnerable space probes that must battle through the darkness to survive.
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