With the lunar horizon in the foreground, the earth will pass in front of the sun on Wednesday, May … [+]
NASA scientific visualization studio
During the total lunar eclipse on Wednesday – the so-called “blood moon” – the lunar surface will take on a reddish-copper color for 14 minutes and 30 seconds.
Because the earth is aligned with the sun and moon, our atmosphere filters sunlight on the moon as it enters the earth's shadow and essentially projects thousands of sunrises and sunsets onto the lunar surface.
This is the bright red full moon that those west of the Mississippi will see in the early hours of Wednesday, May 26, 2021. But what would the event look like from the surface of the moon?
It would be nothing less than a total solar eclipse through the earth!
When the earth hides the sun, astronauts on the surface see a red ring – the sum of all sunrises and sunsets on earth – around the earth's limbs.
“You would look up and see the earth, possibly the lights of the city on earth, and you would notice the ground turning red around you,” said Dr. Noah Petro, project scientist for NASA's Lunar Reconnaissance Orbiter mission, which is currently orbiting the moon.
It would be very cold too.
“The lunar surface then cools significantly during a solar eclipse when the moon changes from the intense midday sun into darkness,” said Petro. “Hopefully you don't feel the cooling in your spacesuit.” However, the floor would cool significantly.
“It would be a pretty haunting effect, just like on Earth during a total solar eclipse, when the shadows look different and the atmosphere around you cools,” said Petro.
A lunar eclipse can be a risky time for a space mission on the moon.
NASA plans to send astronauts to the moon in late 2024, though that could and likely will slide – and there are two total lunar eclipses in 2025 and one in 2026.
“It depends on what you want to survive this mission and its experiments, but it puts your equipment in a completely different thermal regime,” said Petro. “During the Apollo missions, the astronauts weren't on the moon during a solar eclipse, but the experiments they left behind were.”
Day-night cycles are of course normal on the moon. “The only big difference, of course, is that it gets very hot and then cold very quickly, so the thermal change is pretty dramatic – and dramatic temperature shifts can do weird things,” Petro said, mentioning windshields that crack in extreme temperatures.
A lot of science can be done during a total lunar eclipse, and NASA's Lunar Reconnaissance Orbiter (LRO) will measure the temperature change on the lunar surface on Wednesday.
Will it survive the sudden onset of the cold?
Without sunlight on the solar panels that power the heaters, a spaceship in orbit can freeze up during a total lunar eclipse unless it is specifically designed for the challenge.
The LRO is designed to withstand a solar eclipse. “We've been on the moon for 12 years now, so we've gone through a series of solar eclipses and know how the spaceship will react,” said Petro. “But at the beginning of the mission we didn't, so we were very careful.”
NASA's LADEE (Lunar Atmosphere and Dust Environment Explorer) – a robotic probe that orbited the moon in 2013/2014 – was purposely designed not to withstand a total lunar eclipse, but was able to survive anyway.
“Since this is a very brief solar eclipse, we know that the battery will not be fully discharged during the solar eclipse, so we can leave the one experiment measuring temperature change,” Petro told LRO this week. “If it was a prolonged eclipse we couldn't do it”
For missions to the moon, lunar eclipses have become technical milestones that must be planned in advance.
A total lunar eclipse occurs when a full moon passes through Earth's 1.4 million km long shadow in space. This only happens occasionally and can take anywhere from 105 minutes (like 2018) to just five minutes (like 2015).
On May 26, 2021, it will take such a short time because it is not migrating through the center of the Earth's shadow, but through its northern part, just 34 km from its outer edge. So it is predicted that the northern limb of the moon will be quite bright all the time.
Either way, it is an event not to be missed if you are on the night side of the earth at the right time.
I wish you clear skies and big eyes.