On this day in 1959, the Soviet Luna 3 spacecraft transmitted the first ever pictures of the far side of the Moon. I thought we might take this opportunity to discuss why it is that the same side of the Moon is always facing us. Although I should tell you that this is not exactly true. I think we are able to see about 55% of the Moon’s surface, because it jiggles. But for all intents and purposes, we see the same moon each night. This is because it is tidally locked.
The Moon once rotated rapidly. But over time, the Earth’s gravitational field slowed it. The force from the Earth produces a bulge in the part of the moon that is directly facing the Earth — and also directly opposite (just like the Moon created tides on Earth). This has the effect of squishing down the sides, so that the moon looks like a football with the pointy end facing Earth. Of course, the deformation isn’t anywhere near that great. But that’s the basic idea.
While the Moon was spinning fast, the bulge was always slightly after the direct line. As a result, the gravitational field had a net torque on the Moon, slowing its rotation. The effect was very small. But it’s amazing what you can accomplish in a billion years. I used to tell my students to image the Moon (or any other tidally locked object like Mercury) as if it were a frying plan. The handle would always be facing just a little off center from the Earth, and would thus be constantly pulled slightly in the opposite direction of the Moon’s rotation.
Luna 3 was the first mission specifically meant to photograph the other side of the Moon. Luna 1, sent in January of that year, was meant to crash on the Moon. It missed. (Don’t laugh: we missed the Moon the first time we tried.) And it became the first human object to go into orbit around the Sun. Luna 2, sent in September, actually hit the Moon. Later, in February 1966, Luna 9 would be the first spacecraft to make a soft landing on the Moon (or any other place).
The radio signal on Luna 3 was so weak, that the spacecraft had to get almost all the way back to Earth in order to transmit its 18 images. The one above is the first transmitted back. I think we humans have become far too cavalier about this kind of stuff. What we now do in space is mind boggling. It’s always nice to go back five or six decades and see what we were doing and just how hard it was. Oh, and no one knows for sure what happened to Luna 3. But it probably burned up in the Earth’s atmosphere.