The July 20, 1969 The American crew of theApollo 11 He landed on the moon: A historical moment that, still today, is questioned by countless conspiracy theories. Fortunately, the nature of our satellite allows you to keep the traces of human landings and tools on its surface, starting from first human object To touch its soil, the Russian probe Moon-2.
One of the most important traces of these landings is mentioned in episode 23 of the third season of Big Bang Theory, The famous TV series that follows a group of nerd friends. Replicating an experiment capable of measuring the moon-termination distance (an average distance of 384,400 km), the protagonists point A laser towards particular mirrors positioned right on the occasion of the first wing. The Return of the signal it is further proof of the presence of human artifacts on the moon. Unfortunately, however, it is not possible to do it with “home” tools, but specific laser and instruments are needed that also reach 3.5 meters! Are there other ways to “see” the traces of the landings on the moon? Yes, for example, thanks to satellites in orbit we can see the signs of the rover on the lunar soil.
The landing of the Apollo 11 and retrioriflectors
The Apollo 11 mission brought two men to the lunar surface, Neil Armstrong And Buzz Aldrinin charge of collect samples of lunar soil to be brought back to Earth. Their mission also consisted of deposit some back-reflectorsspecial mirrors (arranged like the internal walls of the corner of a cube, then with 3 reflective surfaces at 90 °) capable of Send the accident light precisely in the departure direction.
Knowing the speed of light, through theUse of laser And of these mirrors you can actually Measure the moon-length distance and its variations in orbit: the first laser impulse to reach the mirrors was launched by Lick Observatory in California on August 1, 1969two weeks after the wing, e It was detected after about 2.5 secondsallowing to measure the distance traveled by the impulse: 384,400 km, With a precision of +/- 25 cm.
With the Modern Laser Ranging technologiesthe precision of the measure is currently about 0.1 mmand thanks to the experiments still underway, it was possible to calculate that the moon is slowly moving away from the earth.
Those left by the Apollo mission are not the only retro-reporters: The USSR also sent probes with mirrors similar, as in the case of Lunokhod 1and more recently one Indian mission (Chandrayaan-3) He landed on the moon with the same goal.
How difficult is it to identify back-reflectors?
But it is really possible to hit these mirrors with a “portable” laser from the roof of your own home, as shown in the episode of Big Bang Theory? The answer is, predictably, No.
Modern laser telescopes normally used for these experiments, such as the Apache Point Observatory in New Mexico (USA) have rays of 3.5 meters or morein order to capture the return signal. The interference present in the atmosphere, in fact, “widen” the laser bundle sent by telescopes up to about 1.8 km on the moon’s surface, therefore only one very small part of the light affects the mirrors To go back: to be precise, 1 photon (“package” of energy) out of 30 million (3 x 106).
In turn, this small part, on the return trip towards our planet, it comes dispersed until it covers A surface of 15 km On the earth’s surface: here too the report is only 1 photon revealed by the telescope on 30 million reflected by the mirrors.
The chances that a photon issued can be reflected by the mirrors and captured on the ground therefore become 1 out of 1024 (a quadrilion). Fortunately, modern laser can Send up to 300 quadrilion of pulse photonsobtaining the return of about 1200 photons per minute: an impossible result for simpler equipment, such as those used in the TV series.
Is it possible to see in the footsteps of the astronauts with a normal telescope?
What if we wanted to identify the footprints of astronauts? Also in this case, unfortunately, there is little that can be done from the earth. There resolution of observers and the interference atmospheric make Impossible to see details so minutesand also to be able to identify the largest structures such as the landing form it is necessary to have Tools in lunar orbitlike the Lunar Reconnaissance Orbital Camera.
This satellite, orbiting about 33 km from the surface lunar, is able to give us images with a resolution of about 50 cm for pixelsas in the case of this photo of the sea of tranquility, the Apollo 11 landing site. An adequate resolution to identify the traces of the lunar rover and the main objects, but not enough to see the individual footprintsUnfortunately.
