How do spaceships find their destination

How do you fly to the moon?

Fifty years ago, on July 20, 1969, Neil Armstrong and Buzz Aldrin landed on Earth's satellite. More than eight years passed between the announcement and implementation of a manned moon landing - so it doesn't seem that easy to get from the earth to the moon.

"I believe that the United States should set itself the goal of landing a person on the moon and bringing him safely back to earth before the end of this decade," proclaimed US President John F. Kennedy on May 25, 1961. It was more than eight years before NASA put this bold plan into action.

During this time, the space agency not only developed the necessary technology for a flight to the moon, it also had to find the best route to the earth's satellite. Because unlike on Earth, the start and end point in space change their position relative to one another during the course of the journey. Everything is in constant motion: the earth revolves around itself and circles the sun. And also the target - in this case the moon - orbits around the earth and, together with it, around the sun.

Couldn't a rocket just fly from Earth straight to the moon? There is nothing physically against it. But for such a flight you would need an extremely powerful engine and a lot of fuel. And so the space scientists try to optimally utilize the forces of attraction of the celestial bodies - and thus save energy. It starts at the start: rockets are preferably fired into space near the equator and in the direction of the earth's rotation. This alone allows a rocket to travel at a speed of 1,674 kilometers per hour. For a typical orbit around the earth at an altitude of 300 kilometers, however, a spacecraft must reach a speed of 28,000 kilometers per hour.

Hohmann Railway

In order to get from an earth orbit to a more distant place - such as the moon - with as little energy as possible, there are now different variants. One is the so-called Hohmann orbit: This is an ellipse with the earth at its focal point. The closest point of this ellipse touches the original orbit around the earth, the furthest point of the ellipse is in the desired orbit - in this case in the orbit of the moon. As early as 1925, the German space pioneer Walter Hohmann described this transition between two orbits in his book “The accessibility of the heavenly bodies“.

In order to get onto such an elliptical orbit leading to the moon, the spacecraft must be accelerated to a speed of around 40,000 kilometers per hour. The engines have to ignite at exactly the right moment so that the desired Hohmann orbit actually hits the moving moon at the point furthest from the earth. This flight maneuver was a technical and computational challenge in the 1950s and 1960s. Every smartphone today is a million times superior to the NASA computers used for the Apollo missions. And in order to get into orbit around the earth's satellite or land softly on the moon, an even more complicated trajectory is necessary.

In numerous experiments, the space authorities slowly approached the moon: after several false starts, the Soviet Lunik 1 probe raced past the earth's satellite on January 4, 1959 - at a distance of about 6000 kilometers. On September 12, 1959, Lunik 2 hit the moon for the first time. The first soft landing on the moon was made on February 3, 1966 by the also Soviet probe Luna 9. And on April 3 of the same year, Luna 10 swiveled into orbit around the satellite for the first time. On December 24, 1968, with Apollo 8, a manned spacecraft reached the moon for the first time and orbited it a total of ten times within twenty hours. And just seven months later, NASA managed the first manned moon landing with Apollo 11.

Course of the Apollo 11 mission

Apollo 11 launched on July 16, 1969 and reached Earth orbit twelve minutes later as scheduled. The rocket orbited our planet one and a half times before going on course for the moon. After 76 hours, Neil Armstrong, Edwin Aldrin and Michael Collins reached the satellite, some 380,000 kilometers away. On July 19, the astronauts entered a lunar orbit. A day later, the lander disconnected - with Armstrong and Aldrin on board - and touched down on the lunar surface a little later. On July 21, at 3:56 a.m. Central European Time, Armstrong was the first person to walk on the moon, followed twenty minutes later by Aldrin. The first excursion to the lunar surface took two and a half hours. After about 21 hours on the moon, we went back, first to the Apollo spacecraft and then to earth. On July 24, 1969, the capsule landed with the three astronauts in the Pacific.

The journey of the Apollo astronauts to the moon took three days and four hours. A short flight time is a decisive criterion for manned missions because it means less radiation exposure for space travelers. In the case of unmanned probes, on the other hand, the flight time plays a minor role. Flight routes that take months but cost little energy are also conceivable. Such orbits usually lead far out of the earth-moon system and make use of the gravitational pull of the sun to eventually return to the moon. Instead of the powerful chemical drives, electric drives can also be used for this. Another big advantage: The space probes approach the moon at a low relative speed, which means that only minor corrections are necessary in order to swivel into an orbit around the earth's satellite.