How does gravity help with interplanetary satellites

Gravity Gravity instead of fuel: space travel to other planets

Completely different physics: traveling through space

Prüfer is a mission planner at the German Space Operation Center, part of the German Aerospace Center. His area of ​​expertise: so-called orbital mechanics. What maneuvers does a spaceship have to perform in order to get from the orbit of the earth to that of another planet in the solar system?

On earth, travelers, whether by car, airplane or on foot, can simply repel themselves against the surrounding masses in order to move forward. An airplane paddles through the air, a car rolls over the ground over which pedestrians are walking. In the universe, on the other hand, there is no surrounding medium, only infinite emptiness.

How gravity accelerates bodies

Spaceships themselves have to repel mass in order to accelerate in the opposite direction to the ejection. Another factor is the attraction of all celestial bodies, such as the earth, the moon, the planets or the sun. Because all these masses attract each other through gravity and the closer they are, the stronger.

If researchers consider two celestial bodies and their gravitational force to one another in their calculations, they speak of the two-body problem. There are simple everyday examples of this: If we drop an apple on the ground, it accelerates. Without the resistance of the air or the ground in the way, it would steadily increase in speed up to the earth's core. However: Although this is hardly noticeable due to the extremely different sizes of earth and apple - the apple also pulls the earth a tiny bit towards itself.

How space travel benefits from gravity

These effects of gravity are extremely useful in space travel. Because fuel is extremely scarce on all missions. It has to be brought from earth. However, the weight that a rocket can carry into space is strictly limited.

That is why most research flights, whether to Mars, Jupiter, Saturn or beyond, use the planets' gravity field. If the probe takes the right trajectory, it can, for example, take a flyby, like New Horizons does with Pluto. However, if a probe wants to examine an object for a longer period, it has to pivot into an elliptical orbit. To do this, it would simply have to brake when it reaches the planet.

Space flight in practice: the Hohmann transfer

If you are now planning a practical space mission, you can ensure that you only start when the so-called transfer window is cheap. Mars is in a favorable position to earth every 26 months. The red planet can then be reached relatively easily via a so-called Hohmann transfer. The NASA InSight mission, for example, made use of this.

But unfortunately, physics in space is even more complicated. Indeed, during a flight to Mars, not only Earth and Mars exert a gravitational influence on the probe. During the flight, the sun also pulls on the spaceship. This leads to the so-called three-body problem, which is considered to be one of the most difficult problems in mathematics.

How spaceships can accelerate or brake with gravity

If three bodies mutually attract each other, mathematicians can only approximately calculate the consequences. The orbits of these bodies around each other are then no longer regular ellipses, but a wild, chaotic tangle. Scientists then try to use a few tricks to simplify their calculations and make them manageable.

Different gravitational influences can be used to accelerate or brake a spaceship with so-called gravity assists. In this way, probes can pick up speed with almost no fuel or be captured by their target planet. An example of such a mission is that of ESA and JAXA's Bepicolombo probe, which is currently en route to Mercury and is expected to arrive in its final orbit in December 2025.