Assisted by gravity

The heavier probes, that must face long trips, heading towards planets that are beyond the Solar System, are launched with enormous slingshots. This technique, that was developed in the years 1961-63, by the mathematician Michael Minovitch,  is called Gravity Assist.
In order to better understand what it is, we must once again examine the history of the Cassini-Huygens probe that reached the giant Saturn in 2004, after a journey that lasted seven whole years.  To date in fact there are no launchers that can send a 6 tonne probe, equivalent to a bus that can carry 30 passengers, directly to Saturn. In order to reach it in fact, the probe must have an average speed of 50 km/sec. which is quite different from the 5 km/sec at which it is launched. For this mission, therefore, a particular technique was adopted, which in fact, has been named Gravity Assist, that exploits the mutual gravitational attraction of  the planet and the probe, in which the planet acts as a slingshot  that accelerates the probe.
The Cassini Huygens mission used this natural propulsion four times: on 26 April 1998, and 24 June 1999, Venus provided the first two gravity assists, then it was provided by the Earth on 18 August 1999, and finally by Jupiter on 30 December 2000.
Just to have an idea of the result of a gravity assist, in the case of Venus the couple of probes increased the speed of the module approximately 15 times, from 5 km/s to 75 km/s, while the speed of Venus  decreased by  2 x 10-20 km/s, i.e. it remained practically unvaried. The different collisions with the planets are therefore necessary to change the speed of the module   and to change the probe’s flight path, to guide it towards the desired route.

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