Friday, August 31, 2012

Voyager - The Grand Tour

Voyager spaceships deserve their names. They travel in space farther away than any other object and will continue to do so for many more years. Celebrating 35 years for the Voyager launch, we will dedicate several articles which will present various aspects of this great mission, the longest ongoing active mission that NASA holds.
Note: Although there are two Voyager spacecrafts we will mostly use the singular form, unless necessary to differentiate between the two spaceships.

When the Voyager mission was first planned there were so many question marks about the solar system. Our knowledge was just a fraction of the knowledge we have today. Many details about the gas giants, especially Uranus and Neptune were totally unknown and spaceships only traveled to our near neighbors: the moon, Venus and Mars.

The first problem for space traveling is the great distances between the destinations, and the fact that there is a need to get there in a reasonable time, otherwise the spaceship itself might not work properly. Sending something directly to the edge of the solar system was not possible and required extremely large rockets and quantities of fuel. That was until someone (and there is some disagreement about who exactly is that someone, so I will not write any names) thought of a marvelous idea. There was no need to go directly to Neptune. It was enough to reach Jupiter at the correct time and angle. The massive gravity of Jupiter would accelerate the spaceship even more, and throw it outward toward Saturn. It was possible to do the same at Saturn, throwing the spaceship to Uranus and again to reach Neptune, in a reasonable time of about 10 years only (and not 25) which was less than the spaceship's expected life time. The term for such a maneuver is Gravity Assistance or the Slingshot Effect. This conclusion was reached in 1965.
The calculations of the such trajectories are extremely complicated, as the spaceship must be at the right distance from the planet, at the right angle and at  the right speed, otherwise it might crash on it or go into orbit around it. The Voyager itself had an engine and some fuel but this engine was used for minor trajectory corrections (or to give just a bit more of acceleration). The calculations required many computer hours (the computers at that time were much less powerful than today), but eventually such calculations were finished (by hand or with computers) and they showed that all planets would be aligned for such a tour in the late seventies!
However there were many political issues as well. In the early 70', the Apollo program was coming to an end, NASA started to work on the Space Shuttle programs and the planetary science was neglected a little. However, JPL started to present the idea of the "Grand Tour" to the end of the solar system, not just to near planets, with much better equipment than the Pioneer spaceships. Finally the green light was given.
Since so many details about Jupiter and Saturn were unknown it was impossible to set exact trajectories until more data was gathered, however it was impossible to wait for a spaceship to gather that data as the correct alignment of planets would pass. Therefore, two almost identical Voyager were sent. The first one, Voyager I's goal aim was to go to Jupiter and from there to Saturn only, providing many details which were required for a finer calculation which were used by Voyager II some months later to manage to travel the exact route to Uranus and Neptune.
Due to some technical reasons the first launch was that of Voyager II in August 20th 1977, and Voyager I just two weeks later on September 5th. Voyager I's speed was slightly higher and it quickly passed Voyager II, thus starting the longest journey ever.