Getting spacecrafts from one planet to another is no easy task. Sure it may sound easy, just fire off a rocket aimed towards the planet of your choice and eventually you will get there. There are many problems with this "point and shoot" method. One is that you would need a lot of fuel to rocket yourself all the way from Earth to another planet, both to get you there and to keep the ship aimed properly. The more rocket fuel you have though, the harder it is to successfully launch yourself away from Earth, and the more money it costs to fly the space craft. The second major problem comes about once you've reached your destination. Assume you've aimed properly and had enough fuel to make it to the planet in question, now how do you slow down so that you can either safely land on the planet's surface, or orbit it in a stable orbit? More fuel, high tech gadgets, and lot of luck are needed to succeed then. So how do astronomers get spacecrafts from one planet to another? They use the sun! Since the sun is such a massive body, it can be used both to slow spacecrafts down, or speed them up. This is called a gravity assist or slingshot maneuver. When we want to get to Venus or mercury for example, we send spacecrafts around the sun multiple times. The gravitational attraction between the two objects slows the spacecraft down so that it can casually approach the planets. When we want to travel to Jupiter, or the outer edges of our solar system, we use the sun as a slingshot! By sending spacecrafts around the sun at the right distance, the pull of the sun can give the craft angular momentum and increases its speed. As the craft rounds the sun it shot out towards to outer solar system. The key here is to approach the sun with the correct speed, distance and angle, so you get the increase or decrease in speed you desire. Below is the path the Cassini-Huygens mission took around the sun before it was shot out towards Saturn. Carefully planned gravity assists via the sun and inner planets are what got Cassini to Saturn successfully.
Image Credit: NASA
