NASA is covering a huge distance as Cassini already covered 5 billon miles on less than 4 tons of fuel

The report says that the Cassini spacecraft had drained more than 96 % of fuel by the end of 2010 after 13 years of its launch. 

According to lead propulsion engineer Todd Barber the idea of extending it seven more years was not at all a feasible one, but the navigators in the project chalked out a fuel budget that made the sample Enceladus’s geysers and the Saturn ring. 

To continue this long term mission NASA had to moderate an engineered propulsion scheme for planetary motion, and exploring new horizons.

In the space, speed matters the most.  The spacecraft spend their complete tenure in free fall and around the massive planets in the solar system.  The navigators of every spacecraft measure the fuel budget based on its speed change. In the beginning, Cassini was launched with proper fuel supply to go around Mars, but there was no plan to send it across Venus and Jupiter.

At the time of first Venus Rendezvous the spacecraft increases its speed more than two times contained in three tons of fuel. 

In the later stage it was observed that Cassini use to get connected repeatedly to Titan the largest moon of Saturn.   

In order to produce planetary speed in the space, a spacecraft needs a proper propulsion system. Cassini uses to execute the process in two major ways. At first, it may change the direction of eight heavy thrusters and then take help from the two main engines to move ahead.

The thrusters in the spacecraft are fired when it is aligning the craft for a new orbit or may be taking a snap of the Saturn ring.  Apart from these two activities, there is no cost involved in simple drifting of the spacecraft.

Mileage is not calculated in this sphere, but the propulsion engineers conduct the measurement with a thing known as ‘specific impulse’. It can be explained as a force for the fuel.

Space never pushes off anything, and therefore the vehicles move by shooting all the junk at the back. The calculation of the force of the craft completely depends on how fast it can expel all the materials.

The different chemical explosions that help in major NASA projects can push the molecules out at a faster speed, and its efficiency is still the same. NASA is now turning to design features which are smaller and faster exhaust.