Posted by: Kash Farooq | September 1, 2011

How to send a spacecraft across the Solar System – Cassini

Yes, this blog post has a very optimistic title considering I’ve never really looked into this subject before.

I was going to cover the recently launched Juno spacecraft – a 5 year mission to Jupiter. But the NASA website is lacking in technical details about this at the moment – I guess they were a bit busy preparing to send a spaceship across the Solar System…

Instead, I thought I’d look at Cassini.

Cassini is a spacecraft that is currently orbiting Saturn and sending back stunning, beautiful images of the planet, the rings and the moons.

Saturn imaged from Cassini

Saturn imaged from Cassini

Epimetheus, Rings and Titan

In this image you can see the heavily cratered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene.

Surface of Titan, one of Saturn's moons

That’s the surface of Titan; the site of the most distance spacecraft landing.

Cassini is named after the Italian astronomer Giovanni Domenico Cassini. He was the first person to observe 4 of Saturn’s moons and also spotted a gap in the rings – the gap is now called the Cassini Division. The data provided by the spacecraft has led to some amazing discoveries.

Cassini also dropped the European Huygens probe onto the surface of the moon Titan – this is the most distant landing of any craft launched from Earth.

But how did Cassini get there? How did it traverse the billions of kilometres of space to get to Saturn and go into orbit around the giant planet?

Well, to start with, it didn’t just go straight there. The Cassini was heavy – over 6 tonnes. Even the most powerful launch rockets today would not have been able to provide the spacecraft with enough speed to escape the pull of the Sun’s gravity to reach Saturn.

Instead, of going straight there, Cassini was sent on a tour of the Solar System to take advantage of a technique first proposed in 1961 called gravity assist.

Cassini passed by Venus twice, and the Earth and Jupiter at such trajectories so that it could use the gravity of each planet to gain some speed. The spacecraft (sort of) bounces off the gravitational field of each planet.

Cassini's Interplanetary trajectory

Cassini's Interplanetary trajectory

During a gravity assist, a tiny amount of the momentum that the planet had in their orbit around the Sun is transferred to the spacecraft, increasing the spacecraft’s speed.  So, the planet being used in the gravity assist manoeuvre slows down a little.

And no, NASA isn’t going to make the Earth fall into the Sun and kills us all. The loss of momentum experienced by the planet is vanishingly small.

You can view animated simulations of a gravity assist at the Messenger Education website.

In all Cassini travelled 3.5 billion kilometres to get to Saturn. Next question – how do NASA work out where exactly the spacecraft is?

To work out if the spacecraft is moving towards or away from Earth, Doppler Shift is used. Radio transmissions sent by Cassini are captured by NASA’s Deep Space Network. Depending on whether the radio waves are bunched up or stretched out tells us whether Cassini is moving towards us or away from us.

And to work out how far away the spacecraft is, specially coded signals are sent to the spacecraft which are immediately returned. As the signals travel at the speed of light, the round trip distance can be calculated. The calculations are incredibly precise – they even take account of how long Cassini’s electronics take to turn around the signal and how far Earth has moved around its solar orbit since the signal was sent. The current time taken for the round trip of the signal is about 3 hours.

Around Saturn, optical navigation images are taken of Saturn’s moons against a field of background stars. The positions of the stars are well known and allow NASA to know exactly where Cassini is in the Saturn system and in which direction it is pointing. This information allows NASA to reposition Cassini or re-point the instruments.

The Cassini mission is due to continue until 2017. Eventually the fuel for the thrusters will run out so NASA will no longer be able to change its position. Before that happens they have to decide what they are going to do with it.

Because Saturn is too far away to use solar power, the onboard electronics of Cassini are powered by electricity generated from the heat of nuclear decay; specifically by three Radioisotope Thermoelectric Generators (RTGs).


Jet Propulsion Laboratory workers inspect one of the radioisotope thermoelectric generators on the Cassini spacecraft. RTGs use heat from the natural decay of plutonium to generate electric power. The three RTGs on Cassini will enable the spacecraft to operate far from the Sun where solar power systems are not feasible.

So, what do you do with a spacecraft that has 30-odd kilograms of plutonium and associated decay products on board?

NASA have got about 5 years to work that one out.

I recorded a version of this blog post for episode 99 of The Pod Delusion – a podcast about interesting things.



  1. really usefull the way its broken down makes it a lot more simple than all the other websites which just use sentences which arent easy to follow


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