Planets have moons because early in their formation they were introduced to other space- faring rocks that either crashed into the planet and threw off debris, or were trapped in the gravitational pull of the planet. Over billions of years the orbits of these rocks (or debris), now under the influence of the planet’s gravity, were squashed into a spherical shape that kept them encircling their host planet.

To visualise how a moon becomes ensnared, imagine the earth is a ball placed on a floating sheet of frictionless paper (to represent gravity). The ball depresses the paper and if you roll a coin (to represent a moon) around this depressed area at a fast enough speed, it will circle the ball indefinitely.

So, why don’t moons have their own moons? The answer is that the planet they are orbiting has a much stronger gravitational pull, so over time it would selfishly take other objects in as another moon for itself.

Travelling at 38,000 mph, the Voyager 1 probe was launched on 5 September 1977 with the intention of studying Jupiter, Saturn, Uranus and Neptune. Now, 33 years later and almost 11 billion miles from home, it is going where no Earth object has gone before.

Both Voyager probes are approaching the interstellar medium, with Voyager 1 further ahead than its sister ship Voyager 2 (which is travelling at 35,000 mph). It is here that the nature of the environment is expected to dramatically alter.

Dispatched towards deep space, the Voyager probes have functioned far beyond expectations, with their radioactive power packs continuing to work and send data back to Earth, although at such a distance away any radio message takes 16 hours to reach us.

Speaking to the BBC, Edward Stone, the Voyager project scientist, said: “When Voyager was launched, the space age itself was only 20 years old, so there was no basis to know that spacecraft could last so long. We had no idea how far we would have to travel to get outside the Solar System. We now know that in roughly five years, we should be outside for the first time.”

Our solar system is contained inside an area of space known as the heliosphere. Up to the heliosheath our Sun exerts a magnetic and energetic influence, with its solar wind extending to all corners of the system. Upon reaching the heliosheath, the solar wind slows considerably and begins to heat up as it reaches a shockwave known as the termination shock.

Voyager 1 crossed the termination shock in December 2004. Using data from its Low-Energy Charged Particle Instrument, the velocity of the surrounding solar wind has been constantly measured.

At the edge of the heliosphere is the heliopause, where the Sun’s influence is less apparent. It is here that Voyager 1 – and later Voyager 2 – will enter the interstellar medium, the matter between stars in our galaxy.

It is theorised that at this distance from the Sun, over 11 billion miles, the velocity of particles emitted by the Sun will slow to zero. Although it is not quite there yet, once Voyager 1 reaches interstellar space there should be a sudden drop in the density of hot particles and an increase in that of cold particles. The success of the Voyager probes so far has markedly increased our understanding inside our solar system, and its continuing journey will give us unprecedented information on the outside as well.

1. A titan – Mimas is named after one of the titans, a race of gods and goddesses in Greek Mythology. Saturn (or Kronos in Greek mythology) was the leader of the Titans.

2. Arthurian craters – With the exception of the Herschel Crater, which was named after Mimas’s discoverer William Herschel, all the craters are named after Arthurian characters.

3. Death Star moon – Although Mimas looks a lot like the Death Star of the Star Wars films, the first film was released before images taken by the Cassini spacecraft ever revealed the resemblance.

4. Pac-Man – A map of daytime temperatures on Mimas shows warm areas that resemble the Eighties arcade game character Pac-Man, with the crater as the dot.

5. Perturbation – Mimas’s gravitational pull perturbs, or throws off, the orbits of some smaller moons. It causes the orbit of Methone, just four km in diameter, to vary by up to 20km.

We thought we’d treat the regular How It Works Daily users to an exclusive sneaky peek at one of the questions from next issue’s exciting interview with Professor Brian Cox. We asked him to tell us about his latest series for BBC 2, Wonders Of The Solar System…

Professor Brian Cox: Wonders is a documentary about astronomy and physics. Since the BBC last made a documentary series about the solar system a decade ago, we have lived through what I believe is something of a golden age of exploration. We’ve confirmed that there was once water on Mars, and have seen evidence that there may still be liquid water beneath the Martian surface today. We are now virtually certain that Jupiter’s moon Europa has a liquid water ocean beneath its icy crust that is perhaps 100km deep, and therefore contains more water than all the oceans of Earth. We’ve seen fountains of ice erupting from Saturn’s tiny moon Enceladus, and parachuted to the surface of the giant moon Titan, on which we have discovered lakes of liquid methane, methane snow and methane rain.

To find out what else everyone’s favourite scientist had to say, including the rest of this answer, remember to get your copy of How It Works issue 7, which hits the shops on 22 April – earlier if you subscribe.

Images courtesy of the BBC

They may not be as interesting as the moons around Uranus but Saturn’s rings do bear further investigation…

Saturn – The Roche lobe causes gravitational forces around Saturn to hold rocky particles

Inner rings – Inner rings are made up of rock particles that never formed into a moon

Outer rings – Outer rings are caused by geysers in the south pole of Saturn

Why does Saturn have rings but other planets do not? The answer has to do with something called the Roche lobe, named after a French astronomer. It seems when a planet orbits around a star (eg our Sun) and that planet has its own orbiting objects (eg a moon), a gravitational pull occurs between the objects. Around Earth, orbiting rocks formed into the moon. On Saturn, the rocks never coalesced and are still orbiting.

Interestingly, the rings are only a few miles in thickness because of the highly localised effects from the Roche lobe. Dr Steve Maran, a noted astronomer, says Galileo was the first to discover the rings, but could not explain them. Today, viewing angles from the Hubble Space Telescope reveal an enormous region extending widely around the planet. There’s also one distinct outer ring, which Maran attributes to geysers emitting from the icy southern polar region on Saturn, leaving a more distinct trail.