Ben -   Earth’s orbit is becoming very crowded, but it’s a great question.  I asked Chris Castelli, who said that CubeSats will only be in low earth orbits, only 90 to 100 kilometres above the ground.  And as there’s still an atmosphere up there, that will exert a drag on the CubeSats, and they will quite naturally drop out of orbit, and harmlessly burn up in the atmosphere.  So, existing satellites should be safe.

We put this question to Dr Andrew Pontzen:

Andrew -   Well firstly, normal matter in the universe is really clumped together.  It’s not evenly spread out through the universe, it’s clumped, for instance into galaxies.  Galaxies are collections of around 100 billion stars, and they're relatively compact, and there's a lot of relatively empty space between each different galaxy.  Now, dark matter certainly does clump together in that sense.  We know that for sure, from observations of galaxies.  

This was in fact the original evidence for dark matter, looking at the way that material like stars and gases moves around in galaxies, and inferring from that strength of the gravitational field in galaxies, and from that inferring how much stuff was there, and that’s how we knew that there had to be dark matter.

In fact, because there’s so much more dark matter than normal matter in the universe - there’s around five times more dark matter than normal directly visible matter - its clumping is incredibly important in terms of determining the kind of structures that form in the visible universe, and the existence of galaxies effectively owes itself to dark matter.  So in that sense, there are structures in the dark matter that are similar to the ones you see directly in the normal matter.

On another level though, we don't really know what dark matter is, and so, when we’re talking about dark matter, we tend to be modelling it subject to some simple assumptions about what it’s doing, and you get the best results for the evolution of the universe, matching what we see in the real universe, when you model the dark matter as completely non-interacting, except through gravity.  So, other than the gravitational force which it exerts, and which it’s also subject to, it’s not subject to any other forces, for instance, the electromagnetic force which normal matter is subject to.

In fact, the really interesting structures in normal matter arise through things like the electromagnetic force in all of chemistry for instance, and therefore, life really arises through forces like the electromagnetic force.  And for that reason, the evidence at the moment would suggest that you can't have really complicated structures that will be required to create what you might describe as dark life forms.  So, most likely, there’s nothing quite that interesting going on in the dark sector, but until we really know what it is, we can't say for absolute definite.

We put this question to Dr Carolin Crawford:

Carolin -   Well this is about, not so much from the effects of dark matter on our solar system, but looking at more the effects of dark energy – so staying on the dark side.  Space is expanding and it’s carrying the galaxies along with it for the ride.  They're all receding from us, and we think they're being pushed apart by a force that we call dark energy, and this is currently accelerating the expansion of the universe.

But the curious thing is, that this dark energy, whatever it is, is a property of space.  So the larger the distance between bodies, the stronger they push to drive them apart.  Conversely, gravity - which we’re a bit more used to - is a property of matter, and it’s a pulling force, so that opposes the expansion, and the gravitational pull is stronger the more mass that’s there, and depends on how close you are to it.

So, whether the pull of gravity, or the push of dark energy dominates over a given region of the universe, depends on how much mass is there, and how widely separated it is.  If they're far apart, the push of the dark energy wins, but if they're close together, gravity is going to dominate. 

You have to remember, in astronomical terms, our solar system is absolutely tiny.  The planets and the sun, and all the constituents of our solar system, are very close together, and there’s no question that gravity wins in that circumstance. 

Even on the scales of the galaxy, gravity is the dominating force.  Even between groups or clusters of galaxies, gravity is gluing them together. You're only going to get this expansion of space on the very largest scales, where you have sufficient space that the dark energy can dominate.

We put this question to Dr Dominic Ford:

Dominic -   Well yes, in theory, you could.  The way that any rocket works is that it ejects material backwards and there’s a principle of physics - the conservation of momentum - which says that if the rocket exerts a backward force on that material to accelerate it backwards, there must be an equal and opposite push, pushing your rocket forwards, accelerating it to move faster.  And what matters is how much momentum the ejected material has, and that affects how great the push is forwards on the rocket.

So, could you substitute the exhaust gases of a conventional rocket with a laser or light beam?

Well, yes you could, because light is made up of photons which – although they have no mass, do carry a very small amount of momentum.  So for example, if you put your hand underneath a light, then the light is actually exerting a very tiny downward force on your hand.  You don't notice it because the force is so small, but it is there.

So likewise, a light on the back of a rocket would push it forwards ever so slightly, but the problem is, this force is so incredibly small.  I did a quick calculation this morning of what power of light source you would need to replace the thrusters on the Cassini Spacecraft in orbit around Saturn, which can produce a force of 440 Newtons, and the answer is you would need 130-Gigawatt light source.  So that’s equivalent to the output of several hundred power stations, all going into one light source.  So, this isn’t a terribly practical way of propelling the rocket.

Andrew Pontzen -   But of course, there are actually ways you can use the pressure of light.  For instance, in solar sail technology, and I think there’s just recently been an announcement that the solar sail of the Icarus spacecraft is being unfurled.

Dominic -   So this is a serious question because it’s very inefficient to carry large volumes of rocket fuel around the solar system.  So people will continually looking for new ways of controlling spacecraft and solar sails are one promising idea.  How they work is you have a reflector which the sun exerts an outward force on, due to radiation pressure.  Also, the solar wind exerts an outward force.  What the Japanese Space Agency are doing at the moment, is trialing this experimental solar sail Icarus, which is 20 metres across.  And they're going to try and to glide it down in the solar system towards Venus, using the solar radiation pressure to control their direction as they glide through the solar system.  It will be fascinating to see how it goes.