Put simply it is the vast distances involved, indeed the distances are so great, it is almost impossible to visualise them. The nearest star to us is Proxima Centauri (one of three stars in the Alpha Centauri system) and this is 4.2 light years away. This means it takes light 4.2 years to travel the distance, in everyday units it is approximately 25 trillion miles (40 trillion km).
The advanced rocket concepts that we have been looking at so far are really designed for interplanetary travel. Interstellar travel is a wholly different problem, the distance from the Earth to the nearest star is a few thousand times greater than the distance to Pluto. The only craft that we have looked at that can even be considered are possibly a laser sail (probably only if nantechnology materials are used), one of the forms of enhanced nuclear fusion or the best situation of an antimatter beam core spacecraft.
The reason we choose these craft is that they are the only ones capable of achieving a velocity that is an appreciable percentage of the speed of light, thereby reducing journey times to the order of a few decades.
If we take today's rocket speeds, such as the Apollo spacecraft that went to the moon, the journey to the nearest star would take over 900 thousand years.
Further to this they are the only craft that allow the journey to be made with a feasible amount of propellant. The farther or faster we wish to go the more propellant is required and such amounts can become prohibitive.
A Local Star Map |
| Courtesy JPL |
This is a serious limitation of all Newtonian rockets, unlike an aeroplane that has the air to push against a rocket must use the expelled rocket propellant. As the propellant is blasted out of the back of the rocket it pushes the spacecraft forward (Newton's third law). We must find a completely new way to drive spacecraft forward that requires no propellant, probably at the expense of the action-reaction principle.
Imagine using conventional chemical rockets for the journey and we want to carry a payload in the order of size of a bus. Even if we say that we will not need to slow down and allow 900 years to make the journey the propellant required would still be greater than the entire mass in the universe! Clearly the amount of propellant required is an extremely important factor.
It must also be remembered that the issues and examples we have used above are for our nearest star; consider that the nearest 'sol type' star that is probably accompanied by a planetary system is Epsilon Eridani which is more the twice as far away, about 10 light years (Epsilon Eridani is located at the far left, just below centre, of the star chart shown to the right). After that we could be looking as far as 24 light years at Xi Ursae Majoris B.
Possible Solutions?Clearly even antimatter rockets we will still have velocity and propellant issues, so what is the answer? There are two ideas that are out forward for the near term that can improve the situation, but both have serious limitations.
Time Dilation
Note information on time dilation can be found in TheSpaceSite.com's Special Relativity page.
If we can get over about 90% of the speed of light (a theoretical
possibility with antimatter rockets, though in practice may be
prevented by propellant requirements) time dilation will come
into play. This, however, only exists for the space travellers,
although the several year journey will only be a few months for
them several years will pass by on Earth. With longer exploration
trips the human civilisation will have changed greatly by the
time they return, this is clearly not the answer for the human
development of space.
Imagine if the astronauts set off on a journey that was to take
100 years on Earth but would be only be few months for them.
While they are away new technology is developed that allows us
cross great distances very quickly. By the time they arrived it
could be a fully developed community and the astronauts would be
left 'out of their time' (remember 100 years have passed) for no
reason!
Robotic Exploration
For Robotic exploration there is the problem of observation and control, if the robotic craft is 2 light years away whatever method of observation was used, whether it was by telescope at the base, or pictures beamed back from the craft, the pictures would be 2 years out of date. If the need to make important decisions arose, such as course corrections for unknown phenomena, 2 years will have passes before we even knew of the situation. It would take another 2 years before any response we tried could be received and a further 2 years before we received conformation of what had happened. This really does limit the effectiveness of robotic exploration over interstellar distances. The only hope of any chance of success is the development of advanced AI, but even basic AI is some way off.
Better Options for the FutureWhat we need are new advances in science and there are three in particular:
- Speed, we need to find a way to travel faster than the speed of light
- Propellant, we need to find a propulsion system that does not need propellant
- We need to find new, improved ways of generating onboard energy.
These are the three objectives of NASA's Breakthrough Propulsion Program (BPP).
It is true that the laser sails will not need propellant as they would be powered from energy back home. As we have already discussed there are many problems here both technological and in operation (i.e. very small payloads, time delayed responses and dependency). Of course the maximum anticipated speed is 'only' about half light speed, which truthfully is not going to be sufficient for regular interstellar travel, or for exploration of more remote systems.What type of technology can fulfil these requirements?
Clearly special relativity expressly states that no object with mass can exceed the speed of light due to relativistic effects. It is, however, interesting to note that there is no such limit theoretically placed on spacetime itself by special relativity or anything else. This could be the key to solving all the problems. This implies that a faster than light craft using no propellant, could be achieved by developing a propulsion system that actually modifies and pushes against spacetime itself. This in turn implies developing a method to manipulate gravitational or inertial forces or any other interaction that occurs between matter and spacetime. What's more general relativity actually implies that this is possible.
An advanced system could even perform interstellar travel as quickly and comfortably as science fiction writers have imagined for years. There is no doubt this drive would revolutionise the way we look at space travel, and it is fair to say it may be a necessity for human exploration beyond our solar system.
The third important advance is the production of energy, even with the development of this 'space drive' it would still require a great deal of energy. We need to either a breakthrough in energy production physics, a way to tap into the energy in the space vacuum or ideally a breakthrough where laws of kinetic energy do not apply. We shall return to this point a little later.
Gravity ControlProbably the first thing to point out here is that antimatter is not antigravity. While conformation is still awaited we are fairly sure that antimatter behaves exactly as matter does with regard to gravity (and virtually everything else for that matter!).
Just as mastering sails and riggings did not create the steam ship and the propeller aircraft did not result in the jet aircraft, mastering and refining rocket engines will not lead us to space drives. They will require completely new physics that will be found far away from the Newtonian rocket concepts.
It is interesting to note that gravity/inertia control will not only be used in propulsion, there are many other applications of such control that will also be crucial to deep space exploration craft. We can borrow a couple of ideas from Star Trek to emphasise the point.
- Artificial Gravity - allowing the crew to live normally with no microgravity effects on the human body
- Inertial Dampers - handy devices that eliminate mass-inertial effects on the crew and the spacecraft. Unlimited accelerations, both in speed and directional changes, would be possible with the dampers counterbalancing any forces that would be felt in the spacecraft
- Deflectors - moves objects out of the path of the craft, particularly useful at very high velocities
With the space drive (or gravitation drive) propulsion system and the inertial dampers it is very probable that extremely high accelerations could be achieved. This could be to either to quickly attain relativistic speeds (within a solar system) or to almost instantly 'jump to warp' and accelerate from standing starts to faster than light travel (for interstellar travel).
It should be noted that most scientists believe that any such
control would be impossible, as it does not conform to known
physics. With due respect it must be felt that this is a
narrow-minded approach. How many times throughout history has it
been suggested that something is clearly wrong and the future
proves it to be correct. I would imagine that every scientist
would agree that it is clear that we do not have the complete
understanding of how the universe works and there is great deal
more to understand yet.
For example the major objection is usually the issue of the
conservation of momentum, for this to be satisfied there must be
a reaction mass. We may find this is satisfied if it turns out
that gravitational fields have energy and momentum of their own.
Other possibilities exist to, perhaps the so-called dark matter
or cosmic radiation could provide reaction mass. Perhaps even
spacetime itself could be the reaction mass, similar to old idea
of an '
aether', this time suggesting that the ether is
electromagnet. This is a characteristic of a concept known as
Zero Point Energy. This is extremely important it the space drive
theories and could also be the answer to the third required
breakthrough of energy production. This is a concept we will
examine in detail a little later, but if you are anxious the page
is here.
For now let us move onto the theories behind how a space drive might work.