The origins of sailing through space to explore the solar
system and beyond date all the way back to the early
17th century with Johannes Kepler (see TheSpaceSite.com
Kepler page). Kepler believed
that there was a strong solar wind blowing through space that he
believed he observed when he saw the tails of comets blowing
around. Such a strong wind continuously blowing has been
disproved. The reason for the tail moving though is found to be
radiation pressure and this can be used to propel a spacecraft
with the light administering a continuous 'push'.
So Kepler was actually very close to the truth 4 centuries
ago.
Rather than use rockets, why not use light?
Light creates a slight 'push' on an object, if we use a very
large, flat, reflective surface to capture light, and make it
very thin, supporting it with a lightweight structure (hence low
mass), this slight force is able to propel a spacecraft.
The solar sail produces very low
thrust, this means sailing
through space must begin and end in space. The sail would have to
be launched by some other some other means and deployed
(unfolded) in space, unless we had a space station of course,
then it would require just a short tow!
The greatest advantage to these concepts is they provide
propellant free systems, meaning that they can accelerate
indefinitely. It is also a major saving of mass and together with
the fact they do not to carry an engine means a large amount of
energy per unit mass can be imparted to the craft. Putting the
two together means these craft are capable of very high
velocities, which is crucial for deep interplanetary travel and
especially interstellar travel. This makes the more advanced
sails detailed below candidates for a precursor interstellar
missions. It is unlikely, however, that even the most advanced
sails will open up routine interstellar travel.
Interstellar travel is a concept we will examine in more detail
later.
It should be noted that for a sail to be effective it would have to be extremely vast in order to generate enough thrust to carry itself and any payload. The sail would have to be in the order of kilometres wide, but it would have to be extremely thin, current proposals suggest the use of lightweight carbon fibres that would be only a few microns thick, about the width of a piece of paper. Carbon fibres would also be attractive as they are one of the few materials would be able to stand the heat and radiation from the Sun.
Unsurprisingly the greatest general problem for all the sails is developing such a large, thin, reflective material.
We will examine the solar (or light) sail, the lasersail, the microwave sail and the relatively new concept of the magsail below.
The Solar/Light SailThe basic solar sail works on the principle of using photons
(sunlight) to accelerate towards their destination (provided it
is away from the Sun!).
The theory behind this is that as the light is reflected the
acceleration is imparted from a momentum transfer from the solar
photons, and the more energy is reflected, the more momentum is
imparted.
This is an important design consideration, i.e. the more
reflective the surface of the sail, the better the performance
will be.
The Sun exerts a force of about 9 N/km2 at 1
A.U.
from the
source. This means the
thrust to weight ratio for a solar sailing
craft is extremely small.
This is not surprising though, compare this to the space
shuttle's rockets that produce force in the order of millions of
newtons.
Of course the shuttle will burn its fuel load quickly, which
means that a velocity of about 30,000mph is close to the limit of
chemical rockets. Sails by contrast will be able to continuously
accelerate as they have no propellant to use up directly. This
means that over long distances much higher velocities could be
reached and estimates as high as 200,000mph are not uncommon.
This will obviously result in substantially reduced journey
times.
It is worth pointing out that over short distances chemical
rockets would be the optimum solution as the sail would not have
enough time to build up to high velocity and the faster
accelerating chemical rockets will result in faster journeys.
One of the great disadvantages of the solar sail over other sails is as the craft travels away from the Sun the intensity of light diminishes and thus so does propulsion. As we go deep into the solar system the intensity is so low that the craft will not accelerate, and so will be lost in space.
Laser and Microwave SailsClearly the light sail has problems, from the
thrust being too
small to move any useful payload to the fact that it can not
function once the sunlight energy has dropped to near negligible
levels. Ideas, that have been around since the 1980's, suggest
using banks of lasers or microwave transmitters in orbit around
the Earth or the Sun or even on the lunar surface to accelerate
the craft, rather than using solar photons. This will allow a
substantial increase in performance, due to much higher intensity
energy, and also solves the problems of deep space travel. NASA
has recently suggested that such a craft could reach 1/10th of
the speed of light (about 30,000 km/sec), though there are other,
rather more optimistic, suggestions that as much as half the
speed of light could be obtained.
This estimate may however be accurate for high performance
sails built with advanced materials that will be available with
the arrival of nanotechnolgy.
Robert Forward first proposed the idea of the laser sail,
though his ideas used 1000 km lenses, a laser producing
10-million-gigawatts and 1000 km sails, which was a little
impractical.
In fairness these figures were revised, together with a drop in
performance, to slightly more reasonable levels (although still
fantastic, e.g. a microwave laser of a mere 10-gigawatts).
One of the major problems with these designs are the lasers would have to be prohibitively large to prevent the beams diverging at great distances. Further to this laser technology would have to be greatly improved to hit moving targets millions of kilometres away.
2 Designs for Sails |
 |
| Courtesy: NASA | Courtesy: www.solarsails.info |
Microwaves could be used to achieve the same result, though the design is slightly different in that the sail consists of wire mesh, though the gaps are less than half the microwave wavelength.
In this approach the craft would probably be extremely light weight robotics, accelerated quickly to a coasting velocity as microwaves are much better over short distances, although much less effective over long distances.
Microwave transmitters have the substantial advantage that they are much more efficient than lasers, and required power increases over today's technology for small probes is not that great, this is a potential near term possibility.
This will also allow extremely low mass craft, though the idea may not be practical for interstellar distances, as microwave wavelength is much greater than that of lasers, hence requiring much larger optics.
Problems for Solar/Laser/Microwave SailsOne of the greatest challenges to this technology is enabling the craft to slow to a stop and manoeuvre around the target.
Several ideas for stopping the craft have been put forward,
the most common in the suggestion of using multiple sails. For
example a two stage sail would allow the inner sail to separate
from the main sail, the laser beam would then hit the outer sail
and reflect it onto the inner sail. This causes the outer sail to
slowly accelerate away from the target, but will decelerate the
inner sail, together with the spacecraft. Then the smaller sail
can now be used as the main sail and may manoeuvre about.
A three stage sail could be used for round trip missions, the
stopping functions as above, when it is time to return the third
stage sail separates from the second stage, and laser light from
the solar system is retro-reflected onto the third stage for the
return journey. This would require the sails to be slightly bowl
shaped to redirect and focus the beam.
Another, perhaps more ingenious, idea suggests that to stop, switch off the beams, the craft would then extend charged wired that could interact with a star's magnetic field to turn the craft around through a shallow arc, finally the beams could be switched back on.
One suggestion that could prove fatal to the plan of using these craft for a precursor mission is that the actual sails might not be durable enough to power craft on interstellar missions.
The MagsailThe Magsail could provide answers to all the above noted
problems. It is a relatively new concept that was first
introduced by Robert Zubrin and Dana Andrews.
The design consists of a superconducting cable, millimetres in
diameter, forming a loop. Potentially this could be both lighter
and cheaper than a lasersail.
The current idea is to create a magnetic dipole to divert the
background flow of solar wind; it may also be able to provide a
lift force by correct alignment allowing directional control.
A recent addition has been the suggestion to using a particle
accelerator to fire charged particles at the sail and these can
be produced at much higher efficiencies (approximately 6 times
better) than lasers.
The magsail also offers more flexibility in terms of missions, being able to slow and manoeuvre with much greater simplicity than the solar sail.
There are many unresolved problems at the moment and it will require advances in the sciences of superconductors, thermal control and planetocentric operation to name just a few.
General Problems for all Sail DesignsThere are three major disadvantages for all these beamed propulsions systems:
- The first is the required accuracy of the pointing devices, which have to be virtually perfect over such great distances.
- The second is that the spacecraft are capable of carrying only extremely low payload masses.
- The third, and perhaps biggest disadvantage, is the total
dependence on the beaming technology based back home. If there
were any problems then the sail would be lost in space, virtually
unrecoverable. Further to this to make changes they would have to
radio back to base, get the beam re-aligned and then receive the
new course corrections.
If the craft is only two light years away from the base it would take more than 4 years for the change to occur (2 years for the radio message to be received at base and two years (plus distance travelled in those 4 years) for the effect to catch up with them).
It should further be noted that for regular, routine travel
with these concepts it would have to be presumed that huge banks
of beaming devices would be required at every star port, creating
interplanetary and interstellar highways.
Whether this would be practical is of some doubt.
Go on to
Nuclear Fission Propulsion
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