I was watching a
programme on sky earlier about the possibilities of using the 'solar wind' to propel spacecraft
through space. I thought that the density of particles would be less the further away from a
planet you are, i.e., the number particles emitted from the surface area of the planet, will be
half as dense (i.e., number particles per unit volume) as one were to reach a distance from
the star that equates to double the surface area, so the propulsion will be less (50% than at
the star), but in terms of maths + probability, still a small force acting on the craft
.

So at the mid point between two stars, why does the craft not reach an equilibrium
where it does not move, and does it mean (that unless it harbours some sort of manipulable
sail) that it will always be deflected from it's target (obviously assuming the destination is a
star or solar system etc.)

The solar wind, or emission
of particles like it, will come from a star and not a planet.

I agree about the distance
factors and the force per unit area, doubling the distance will increase the surface areas by a
factor of four. If you move from distance R to distance 2R the sail on the spacecraft will only
intercept one quarter of the amount of solar wind at 2R as it would at R and so the force will
be one quarter as great.

However it seems that the force of the solar wind is less
than that due to the radiation pressure from the Sun. It is often though that solar sails are
pushed by the solar wind just as sailboats are propelled by the wind on Earth. This is not
true. The solar wind is an extremely tenuous flow of particles streaming away from the Sun
and it exerts very little force on anything it hits. (However we will look at the effect on the tail
of a comet later).

Lets start by looking at the effect of the solar radiation on a
spacecraft.

Sunlight at 1 Astronomical Unit (1 AU is Earth's distance from the sun =
150 million km or 93 million miles) exerts a force of 9 Newtons per square kilometre (0.78
pounds per square mile) on a solar sail.

Imagine a space craft with a mass of 1000
kg and with a 'solar sail' of area 1 km^{2}.

The acceleration of such a spacecraft due to
solar radiation pressure would be 9/1000 = 0.009 m/s^{2}.

If this started from rest just
outside the Earth's atmosphere it would reach a speed of about 5440 m/s within a week and
have travelled a distance of 1.6x10^{9} m or over one and a half million km. (Just about one
hundredth of the distance from the Earth to the sun). Of course we should allow for the
intensity of the radiation falling off with the inverse square of the distance from the
Sun.

So looking at it another way, how long would it take our spacecraft to reach
the orbit of Mars (a distance of 8x10^{10} m). The answer (at a constant acceleration of 0.009
m/s^{2}) is just under seven weeks. However as I said the force will be decreasing and at the
distance of Mars it will have fallen to 0.004 N/square km. It will then be travelling at 38 km/s
and if there was no further acceleration it would take 7900 years to reach the nearest
star!

Try looking at this web site: www.planetary.org/solarsail

You will
probably be able to find others by typing in solar wind powered spacecraft, or solar powered
spacecraft.

**Solar wind powered craft**

The solar wind powered space craft was
proposed by the American geophysicist Robert Winglee. This system employs a huge
plasma field around a satellite. The field catches solar wind, like an enormous
electromagnetic sail. It uses solenoids on the spacecraft to generate an electromagnetic field
40 km across, with the spacecraft at the centre.

The electromagnetic field is then
filled with a cloud of magnetized plasma, or ionised gas, generated by a small plasma
chamber about the size of a pickle jar. The whole thing is rather like a giant balloon with the
electromagnetic field behaving like the fabric of the balloon and keeping the plasma in
place.

If the spacecraft is moving along a line at right angles to a line joining the
two stars then it will receive equal and opposite forces from each star (assuming that they
both give out exactly the same 'solar wind' and radiation). However these two forces act at
right angles to the velocity of the spacecraft and so will have no effect on its motion.