Dave suggests:
> No, you can't tack into the solar wind as long as
> you are flying in a vacuum.

A quick STFW finds:

   http://newscientist.com/ns/19991023/letters9.html

which includes a letter:

"I read with interest and enjoyment the report on
using "plasma sails" to harness the solar wind for
spacecraft. It sounds most impressive, but I was
puzzled by the statement that it would be possible
to "tack towards the Sun".

As I understand it, a sailing boat can sail upwind
by only combining the effect of a well-trimmed sail
with that of a properly designed keel or
centreboard and rudder. Without the resistance of
the water, the wind will simply blow the boat
sideways. A spacecraft could not therefore tack
into the solar wind without a solar keel and a solar
sea in which to dip it--or am I missing something?

CLIVE BERRY
Witney
Oxfordshire


COLIN MCINNES, an aerospace engineer at the
University of Glasgow, explains how to steer a
spaceship with a solar or plasma sail towards the
Sun:

There is indeed, roughly speaking, a kind of solar
keel and solar sea--a spacecraft's mass and the
Sun's gravitational field. A spaceship leaving the
Earth is, like our planet, in orbit around the Sun. If
it uses its propulsion system to reduce its angular
momentum, it will move into a closer orbit.

A sail can be used to do this because, when
photons of sunlight hit the mirrored surface of the
solar sail--or when charged particles strike a
plasma sail--they don't simply move it directly
away from the Sun. If the sail is angled, the
photons will also push the sail to one side or
another as they are reflected.

The result of the momentum gained from the
reflected particles is that a force is exerted on the
sail at right angles to its surface. So
by simply turning the sail, the direction of the force
can be controlled and as a result, the sail can
travel almost anywhere in the Solar System.

To move away from the Sun for instance, the sail is
angled so that the force acts along the direction of
motion of the solar sail. The solar sail then gains
energy and spirals away from the Sun.

Similarly, to move towards the Sun the sail is
oriented so that the force acts in the opposite
direction to the motion of the sail. This slows the
sail, and to balance its kinetic and potential
energy, the sail spirals inwards towards the Sun
under the influence of its gravity.

Even though the force from the sail is acting in the
opposite direction to its motion, the sail's speed
increases as it spirals towards the Sun. This
seeming contradiction comes about because what
the force is really doing is reducing the orbital
angular momentum of the sail. Angular momentum
depends on the radius of the sail's orbit as well as
its speed, so as the sail falls towards the Sun, its
angular momentum can decrease even though its
speed is increasing. (The same confusion arises
with Earth satellites--as they spiral in because of
air drag, their speed actually increases.)

Changing the angle of a solar sail can be done in a
variety of different ways: thrusters could be used
to move it, or small reflective vanes at the edges
of the sail could do the job. By adjusting these
vanes so that they reflect more or less photons, a
torque can be applied to the sail and it will rotate.

Another way to do this is to mount the payload on
a rotating boom. By moving the boom, the centre
of mass of the sail will be displaced away from the
centre of pressure, again creating a torque that can
be used to turn the sail.