One possible application of a solar sail is a spacecraft to warn Earth of approaching interplanetary disturbances and shocks. Such events originate on the Sun and greatly affect the "space weather," the magnetic "storms" and "substorms" which disturb the Earth's magnetic environment, whose occurence and intensity depend on interplanetary conditions. Such events can impact communication satellites and even affect electric power grids on the ground, in Canada and Alaska.

Spacecraft that can provide such a warning are currently stationed at (or near) the Lagrangian L1 point, between the Earth and the Sun--at about 4 times the distance of the moon and 1/100 times that of the Sun. A spacecraft at that point is in a special kind of equilibrium. It orbits the Sun just as the Earth does, but being closer to the Sun, it should move faster, like Venus and Mercury (which are also closer to the Sun), and have a shorter "year", a shorter orbital period. If this were actually happening, the spacecraft would pull ahead of the Earth and would quickly become inconveniently distant.

However, a satellite placed on the line between Earth and Sun is pulled outwards by the Earth, canceling some of the Sun's pull. The distance of L1 is such that the Sun's pull is reduced just enough to slow down the motion of the spacecraft to where it matches the motion of the Earth. The satellite then keeps its position between the Earth and the Sun, intercepting disturbances before they reach the Earth. Currently ACE is stationed in the vicinity of L1; in the past ISEE 3, WIND and SOHO have also performed observations there.

Four times the distance of the Moon sounds far, but the solar wind, the stream of fast particles which carries "space weather" earthward, can cover it in just about 1 hour. Even that hour is not guaranteed: the most damaging kind of "space weather" tends to be associated with unusually fast streams of solar wind, which can cover the distance in half an hour or less.

A proposed scheme for providing a longer lead time used a spacecraft equipped with a solar sail and stationed sunward of L1. Because of the greater distance from Earth, the Earth's pull there is weaker, not enough to make the spacecraft match the Earth's motion around the Sun. However the pressure of sunlight also pushes the satellite away from the Sun. At a suitable distance--always further sunward than L1, thus allowing an earlier warning--the two forces together, again, take away just enough of the Sun's pull to allow the satellite to match the motion of the Earth.

Such a spacecraft was actually among the options for NASA's "Deep Space 5" (DS-5), the fifth in a "New Millennium" series of small experimental spacecraft. The first of that series, DS-1, was launched in late 1998 to demonstrate the use of an ion engine and is described in the next section. The plan (not realized) was to equip DS-5 with a 70-meter solar sail, which would have allowed it to be stationed at twice the distance of L1