Cosmos-1 will turn its sails fully face-on to the Sun, to receive a push during the portion of its orbit while the spacecraft is moving out to apoapsis in its orbit. The result is that the altitude of apoapsis increases. The spacecraft flies to a higher point.

When the spacecraft is out near apoapsis, it can "tack" like a sailboat does, by articulating the sail blades to glancing angle, and increasing its orbital speed slightly. The effect of this is to raise the periapsis altitude on the other side of its orbit.

As the spacecraft moves around toward its periapsis, the sail blades will be angled edge-on to the Sun to minimize the push it can receive via photon pressure. Recall that you don't want to slow down here, or it would have the effect of decreasing periapsis alttitude.

This animation illustrates the whole process, so you can watch the blades change pitch in various parts of Earth orbit. It also depicts the initial deployment and inflation of the sail blades. The orbital part of the animation moves quickly, but if you watch carefully as the orbit repeats, you'll recognize why the blades are changing pitch when they do. The word "enter" takes you to the Cosmos-1 solar sail website.

It's the interplay between solar photon pressure and the mechanics of the spacecraft's orbit around the Earth, that Cosmos-1 will work to its advantage, increasing the size of its orbit.

Use your Scale Model to Illustrate Cosmos-1's Operations

1. Move your scale model out from there, around the globe away from the Sun, with its sail blades flat on towards the Sun. Note: The Earth is turning such that any point on the surface is moving toward the east. Cosmos-1's orbit will take it from the northwest toward the southeast.

2. Continue moving the spacecraft out to a high apoapsis.

3. Near apoapsis, the spacecraft will alter the angle of each of its blades to a glancing angle, causing the solar "push" to help speed up the spacecraft in its orbit as it passes through apoapsis. If you built the Advanced Version of the model, you can adjust the sail blades to illustrate this.

4. After passing apoapsis, the spacecraft will move back around toward periapsis. It's during tis portion of the orbit that you don't want any push from the Sun. The spacecraft will articulate its sail blades so they're edge-on to the Sun, providing as little area as possible for the Sunlight to puch against.

5. Reaching periapsis, the spacecraft will articulate its blades to another "glancing" angle to help speed up the passage through periapsis.

6. Lastly, the spacecraft will turn all the blades flat-on to the Sunlight again, receiving maximum push from sunlight, and achieving an even higher apoapsis this time.

Cosmos-1, the first-ever solar-sailing spacecraft, will be confined to Earth orbit. It is a rudimentary spacecraft, with sails much smaller than those we may see on future sailcraft. However, you can extrapolate and imagine that some larger sailing spacecraft can use these same principles to gain Earth-escape velocity. Once separated from Earth's gravity this way, the spacecraft would find itself in an independant solar orbit, just like a planet. From there, the same techniques would be able to change apoapsis and/or pariapsis altitude, achieving trajectories to take the sailcraft to other planets and beyond. To achieve Earth escape velocity, Cosmos-1 would have to work for about two years -- far beyond the scope of its short mission and relatively simpler objectives.

For more on interplanetary trajectories, see Chapter 4 in JPL's Basics of Space Flight.