Interplanetary Spaceflight - Economical Travel Techniques

Economical Travel Techniques

Interplanetary travel has to solve two problems, other than escaping from the planet of origin:

• The planet from which the spaceship starts is moving round the Sun at a different speed than the planet to which the spaceship is traveling, because the two planets are at different distances from the Sun (due to Kepler's third law). So as it approaches its destination, the spaceship must increase its speed if the destination is closer to the Sun, or decrease its speed if the destination is further away (assuming a Hohmann transfer orbit).
• If the destination is farther away, the spaceship must lift itself "up" against the force of the Sun's gravity.

Doing this by brute force – accelerating in the shortest route to the destination and then, if it is farther from the Sun, decelerating to match the planet's speed – would require an extremely large amount of fuel. And the fuel required for deceleration and velocity-matching has to be launched along with the payload, and therefore even more fuel is needed in the acceleration phase.

The change in speed (delta-v) required to match velocity with another planet is surprisingly large. For example Venus orbits about 5.2 km/s faster than Earth and Mars orbits about 5.7 km/s slower. To put these figures in perspective, Earth's escape velocity is about 11.2 km/second. So matching a space shuttle's velocity with that of Venus or Mars would require a significant percentage of the energy which is used to launch a shuttle from Earth's surface.