Space rendezvous is a precise orbital maneuver, which requires two spacecrafts to arrive at the same orbit, and move at the same speed. Since the orbital velocity is usually not less than 7 km/s, a 1-second error will case a distance of 7 kilometers between the spacecrafts. Such an error is unacceptable, as rendezvous is a prerequisite of docking, which requires spacecrafts to rendezvous as close as some dozen meters.
Initially, astronauts were not aware of principles of the space mechanics. Engineer André Meyer says that “to catch something on the ground, one simply moves as quickly as possible in a straight line to the place where the object will be at the right time”. Meyer notes that they found out that such a technique “will not work in orbit”, and explains that “adding speed also raises altitude, moving the spacecraft into a higher orbit than its target” (Gemini 4). Paradoxically, it will cause the faster spacecraft to move relatively slower. Therefore, for an orbital rendezvous between two spacecrafts, their orbit has to be adjusted in such a way that they catch up with one another at first, and then in a precisely calculated moment one of them has to change its orbit to eliminate the relative motion.
Usually to achieve the same orbit, one of the spacecrafts would have to transfer to a higher or a lower orbit. It is achieved by thrusting tangent to the orbital velocity, coasting halfway along the orbit, and thrusting tangent again to return the orbit shape back to circular. In the case of non-coplanar objects, the situation is complicated by the difference in their orbits’ inclinations. To change the plane and make spacecrafts’ orbits coplanar, the spacecraft has to add a step of thrusting perpendicular to its orbital velocity.
Work Cited
Gemini 4. Encyclopedia Astronautica. Accessed April 28, 2016. http://www.astronautix.com/flights/gemini4.htm.
