Why are space probes always launch in pairs like Mars 2 & 3 and Mariner 8 & 9?

Question

Why are space probes always launch in pairs like Mars 2 & 3 and Mariner 8 & 9?

Answer 1

In the case of Mariner 8 and 9, they were two seperate space craft doing a complimetary mission, although it didn't end up that way.

Answer 2

That has generally to do with how you test spacecraft and also often R&D costs.

The classic approach is building two identical craft, and use the first one as test article for all important ground tests. When these tests are done and the flight article gets tested based on the information from the test article. And the flight article gets launched.

When something fails, the test article can quickly be turned into a flight article after a short overhaul.

Another reason is saving costs: Research and Development is the most expensive piece in most space probes. Launching two identical crafts thus only increases the costs slightly.  Launching more crafts on the other hand is also good for science - it gives you more data, the probes are often on slightly different trajectories covering more space... and that for only little money more.

That's why there are two mars rovers, two voyager probes, two Viking landers, etc...

Additionally, launching two crafts can give you the chance to fix problems in the second craft, which you detect after launch in the first - for example when Pioneer 10 passed Jupiter, Jupiter's radiation caused more damage as expected and damaged the computer. Pioneer 11 thus got equipped with better shields.