# Why do space missions use a single vessel and not a convoy?

In designing space missions, probes that have traveled to the moon or other planets are typically a single unit. A lone probe travels a long distance and if/when it fails the mission is shut down. How come space missions do not send a convoy of 2 or more ships on a mission as has been used in ocean exploration?

While the cost and weight of additional vessels would increase, the redundancy and likely life of the mission would increase.

• Redundancy is important, but you may save a lot of money by using redundancy only in one ship for critical parts.
– Uwe
Commented Apr 27, 2018 at 8:46
• Voyager 1 and 2 were not send in a convoy. A very successful decision as we know now.
– Uwe
Commented Apr 27, 2018 at 8:52
• To start a convoy for ocean exploration, you only need a harbour big enough for all ships of the convoy. But to start a space ship convoy, you need a large launch facility with all facilities needed to prepare and execute the launch of all needed rockets at the same time or within a short time interval. The start window of a long distance mission with swing by maneuvers may be very short.
– Uwe
Commented Apr 27, 2018 at 22:07

• It's mathematically flawed bet:

Case of sending 2 ships on the single mission:

+--------+--------+-----------------+
| Ship A | Ship B | Science outcome |
+--------+--------+-----------------+
|   OK   |   OK   |       100%      |
+--------+--------+-----------------+
|   KO   |   OK   |       100%      |
+--------+--------+-----------------+
|   OK   |   KO   |       100%      |
+--------+--------+-----------------+
|   KO   |   KO   |         0%      |
+--------+--------+-----------------+


Case of sending 2 ships on 2 missions

+--------+--------+-----------------+
| Ship A | Ship B | Science outcome |
+--------+--------+-----------------+
|   OK   |   OK   |       200%      |
+--------+--------+-----------------+
|   KO   |   OK   |       100%      |
+--------+--------+-----------------+
|   OK   |   KO   |       100%      |
+--------+--------+-----------------+
|   KO   |   KO   |         0%      |
+--------+--------+-----------------+


I'd rather take my chances and get 200% given the same initial investment.