# 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
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
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
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.

• There is already redundancy:

Lots of redundancy is built-in inside the probe, to ensure it will not fail. Multiple sets of thrusters, reaction wheels, CPU, ... Basically we are already sending 2 ships on one package.

• Same causes causes same effects.

If there is a design flaw in your twin spacecrafts, it's likely that both your spacecraft are doomed: you couldn't learn from your mistakes.

• Current robot spaceprobes could not "help one another out" in the way that manned exploration ships on Earth could an did. They are far too specialised. For manned exploration of Mars, say, it might make more sense. Apr 27, 2018 at 7:28
• Good point. One example I had considered was the Mars Climate Orbiter, but the nature of that failure would have put it into the 0% outcome in your table. Apr 27, 2018 at 12:13
• Resupply missions could be an exception as "science outcome" is not the main objective. In the case of the ISS, it is close enough to try again upon failure. Apr 27, 2018 at 12:15
• Unlike in KSP even unsuccessful missions will increase your science. Take for instance the F9 launch pad explosion. Blowing up two F9 on the launch pad would not be very productive. But blowing one up can tell you F9 fuel tanks can leak.
– Aron
May 17, 2018 at 14:56