One issue of deep space exploration is that once the object (like Voyager) is far out in the solar system the physical distance is so great that the signal reaching Earth is very weak. I was wondering if the time has come for us humans to invest in satellites around the outer planets that could act like amplifier for remote Voyagers and Voyager-like future missions. Their job would be to orbit around say Neptune and listen to any signal from these extra far away systems and relay them back with amplified signal to Earth. As a backup every planets like Jupiter, Saturn, Uranus, Neptune or even their numerous moons could have such satellites. So, any issue in one satellite won't be an immediate issue and would not need immediate fix.
There are several factors that make this difficult:
- Antenna size. The largest dish antenna in space has a diameter of 10 m (Spektr-R). The DSN has 70 m antennas. A satellite with a 70 m antenna would be very heavy, maybe too heavy to get to the outer solar system. It takes a lot of energy to get to Neptune's orbit: the Voyagers are the heaviest spacecraft we've ever sent out there at ~700 kg. Even an SLS can't send a 10-ton satellite to where you need these relays.
- Orbital mechanics. If you put such a satellite in orbit around Jupiter, it'll follow Jupiter's 12-year orbit. Half the time, it'll be further away from Voyager 1 than we are. So you need 4 of them: one near Jupiter, the other 3 in orbits leading or trailing Jupiter. In fact, you don't want to be orbiting Jupiter at all, because Jupiter and its moons will be in the way (and because of the radiation environment). Better to put all 4 of them in leading/trailing orbits. And they shouldn't be around Jupiter.
- These would be major interplanetary missions in their own right. Jupiter is only 5 AU away from us, the Voyagers are at 140 AU. To make a difference, the relay satellites should be halfway, which is twice the distance to Pluto. It takes 15 years to get there.
- Cost. A single satellite easily costs $250-500 million, which is more than a 70 m DSN antenna costs.
- Limited use. We only have a few missions that could use these relays: 2 Voyagers and New Horizons. These were 30 years apart. There are plans for 2 more: a Pluto orbiter and an interstellar mission. The cost of the antenna network would have to be split between these missions. And New Horizons has shown that flyby missions can work with really tiny bandwidth requirements (1 kbit/s).
- Regular replacement. Because these antennas have to be pointed at Earth and the satellite and those directions change constantly, the satellite needs to use thrusters to maneuver. That limits the lifetime of the satellite. Each one also needs RTGs for power (also limiting lifetime, and making the mission much more expensive, and we don't produce enough Pu-239 to supply all these missions)
Relays around a planet are another story. For Mars, this is already being done. Every Mars orbiter is capable of relaying signals from any of the Mars surface missions. This enables the surface missions to be smaller and lighter (they only need to carry a small dish to communicate with an orbiter overhead).