# Why didn't NASA launch communications relay satellites outside the asteroid belt to communicate easily with interstellar satellite?

I came across with nasa's voyager and other interstellar satellite, just thought that if they build a relay satellite which situated outside asteroid belt or nearby to Saturn & Jupiter, to transmit the signal from that interstellar satellite to that relay satellite and from that it transmit back to earth.

• There are no interstellar satellites (except perhaps Voyager), so that relay satellite would have nothing to do. Commented Feb 22 at 9:59
• What problem would this solve? We can communicate with distant probes already.
– GdD
Commented Feb 22 at 11:47
• The satellite would have to have its own orbit of the Sun, independent of both Earth and the remote object, so it would only be useful on the rare occasion where the three happened to line up. Commented Feb 22 at 18:42
• The Voyagers are not satellites. They're probes, or spacecraft, but they're not in orbit around anything, so they're not satellites. Commented Feb 23 at 11:29
• It would probably make more sense to send the relay (or a series of relays, sent every few years) on a similar trajectory to the probe, but given that the probe's trajectory depends a lot on the position of the planets as it flies by them and gets gravity assists and course corrections from them, that's probably a difficult proposition. Note that "more sense" still does not mean it actually makes any sense. Commented Feb 23 at 11:30

There is no relay in place because we can communicate with probes at long distances already, and a relay at the orbit of Jupiter or Saturn wouldn't make much difference due to the distance involved. Look at the table of distances below:

Planet/Object Distance to the Sun
(millions of kilometers)
Distance to the Sun
(Astronomical Units)
Mercury 57 0.38
Venus 108 0.72
Earth 149 1.00
Mars 228 1.52
Jupiter 780 5.20
Saturn 1437 9.58
Uranus 2871 19.14
Neptune 4530 30.20
Pluto 5360 35.7
New Horizons 8800 60
Voyager 1 24400 163

The most distant probe is Voyager 1, which is between 162-164AU away, that's 24.4 billion kilometers. The orbit of Saturn is just over 9.5 AU, which only shaves 8.5AU off (5%) of the transmit distance, if the relay station is in the right position, i.e. between the probe and Earth. If it's on the opposite side of the sun to the probe it adds up to 9.5AU to get to the relay station.

Add to that the relay would have to be huge. The farther away a probe is the bigger the dish receiving it has to be. The antennas in the NASA Deep Space Network are up to 70m in diameter, there are several of them and they are networked together. Space based antennae could be somewhat smaller as there wouldn't be atmosphere to contend with, they would still need to be very large. The relays would need a good deal of power as well, keep in mind they would have to re-transmit signals back to probes vast distances away.

It's probably doable with current technology, but it would be extremely expensive all to solve a problem we don't really have. It's far less expensive to build earth-based infrastructure.

• And if the Earth-side station breaks, it's reasonably possible to fix (Arecibo notwithstanding). Commented Feb 22 at 17:54
• Even if we wanted to eliminate atmospheric interference, parking a relay satellite in Earth's orbit would get a lot more performance for the same budget than pretty much anywhere else Commented Feb 22 at 22:19
• @Ashaybaghele The question is not why can't they, but why would they. Even assuming the same could not be much easier achieved by building slightly larger antennas on earth, if you have the funds to put a large satellite around one of the gas giants, then sending another research probe would result in much more science, then using the same payload for a big antenna. Additionally such a mission would easily take a decade or more to get done, by which time it is not unlikely that both of the Voyager probes will already have failed due to lack of power and general age of the components.
– mlk
Commented Feb 23 at 7:29
• @Ashaybaghele: For what it's worth, Voyager 1 isn't doing so great since late last year. It's entirely plausible that it will be functionally "dead" at the end of this year, never mind a decade from now. Commented Feb 23 at 16:27
• @jpa Aperture synthesis gives you the resolution of a big dish, but it doesn't give you the sensitivity. For weak signals, there's no substitute for a large collecting area. Commented Feb 23 at 19:15

We don't do this because we don't need to. According to the DESCANSO book series on the DSN (specifically volume 4), the current DSN could stay in touch with the Voyagers until 2057, i.e. long after they'll go offline due to lack of power.

Table on page 6:

i.e. a single 34-m antenna could downlink data at 40 bps until 2057 for Voyager 2.

Antennas can also be used in an array. Page 35 of the same document:

By simultaneously tracking Voyager from these three antennas during the Neptune encounter period, the DSN and Parkes radio observatory achieved an increase in the combined signal strength roughly proportional to the combined surface areas of the arrayed antennas. Other factors being the same, the DSN-Parkes array provided double the bit-rate capability of a single 70-m antenna.

By far the greatest signal strength improvement for Neptune resulted from arraying the twenty-seven 25-m dishes of the National Radio Astronomy Observatory’s (NRAO) Very Large Array (VLA) near Socorro, New Mexico with the 70-m DSN antenna at Goldstone, California. The received signal power (or data rate capability) with the VLA arrayed with the 70-m DSN antenna was nearly triple that of the 70-m antenna by itself. An array of a 70-m antenna, two 34-m antennas, and the VLA increased the downlink capability by 5.6 dB relative to the 70-m antenna alone, almost a factor of four in bit rate.

The DSN already routinely uses arrays: once every 6 months, data from the tape recorder on Voyager 1 is played back at high speed, and all of the antennas at one DSN complex (one 70-meter antenna, and 3 or more 34-m antennas) are used in an array to receive this.

Ground stations are also a lot cheaper. A 34 meter antenna costs around \$60 million, and can be used for all missions, not just Voyager.

A mission that can carry an antenna that size to Jupiter and operate it for 25 years would cost several billion dollars: the spacecraft would be larger and heavier than e.g. Cassini, although it would be less complex. It would need a big power source: those DSN antennas have 20 kW transmitters, so you end up with either acres of solar panels, or a nuclear reactor to provide that much power. Then there's the fact that antennas that large have never been put on a non-classified spacecraft. There are rumors that some signals intelligence satellites use deployable antennas that large or larger. A spacecraft that large and heavy also requires a huge rocket to launch to the outer solar system, let alone on an escape trajectory.

As GdD indicates, you need more than one of these relays, if you put them in orbit, otherwise your relay will be farther from Voyager than Earth is for half the time. If you don't put the relay in orbit but set it on a course to follow Voyager 1 or 2, you'd need at least one relay per spacecraft (the Voyagers are almost as far apart from each other as they are from Earth).

• Those are the (copy-pasted) numbers from the original version of the papers in 2002, though. But I think they are clearly in need of an update, considering if not any that (news flash) it's 2024 and the probes are definitively still powered. I see the year 2036 being frequently tossed around (including on the JPL website) but it's unclear if we are even talking about the RTG running out of juice or the DSN being too weak. P.s. I'm not exactly sure how much that 40 bps of engineering data could be considered "useful" btw.
– mirh
Commented Apr 9 at 14:35
Planet/satellite Astronomical Units (to sun) Million Kilometers (to sun)
Mercury 0.38 ~57
Venus 0.72 ~108
Earth 1.00 ~148
Mars 1.52 ~228
Jupiter 5.20 ~780
Saturn 9.58 ~1437
Uranus 19.14 ~28771
Neptune 30.20 ~4530
Pluto 35.7 ~5367
New Horizons 60 ~8800
Voyager 1 163 ~24400
Voyager 2 137 ~20448

We can't put it beyond/in the Kuiper belt as it is very risky and expensive. Also, it wouldn't improve the time taken to transmit the message. It might improve the transfer rate, but at what cost?

Also if you look at the table above it doesn't make any sense to put a relay anywhere behind the Kuiper belt as it is only a fraction of the distance that interstellar probes(also in the table) have covered.

We also have gigantic radio towers all over the world, which turns this planet into a radio transmitter. Even if we did do that, how would we even get it there and what about the power required for the dish?

An enormous radio dish on Earth might cost a few hundred million, maybe even a billion, but in space? It would cost a few hundred billion dollars, maybe even more.

• What do the "gigantic radio towers" have to do with the question? Commented Feb 25 at 12:58