Deep-space communications relay stations

After I wrote yesterday about the possibility of "mining" Jupiter's atmosphere to obtain fuel for deep-space travel, another idea has come to me. First, I want to mention two existing situations. I think there might be a connection between them.



One, as any object gets farther and farther away from the earth, it becomes more and more difficult to communicate with it. That means that communications from us that are intended to transmit course changes, to correct faulty programming, or to give instructions to a piece of machinery that has landed on, say, Mars is harder and harder to do. The following two paragraphs are from NASA's website, dated March 2010.

For the last thirty years, NASA has sent probes to explore the outer solar system-home to gargantuan worlds like Jupiter and Saturn and icy midgets like Pluto and Eris. More deep space probes are on the drawing boards. Although solar power is abundant in the inner solar system, Earth's neighborhood, it has its drawbacks as we go farther and farther away from the green hills of Earth.

As one travels farther and farther from the Sun, it grows dimmer and dimmer. By the time a spacecraft arrives at Neptune or Pluto, the Sun looks more like a bright star than like the great glowing globe we are familiar with on Earth.​

March 2010 NASA article

The only "solution" that is mentioned on the page is this, also copied and pasted from the same March 2010 NASA article:

An array of solar panels that would run a probe, a base, or a very complex rover on the Moon or Mars would be bulky and an easy target for meteorites. On Mars, the array would also face sandstorms.​

The second "situation" is that nuclear-powered electricity generators are already being planned for robotic missions.

May 2011 Spaceflight Now article



Here's my idea, and part of my inspiration was the network of cell-phone towers across the USA. Could we launch and set up a series of deep-space "repeating towers"?

Here's how they would work. Whenever a probe arrived at a distant planet, or one of its' moons, instead of sending data directly to the earth, using a solar-powered antenna that can't even transmit one watt's worth of power, the probe sends its' data to one of the permanently located nuclear-powered communications satellites that is located closer to it than we are, requiring less of it's very limited energy supplies. The nuclear-powered satellite then sends the data to us, using many times more power than the probe can supply.

For very deep-space probes beyond Saturn, I can imagine that one nuclear-powered communications satellite in between Saturn's orbit and Neptune's can send the data to another "relay station" located in between Mars' orbit and Jupiter's, which would then send the data to us.

I know there would be a high initial cost to set up such a network of communications satellites, and I know that there would be some risk of a collision with a comet, but here's the question I am posing. Which is the bigger risk, losing a relay station or losing the data from the probe?

Benny, an admirer of Mr. Franklin

 

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When the relay station gets knocked out where's the data?
How do we get hold of it?

 

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The data would still be on the original low-powered probe, which is still doing experiments a long way from us. If one of the relay stations gets "knocked out" by a comet, I''m thinking that the data could be sent to another nuclear-powered "relay station".

Note: When a probe "sends" the data, it would only send a copy of the information. If the memory space for data on the probe is that limited, the probe wouldn't erase its' memory to prepare for new data until after it has received a signal from us that the old data was received by NASA.

 

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In which case you're increasing the cost of the probe (storage capacity) AND having mulitple relay stations. And two-comms between them.
Wouldn't it just be cheaper to send multiple probes?

 

Multiple probes, all located in the same area of deep space? That would be idiocy by redundancy, and none of them would be nuclear-powered, or it would add much more to the cost of a one-time-use space probe.

The cost of multiple nuclear-powered repeating stations, all set up for one purpose (transmitting data from any space probes that are nearby) and all permanently located in strategic parts of deep space, would be, in my opinion, a much less costly alternative.

 

Yeah?
The more probes you send the more data you get.

In other words you're expecting there to be multiple probes in range. :shrug:

So you want multiple (nuclear-powered and therefore costly) repeating stations in fixed places as opposed to individual probes sent as and when (and where) required?

 

And not a single one can transmit more than one watt of power because their solar-powered antennas are located so far away from the sun.

Remember, each "repeating station" is permanently located, nuclear-powered, and reusable for any and all probes that are nearby at any time from the time they're set up until either they're hit by a comet, until there's a mechanical breakdown, a deep-space nuclear meltdown, or until their nuclear fuel runs out, whichever comes first.

As I said, every deep-space probe has a weakness, and it's the same weakness for all of them. The further each one gets from the sun, the less power its' solar-powered antennae has available to transmit data to us or to receive instructions from us.

A network of nuclear-powered comm stations would be a good investment if you're looking at a century (or more) of useful life, which would be determined mostly by the chance that it would be hit by a comet. There's also some chance of a mechanical failure or a meltdown, but that risk is not as easily measurable.

 

you are missing something very important.
the time required to get or send data, especially course corrections.

 

We do that now. TDRS satellites relay communications from the Space Shuttle. The Mars Reconnaissance Orbiter can serve as a relay between Earth and vehicles on the surface of Mars, and the Mars Telecommunications Orbiter was dedicated to that function as well. (The MTO was cancelled.)

That could work. The problem will be placement. There are very few places that will be between us and any desired destination. The Mars orbiters work because they stay very close to Mars, and thus serve as good relays.

Again, problem there is positioning. If the relay satellite is in an orbit near Saturn, then for 2/3 of its orbit it will be farther from a probe near Neptune than the Earth is. You'd need 3 such satellites to maintain coverage.

And even then you don't get that much of a benefit overall, unfortunately. The Earth is 1 AU from the sun. Saturn is 9 AU, Neptune is 30 AU. So you'd go from having to design a probe that can transmit 21 AU to one that can transmit 29 AU (best case.) That's not much savings.

The problem is redundancy. Right now only one vehicle has to work to be able to send data back. In your proposed system, two vehicles have to work. Overall reliability would go down unless the vehicle could transmit all the way back to begin with, in which case you wouldn't need the relay satellite.

 

Here's a refinement of my original idea.

Imagine this. NASA sets up a network of nuclear-powered comm satellites, capable of transmitting to us, at a high power level, any and all the data that a probe has collected from a planet, a moon, a comet, or anything else. They're all capable of relaying data from probes to us, and they're also capable of relaying instructions from us to any nearby probes.

Now a probe arrives. It notifies a nuclear-powered comm satellite, located nearby, that it's ready to carry out its' programmed scientific experiments. The probe gives it's position to the comm satellite. The comm satellite then aims a nuclear-powered laser beam at the probe, which receives it and converts the beam into electrical energy. This electrical energy recharges the batteries on the probe, and the probe then uses this stored energy to power it's onboard scientific apparatus. If electrical energy for the experiments can be supplied repeatedly by the nuclear-powered comm satellites, the probes may not need big batteries in the first place! The biggest risk I see in this situation is that one of the comm stations may be damaged by a comet before the probe arrives nearby.

When the experiments are completed, a copy of the data is sent by the probe, using it's fractional-watt antenna, to the repeating station, which transmits it, with multiple watts of power, back to NASA, just as I've described.

This refinement of my original idea would deplete the nuclear-powered repeating stations faster than my original concept, in which all the comm satellites did was to transmit data to and from deep-space probes, but I'm confident that a nuclear-powered repeating station (now also tasked to give electrical power to probes) would still have a long time before its' fuel supply ran out.

Benny

 

So put three comm satellites in orbit between Mars and Jupiter, and three more in orbit between Saturn and Neptune. All six would be reusable for any and all probes that can send their fractional-watt signal to one of them, all six would be nuclear-powered, and all six would probably still be there a century from now. That's six separate launches but potentially hundreds of probes that could benefit twice from the network, once because data from the probe would be sent with much more power to NASA, and once because any one of the six comm stations could also serve as a nuclear-powered energy-generating station, sending its' energy to the probe in the form of a laser beam.

 

A few notes -

You may be thinking that space-based nuclear power sources are like nuclear reactors. They're not; they are radioisotope thermal generators that work via decay of isotopes. They are much safer than reactors, but also much lower power; RTG's are typically a few hundred watts. Really big ones (a few tons) can give you a few thousand watts. The more power they output, the faster they run down, and there's no way to "slow them down" - use the power or lose it.

That being said they are pretty good sources for low to medium power applications. The next Mars rover will use an RTG to power itself.

"Aiming a laser at the probe" sounds easy in theory, but we do not currently have the technology to focus or aim lasers that precisely. You'd have to send gigawatts of power in a laser to ensure covering a small area with enough power for a few watts.

 

OK, here's a further refinement of my idea. Three satellites go into orbit between Mars and Saturn, three more go into orbit 10 AU away from the sun (1 AU further out than Saturn), and the final three go into orbit 20 AU from the sun (10 AU closer than Neptune). If you really want "universal coverage", put another set of three at the 40 AU mark (10 AU further out than Neptune), for use by any probes that want to explore Pluto.

If you can forgive a basketball analogy, a player on one team takes a shot that misses. A player on the other team gets the rebound. He passes the ball to a teammate at the foul line, who passes it to a third teammate at half-court, who passes it to a fourth teammate at the far foul line, who gives it to the fifth teammate, who goes in for an easy layup.

 

The MMRTGs for Mars will be 64 cm (25 in) in diameter (fin tip to fin tip) by 66 cm (26 in) long and weigh about 43 kg (95 lbs).

They will produce (initially) 125 watts (100 watts after 14 years).

A thousand watts would only be about 800 lbs.

Each contains 4.8 kg (10.6 lb) plutonium dioxide, made from Pu-238 with a half life of 88 years.

http://en.wikipedia.org/wiki/Multi-Mission_Radioisotope_Thermoelectric_Generator

As far as the need for these relay stations, seems unlikely.

We still can get the signals from Voyager, and it's over 110 AU from the earth and it's power is now about 250 or so watts, about half what it was launched with, and it only has a 3.7 meter Antenna.

 

I didn't "miss" it, I simply made an amateur calculation that the benefit of a multi-watt communication of data outweighed the slight loss of time involved in the use of a network of comm satellites.

If you want me to state it for your benefit, fine. There will be a delay in the receipt on Earth of any data that is sent by any probe that uses a network of comm satellites, whether nuclear-powered, fuel-cell powered, or battery-powered. There will also be a delay in sending any course corrections to a probe if the course changes have to go through a network of comm satellites. I happen to think that even a delayed course correction would be better than a course correction that is necessary but never made because the probe has lost the ability to receive accurate information from us.

Here's the bottom line, as far as I'm concerned. If a deep-space probe arrives at its' destination and gets more static on it's comm link to us than useful information (like course changes or changes to its' computer program), or if it puts out more static than useful data from it's experiments, what good is it?

 

Sure, that could work. But is it cheaper to build one small probe and 12 big RTG-powered spacecraft and send them to distances as far as Neptune, or just build one bigger probe?

Well, you can't easily design a system that can catch a "missed" shot. At those distances you have to direct signals with high gain directional antennas. If you miss it it's gone.

 

You're quite right. I did think that space-based nuclear power sources were like nuclear reactors. The two ideas I proposed, a network of deep-space comm satellites and a network of energy-generating stations, both grew out of my concept of a nuclear reactor in space. Is this idea not technically possible, or has it been shelved because our political leaders don't like the idea?

If a true nuclear reactor was aboard one of my hypothetical comm satellites, could it put out that much power?

 

When you say "a bigger probe", do you mean one that has a bigger solar panel, or one that has more batteries? I'm trying to weigh, admittedly as an amateur, the pros and cons of launching six, nine, or even twelve nuclear-powered and reusable comm satellites (potentially reusable for decades) against the cost of launching hundreds of probes over the next century, all with bigger batteries and/or bigger solar-powered antennas.

 

Are you also getting a lot of static from Voyager, now that it's 110 AU away from us?

How easy is it to listen to a radio station broadcasting from Los Angeles if your radio is in New York City?

 

Sure, but it would be massive, and nothing we have today could lift it even to orbit. Just the radiators alone to get rid of that much heat would cover football fields.

Whatever. Might even be a bigger nuclear generator, like the next Mars rover will have. I'd also suggest that even if you don't want to land all that mass, launching an orbiter/lander combination (to do the comm relay) would still be a lot cheaper than launching those 12 spacecraft.