Update on Voyager 2 status | Hacker News Reader

Update on Voyager 2 status

planetary.org

56 pointszargon29 comments

It's interesting to note that although the Voyager probes are sometimes depicted as small (compared to satellites like the Hubble, where images of it next to shuttle astronauts are common place), they're not. The diameter of the main dish, 3.7m, is nearly twice the height of an average adult, and height of the magnometer boom is comparable to that of a 3-storey house.

Even despite this size, the article mentions the current transmission rate, a measly 1.2kbs, despite what is probably the most ideal antenna size & placement feasible, both on Earth and on the probe. Doesn't this suggest there are pragmatic limits to the operational range of 2-way radio transmission throughout the solar system?

I am curious if future such deep-space science missions would be more effective if they instead launched swarms of micro-probes, where the probes arranged themselves radially around a center-point to effect a very large antenna, possibly up to 1km in diameter.

Also, having so many micro-satellite could allow for a substantial degree of redundancy to tolerate more rad damage, and ideally have less of the mission control acrobatics described in this article. (Which is not to discount the scientists' effort in any way, for it is amazing.)

Voyager 2 uses X-band radio for communications, more modern systems use Ka-band radio, so with the same size antenna that would increase received power by about a factor of 10. Also, more modern communications technologies such as turbo-codes, which are significantly more efficient than the Reed-Solomon codes used by Voyager 2, further increase throughput for modern systems.

Overall though, yes, there are some pretty harsh limits to radio communication across the outer solar system (Voyager 2 is nearly 3 times as far away as Neptune, and well over twice as far away as Pluto's maximum distance from us). But this is fairly easily remedied by switching to yet higher frequencies or using optical communication.

> But this is fairly easily remedied by switching to yet higher frequencies or using optical communication.

Isn't this the same?

Yes and now. Switching to higher radio frequencies uses a lot of the same systems, switching to optical communication would use entirely different systems (some of which have not even been built yet).

Antenna size is not the problem with reception. It's white gaussian noise over the deep space channel progressively overcoming signal power as the spacecraft recedes. The channel capacity never drops to zero, but as power drops, data rates get lower and lower.

Some other relevant details from http://voyager.jpl.nasa.gov/Proposal-2008/Voyager_SR_2008.pd...:

"Voyager 1 is escaping the solar system at a speed of about 3.6 AU/year while Voyager 2 is leaving at about 3.3 AU/year. Power is the limiting lifetime consumable. The two spacecraft have power to continue returning science data beyond the year 2020. It is likely that at least one of the spacecraft could enter interstellar space while adequate power is still available. All other consumables are adequate for continued operations well past 2020."

"Electrical power is supplied by three Radioisotope Thermoelectric Generators (RTGs) which are performing nominally. The current power levels are about 286 watts, with power margins of about 34 watts and an average degradation rate of about 4.3 watts per year."

"Real-time telemetry data capture is accomplished using 34- and 70-meter tracking antennas of the DSN. Capture of the recorded high rate plasma wave data from Voyager 1 requires the use of an array of 70- and 34-meter antennas."

"Uplink communication is via S-band (16-bits/sec command rate) while an X-band transmitter provides downlink telemetry at 160 bits/sec normally and 1.4 kbps for playback of high-rate plasma wave data."

Just wondering, is voyager going at the maximum speed we could we realistically go today or would a newer craft be able to exit the solar system faster? would that even be useful to science?

Well, rocket technology hasn't really advanced all that much since Voyager was launched. We could certainly send a faster probe out by spending more money to build a bigger rocket. On the other hand, they could have done so back then, as well.

On the other hand, Voyager picked up an awful lot of its speed not from its initial launch but from slingshotting around the various planets it visited -- essentially picking up some free energy at the cost of infinitesimally perturbing the planet's orbit. If our goal was to send a probe out of the solar system as quickly as possible, we might be able to do a better slingshot than was done back then.

On a third hand, Voyager 2 benefited from a rather fortuitous arrangement of planets which enabled it to visit all four giant planets and presumably pick up some momentum from each. This kind of configuration only comes around once every few centuries so if we launched it now then it would have to miss some planets. I don't know how much of an effect this would have; did the Uranus/Neptune visits "pay for themselves" momentum-wise? (Edit: I looked it up and apparently not, because Voyager 1 which skipped the last two planets is further out and travelling faster than Voyager 2).

One thing we can do now which we couldn't do back then is build a good ion drive. A probe with an ion drive and a nuclear fuel cell could keep accelerating for many years, so it would probably be able to outpace Voyager eventually.

So in conclusion, if we wanted to build something faster than Voyager I think we probably could. But in answer to your second question, no, I don't think it would be very useful to do so. There's not much that's particularly interesting beyond the solar system. We're still keeping Voyager alive because we might as well, but sending another probe out there with it would be a fairly low priority for missions.

With your three hands you clearly support the "Aliens messed with it" theory.

Or he's read The Mote in God's Eye/The Gripping Hand.

That will be Voyager 6, after it falls into a black hole and ends up being rescued by machines.

If you want to go to away fast, you could ride an ion engine. It gets you faster for the same mass than chemical rockets can. The concept was tested on the Deep Space 1 probe with great success. IIRC, a Vasimir engine (a variation on the ion theme) will be tested on the ISS. Ion engines are pretty much off-the-shelf (or, as much off-the-shelf as deep-space probe propulsion go) now.

Another alternative (that still would require some work) would be a solar sail, to be deployed after the probe uses the Sun as a gravity assist.

You could, conceivably, use both. You would shot the probe towards the Sun with ion engine on. Ideally, it would reach escape velocity from the Sun during this first leg. When passing close to the Sun, it would deploy the solar sail and get as much impulse from that as possible - discounting the sail mass, it's essentially free. When the solar sail is spent (it's a very thin foil, degrades and becomes less efficient as it gets farther from the Sun) the probe jettisons it, turns on the ion engine again and rides it until all the propellant is spent.

That should give it some serious speed.

Mind you, speed was not Voyager's priority. Voyager 1 and 2 were great sightseeing tours of the outer solar system. The fact they still work is a testimonial of the genius of those who built them.

Doesn't it seem like, as part of the engineering part of this project, that they would have a system in place that would sweep through and do a comparison for "flipped bits" and, if found, would keep a clone of what the memory should look like and reset those bad bits? What happens when the flipped bit causes the machine to rotate out of communications view? You would basically be dead in the water without a failsafe system like this. What am I missing?

This is Voyager 2. Launched 1977. This wasn't an era where you could accidentally lose 8GB because it fell out of your cell phone and subsequently the SD-mini card flew out of your pocket when you went to answer it. There were various failsafes (wikipedia mentions one triggering accidentally after launch: http://en.wikipedia.org/wiki/Voyager_2#Launch_difficulties ) but keeping around a few copies of everything would probably have been a bit expensive.

Furthermore, JPL uses rad-hardened electronics that's far behind the ordinary earthbound stuff of the time.

And besides, it's run just fine for 33 years, so the original design obviously wasn't too badly flawed.

What if the system to guard against this itself got a flipped bit? What if the malfunction caused it to think bits in "normal" memory were flipped when they weren't, really?

http://en.wikipedia.org/wiki/Fault-tolerant_system#Fault-tol...

Quite true. One standard solution is TMR:

http://en.wikipedia.org/wiki/Triple_modular_redundancy

I doubt that Voyager used TMR, they probably used the best rad-hardened hardware they could build at the time (within power and weight constraints).

"Ed told me that that software dump has been completed, and it's 'just a matter of time to determine which bit is flipped.'"

Can they not just diff it?

Against what, Voyager 1? :)

They've got a live memory image, tracking down a possibly bad bit is probably not trivial. Given the era the code was written there could be self-modifying code in there too.

Against the original binary uploaded to the spacecraft, I would hope.

They've got to be simplifying this a lot for the sake of making a press report understandable. They don't even know there's only one bit flipped. If there is one, they will have to do some analysis to figure out that it would really cause the problem.

I wonder if they have a Voyager VM.

Even if they did, it might prove challenging to feed it exactly the same input as Voyager 2 itself in order to ensure it's in the same state.

I'm going to presume that they know what they're doing rather than pretending I know anything at all about operating a 30 year old interplanetary spacecraft.

What I would guess is that it's a bit tricky to "just diff" an entire live memory dump (voyager doesn't have a hard drive, just ram and a no longer used tape drive), they basically need to debug the whole thing and figure out exactly what code lives where in memory, down to every last bit. I imagine this might be a non-trivial task.

And they need to figure out which errors are critical. Some of the bits might be flipped without causing any damage.

Does somebody know how long it takes to send a message to Voyager 2? I calculated roughly 12 hours...

From http://en.wikipedia.org/wiki/Voyager_2, There are regularly posts of the current distance of Voyager 2 to earth in light-travel time to twitter. And Information on the current location of Voyager 2 can be found at HeavensAbove. at http://www.heavens-above.com/solar-escape.asp which looks like a seriously interesting page in its own right.

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