Hacker News Reader

Comment by hapless | Hacker News Reader

Transit 5B-5, one of the two very early satellites mentioned, seems to have carried a very early RTG.

I wonder if it is still running on its nuclear power.

For those that were wondering:

> Radioisotope Thermoelectric Generators, or RTGs, provide electrical power for spacecraft by converting the heat generated by the decay of plutonium-238 (Pu-238) fuel into electricity using devices called thermocouples. Since they have no moving parts that can fail or wear out, RTGs have historically been viewed as a highly reliable power option. Thermocouples have been used in RTGs for a total combined time of over 300 years, and a not a single thermocouple has ever ceased producing power.

Source: https://solarsystem.nasa.gov/rps/rtg.cfm

I had to look it up as well. I am amazed that voyager 1 is running on about 470 watts of power and is still able to transmit an intelligible signal to earth. Im also wondering how much a transmitter that was capable of the Ghz range cost back in the mid 70s when it was built. Incredible stuff.

GHz transmission in the 1970s is not special. The cavity magnetron was invented during WWII and used for RADAR purposes and not long after microwave connections were being used for long distance telephone connections.

The magnetron is a good point! But for RADAR applications you just need a big source of emissions that can be pulsed on and off. The magnetron does that well enough. I have never heard of anyone using a magnetron modulated in some scheme to transfer information at any some useful rate. It's definitely possible with gunnplexers. You could certainly do more code with a magnetron, but even that would be difficult given their unstable output frequency.

The transmitter uses only 22 watts. A cell phone is about 2 watts. It's striking how little it takes.

The choice of antenna is a lot more important than transmit power. Especially for an application like this where high-gain is needed.

It also helps to be in space, where there's nothing to obstruct your signal. Barring obstructions, radio waves propagate indefinitely.

There's still inverse square law at play, although it's much more forgiving than atmospheric attenuation (logarithmic db loss with distance versus linear).

Microwave towers were pretty common in the 70s, and were used before then for long distance telephone links. I can't imagine that they were all that more pricey than any other transmitter.

As for signal transmission, consider that the radio noise is 100 times stronger than the GPS signal your phone's nav software is trying to decipher. Now imagine a humongous antenna with much more processing power than your Samsung Galaxy. Though amazing that we can still make sense of Voyager's signals, it's not quite to the "miraculous" category.

The two Voyagers are often seen downloading data through NASA's Deep Space Network. Here's a real time DSN status page: http://eyes.nasa.gov/dsn/dsn.html

What a neat website! Anyone who is reading down the comments and hasn't checked it out, you should!

Thank you. I knew about the DSN but didn't know there was a status page. Quite fun.

That's pretty amazing. I am guessing that this wins the reliability vs power tradeoff. I wonder how much more efficient a more traditional system is vs an RTG.

5B-5, specifically, is not powered by an RNG, however other satellites in the same series were[1]

[1] - Satellites By Michel Capderou, pg 214

Reply on news.ycombinator.com