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Comment by tectonic | Hacker News Reader

From the last issue of Orbital Index (https://orbitalindex.com):

> Today’s mind-blowing stat: Voyager’s transmitters use just 23 watts, roughly the same as an incandescent refrigerator bulb, yet we are able to interpret the 0.1 billion-billionth of a Watt that makes it to our 70 m dish from 11 billion kilometers away. Both spacecraft are expected to last another 5 years, until their plutonium batteries decay beyond usefulness and they drift with our golden record more or less forever among the stars.

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23W with extremely high gain though. It was difficult to find a number, but this source [0] gives the gain at 47dB (dBi or dBd isn't specified). With that high of gain, the effective radiated power is over 1 million Watts (because it is concentrated into a very narrow beam).

Also using phrases like "0.1 billion-billionth of a Watt" is misleading, and not how radio signals are actually described. For example 0.1 million-billionth of a Watt sounds pretty small too, but describes the power level of a usable, and not particularly uncommon LTE signal level.

[0] https://www.quora.com/How-can-Voyager-send-a-signal-strong-e...

Great quora link, thank you for posting.

Here's a related thought: if we ever manage to send a probe to the closest star, what a tough problem communication with home will be. The distance is about 2000 times higher than where Voyager is now. So the signal will be about 4 million times weaker due to the quadratic attenuation alone, and maybe 100 million if we consider the absorption by the interstellar medium. On top of that, when the probe will phone home, it's going to be a few seconds of arc away from a phenomenally bright source of electromagnetic radiation: the star itself.

You can fix this by using a bigger transmitting antenna (but Voyager's is already 3.7m, not small at all), or a bigger receiving antenna (the current ones are 20m). Or a stronger signal. But it appears we'd need to go from 23W to GigaWatts .

One thing is sure: we won't be able to send a pound-size probe.

The gain of a 3.5m parabolic antenna on the tx end, and they're using from 45 to 70 meter size dishes on the earth end to communicate with them. The NASA goldstone 70m dish has some serious gain.

I heard a rumour that by the time they've gotten to another solar system, they will have evaporated into a metallic cloud; that's how long this next part of their journey is.

Why would they? They will be very cold. Occasional cosmic rays aren't enough to evaporate them. They can last hundreds of thousands of years, if not millions.

Space is mind boggling large and sparse. Millions of years is not a long time to be traveling through it.

* even at light speed

And how long until they reach the next star? They're not pointed anywhere in particular. We are talking about millions of years here.

Millions of years? Fat chance. A few days ago this post https://news.ycombinator.com/item?id=21327269 explained that during the galactic collision of the Milky Way and Andromeda in 4B years, no two stars will likely collide because of the sheer amount of empty space. Keeping that in perspective, I highly doubt that Voyager would find its way anywhere in some mere millions of years.

And even if they were.

Think we can point the James webb at one and get a farewell Pic?

do you mean the plutonium batteries or the probe itself?

The whole probe itself.

Technically you're talking about sublimation rather than evaporation. And no, it won't be very likely for the Voyagers to disappear. Most likely they will be damaged by impact or get absorbed by massive object, but sublimation at 10K will take a very very long time.

I don't have the data, but if you want to calculate the rate of sublimation, you need to look at the vapor pressure of various metals at interstellar temperatures.

Some very low numbers indeed.

But they're in interstellar space. I imagine high-energy collisions (with hydrogen atoms or loose protons) would be a more likely cause of evaporation, and I don't know the numbers for that.

If there’s 1 atom per cubic cm in interstellar space, I’m finding it takes about half a million years to collide with a mole of hydrogen (at 17km/s)

Google says that's about a gram of hydrogen. Is that enough to destroy a craft? I have little intuition, but I imagine touching a gram of 55,000-degree (Farenheit) anything would be destructive.

Will the craft be radiating heat away from itself?

Makes a mockery of the amateur radio “miles per watt” awards :)

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