Not of Arecibo, of the satellite. Do you need to authenticate with the probe?

I doubt it's more complicated than knowing the language and having the proper transmission hardware. 1978 seems a little early to be worrying about hacks of that variety. That said the hardware could also have some kind of encryption built in. I'll ask if I get a chance, though - I hope to get some details from the project soon.

> 1978 seems a little early to be worrying about hacks of that variety.

Interesting, then, that the scenario already appeared in fiction by 1950 (in the Tintin story Objectif Lune/Destination Moon)

I think pointing at it is probably the most difficult part.

No, the probe has no onboard computer. You just need a big enough dish (or network of dishes) and Arecibo is the only thing big enough right now.

> Arecibo is the only thing big enough right now. Not entirely true. How did they control it in the 1970's?

They chose Aricebo because it allows them to use very low power antennas to communicate. The size of the dish makes up for the lower power transmission.

In the past, they used high-powered antenna arrays.

Arecibo's main transmitter is in the Megawatt range, hardly 'lower power'.

For short pulses momentary power output reaches Terawatts!

Very true. Keep in mind, though, higher power is more expensive.

They've said in the past (I'm failing to find the reference right now) that they chose Arecibo because of the high-gain of the antenna. I believe that the amplifiers that they're actually using above the dish are somewhere around 700 Watts.

Antennas with large effective apertures (of which Arecibo's is the largest) are considered high gain antennas, which have small angular beam widths. Friis' Transmission Equation shows that you could use low-power transmitters, but still maintain high boresight with a sufficiently large antenna surface. :)

Yep, here's the amplifier:

http://spacecollege.org/isee3/isee-3-reboot-amplifier-instal...

It is an AR 700S1G4 with rated minimum output power of 700W, and will be used in S-band. Here's the datasheet:

http://www.arww-rfmicro.com/post/700S1G4.pdf

At these frequencies, the Arecibo dish has a gain of around 72dBi, putting the EIRP at about 7 gigawatts.

For scale, a typical wifi router has an EIRP of around 50 milliwatts, so this is about 140 billion times more powerful.

Thanks for digging that up!