Sending Info at Relativistic Speeds? | Doppler Effect | Horizons Photo

[MostlyHarmless]

I just read this article:

http://abcnews.go.com/Technology/pl...aze-revealed-photo-horizons/story?id=33832751

The article itself is really cool, but something at the bottom of it caught my attention.

Traveling at the speed of light, signals take 4.5 hours to travel 3 billion miles to reach Earth, meaning the spacecraft has an enormous undertaking ahead of it. With data downloading at a rate of approximately 1 to 4 kilobits per second, it's expected the entire bounty of science from the encounter will take one year to be transmitted back to Earth.

I don't know how fast New Horizons is traveling, but say, for example it was traveling at relativistic speeds. How would the information be effected, in terms of the Doppler Effect. The information is being transmitted via some form of EM waves, right? So they should be susceptible to such an effect. Suppose they were effected, then would the information actually be effected or just the waves?

Also, I understand the satellite is billions of miles away, but why is the transfer rate so incredibly slow? Why is that it just takes the initial bits of information several hours to arrive, a reasonable transfer rate is achieved? Is this due to the Doppler effect?

I guess its an incredible feat to just be able to send a signal from that far away, it just seems like the receiving part should be the challenge, not transfer rates.

I'm having a hard time wrapping my head around sending a picture 3 billion miles across space to a "pale blue dot". And then reconciling my tenuous grasp on how waves behave.

 

[rootone]

Traveling at relativistic speeds away from Earth would cause the wavelength of the transmission to get longer, (aka redshifted), as far as the receiver is concerned.
The signal would still travel to Earth at the same speed since the speed of light is constant, but the encoded data would be stretched out to the same extent as the carrier wave, so the data rate would be reduced.
New Horizons is not traveling anywhere anywhere near fast enough for this effect to be significant though.

 

[phyzguy]

The slow data rate is caused by the weakness of the signal. This is caused by the inverse-square law, not the Doppler shift. The spacecraft can probably only transmit a few 10's of watts (I don't know exactly). At 3 billion miles away, this makes the signal extremely weak, so slow data rates are needed in order to be able to integrate long enough to distinguish the signal from noise. If the spacecraft were traveling faster, this would just shift the signal to lower frequencies from that at which it is transmitted. I don't think this would be a big problem until it got going a significant fraction of the speed of light.