# Another insterstellar space travel question: radio transmission? **URL:** \ **Category:** Factual Questions\ **Created:** [April 1, 2010, 8:24pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641 "2010-04-01T20:24:20Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [April 1, 2010, 8:24pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/1 "2010-04-01T20:24:20Z")
Let’s say we find a way to fuel a spacecraft and can accelerate it to some significant percentage of the speed of light. Assuming that it’s sending back information the whole time, would there need to be any special considerations for the recievers on earth to account for the signal being “red-shifted,” so to speak? Presumably if so, the probe would need to allow for it too, right? And secondarily. . . What was the frequency did the Moon Missions? ---
**Author:** ![appleciders](https://avatars.discourse-cdn.com/v4/letter/a/85f322/32.png) [@appleciders](https://boards.straightdope.com/u/appleciders)\ **Post date:** [April 1, 2010, 8:31pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/2 "2010-04-01T20:31:42Z")
You could adjust your transmitter on your spacecraft to broadcast on a frequency that will be red-shifted to what the receiver on Earth wants to hear, or you could adjust your receiver on Earth to the frequency that you expect to hear. You shouldn’t need to do both. This is a pretty trivial problem; we solved it a few times in my high school physics class, I recall. The only thing you’d probably want to do is pick a frequency that doesn’t have a lot of noise on it; there are frequencies that are pretty empty compared to the rest, and so you’d be able to detect a weaker signal on them. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 1, 2010, 10:37pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/3 "2010-04-01T22:37:39Z")
In practice, you’d probably want to make the adjustment (if it’s needed at all) on Earth, because everything’s cheaper here than it is in space. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [April 1, 2010, 11:22pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/4 "2010-04-01T23:22:51Z")
Slight side-track. What would be the limits on range of a broadcast like that, given technology we can reasonably expect to have in the next few hundred years? Could we really blast out a signal ‘loud’ enough that it could be received clearly on earth? Do we have any idea how much bandwidth would be available? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [April 2, 2010, 11:17am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/5 "2010-04-02T11:17:02Z")
We already have to make adjustments for Doppler shift just to hear solar system probes. There was a problem involving this with the Cassini-Huygens mission. Cassini-Huygens is a mission to Saturn and its moon Titan. Cassini is still active in orbit around Saturn. Huygens was a probe of Titan. It was released as the probe approached Saturn and directly entered Titan’s atmosphere. It was too small to transmit data back to Earth, so Cassini had to act as a relay. But they didn’t properly allow for the Doppler shift between the two craft: > [@Wikipedia](#): > > It turned out that Cassini was unable to relay the data correctly. > > The reason: under the original flight plan, when Huygens was to descend to Titan, it would have accelerated relative to Cassini, causing the Doppler shift of its signal to vary. Consequently, the hardware of Cassini’s receiver was designed to be able to receive over a range of shifted frequencies. However, the firmware failed to take into account that the Doppler shift would have changed not only the carrier frequency, but also the timing of the payload bits, coded by phase-shift keying at 8192 bits per second. > > Reprogramming the firmware was impossible, and as a solution the trajectory had to be changed. Huygens detached a month later than originally planned (December 2004 instead of November) and approached Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift. From [here](http://en.wikipedia.org/wiki/Huygens_probe#A_critical_design_flaw_resolved) ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [April 2, 2010, 11:35am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/6 "2010-04-02T11:35:25Z")
> [@chrisk](#): > > Slight side-track. What would be the limits on range of a broadcast like that, given technology we can reasonably expect to have in the next few hundred years? Could we really blast out a signal ‘loud’ enough that it could be received clearly on earth? Do we have any idea how much bandwidth would be available? The main limit is how large an antenna dish we can send along on the mission. And also how large of one we can use back here. With sufficiently large dishes, we could send info back from the destination star. Ideally, we’d use a very large radio telescope for the home station, preferably one placed in a radio-quiet environment. The far side of the Moon, for example, or orbiting around some other planet. Somewhere away from Earth, though, since this planet is a very radio-noisy place. The bandwidth will probably not be very high but exactly how high depends on the size of the dishes. However, it should easily beat the single-bit replies from the interstellar probes of Niven’s Known Space series. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [April 2, 2010, 2:38pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/7 "2010-04-02T14:38:58Z")
> [@dtilque](#): > > However, it should easily beat the single-bit replies from the interstellar probes of Niven’s Known Space series. What? What Known Space book or story is that from? ---
**Author:** ![IAmNotSpartacus](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@IAmNotSpartacus](https://boards.straightdope.com/u/IAmNotSpartacus)\ **Post date:** [April 2, 2010, 3:22pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/8 "2010-04-02T15:22:47Z")
> [@Earl\_Snake-Hips\_Tucker](#): > > And secondarily. . . What was the frequency did the Moon Missions? Depends on what transmissions you are referring to. Voice? Telemetry? Video? Data? ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [April 2, 2010, 5:07pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/9 "2010-04-02T17:07:10Z")
> [@chorpler](#): > > What? What Known Space book or story is that from? In Known Space, systems were first scouted by ramrobots. They transmitted back whether there was a planet suitable for human habitation. If one was found, then slowboats filled with colonists in cold sleep would be sent to the planet. The problem was that the the probes were inadevertently programmed to find a habitable point, not a habitable planet. So the colonists often arrived to find planets that presented some severe challenges. ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [April 2, 2010, 5:53pm UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/10 "2010-04-02T17:53:48Z")
> [@Lemur866](#): > > In Known Space, systems were first scouted by ramrobots. They transmitted back whether there was a planet suitable for human habitation. If one was found, then slowboats filled with colonists in cold sleep would be sent to the planet. The problem was that the the probes were inadevertently programmed to find a habitable point, not a habitable planet. So the colonists often arrived to find planets that presented some severe challenges. Yeah … I know. But why does that imply a single-bit reply? I mean, isn’t it more likely that the ramrobots sent back replies like “Hey! Come visit this planet in orbit around Procyon! It’s great! The winds are just right! Here’s all the data I’ve gathered!” because it landed in the winter and not during the “windy season”? Or “Found an awesome planet around Tau Ceti, very earthlike, and great views!” because it landed on Plateau’s single habitable plateau? ---
**Author:** ![Lemur866](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lemur866/32/434_2.png) [@Lemur866](https://boards.straightdope.com/u/Lemur866)\ **Post date:** [April 3, 2010, 4:12am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/11 "2010-04-03T04:12:19Z")
Well, the 1 bit transmission would be “habitable point found”, presumably if no habitable point is found, no transmission is made. Since in the Known Space stories the colonists knew nothing about what they would find at the end of the journey, the 1 bit transmission seems to be a reasonable inference. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [April 3, 2010, 4:23am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/12 "2010-04-03T04:23:02Z")
The way I remember it (and it’s been years since I read any of those stories, so I may be misremembering) is that once the robots found an inhabitable location, they sent back a single message saying “Yes” and no other details. I don’t think it ever said what they did if they never found one. They either never replied or eventually timed out and sent a different message saying “No”. Either way it sounds like a single bit to me. ---
**Author:** ![figure9](https://avatars.discourse-cdn.com/v4/letter/f/f0a364/32.png) [@figure9](https://boards.straightdope.com/u/figure9)\ **Post date:** [April 3, 2010, 4:24am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/13 "2010-04-03T04:24:30Z")
If the ship was traveling at 0.9c wouldn’t we sound like chipmunks to members of the crew when they got our message? Wouldn’t their time be moving slower than our time? ---
**Author:** ![chorpler](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chorpler/32/2888_2.png) [@chorpler](https://boards.straightdope.com/u/chorpler)\ **Post date:** [April 3, 2010, 5:18am UTC](https://boards.straightdope.com/t/another-insterstellar-space-travel-question-radio-transmission/534641/14 "2010-04-03T05:18:51Z")
I don’t think we can infer that it was only a single bit, or that they knew nothing about what they’d find. All we know is that the probes didn’t survey the entire planet over the course of a year like they should have; they just found a habitable point. They may well have sent back tons of data about that point (along the lines of “The weather’s great! Here’s the atmospheric mix and gravity!” and whatnot) and then shut down. On the other hand, they probably didn’t send back images of the planet, or Plateau, at least, would have raised big questions, looking exactly like Venus…