Voyager 2 Still Chugging Along

Voyager 2 is in the science news again today. It seems our little probe launched back in August 1977 is crossing the termination shock boundary of the solar system on its way out into interstellar space. That's a region around 7.8 billion miles away from the sun where the solar wind begins to slow dramatically. Voyager 1 crossed this boundary already but apparently had instrumentation problems and didn't send back any cool data about it.

Apparently this crossing was somewhat unexpected. The boundary was thought to be another 1 billion miles out. This has led scientists at NASA to conclude that something "out there" is pushing against it. No doubt there's an interstellar current of some sort which is pushing against the flow coming out from the sun that caused the Heliosphere to get squished. This was once thought to be more or less spherical, but the new data suggests it's more egg shaped.

NASA is hoping the batteries on board the Voyager probes will last long enough to gather data upon crossing the Heliopause, which is the boundary at which the solar wind is stopped by the flow pushing against it from interstellar space. The estimates on arrival with this are another 7-10 years, with the batteries expected to last until 2025 or so.

When you stop to think about it, the fact that the sun exerts this much of an influence on things 10-15 billion miles away is just unreal.
.........................
"It is pointless to resist, my son." -- Darth Vader
"Resistance is futile." -- The Borg
"Mother's coming for me in the dragon ships. I don't like these itchy clothes, but I have to wear them or it frightens the fish." -- Thurindil

Well. I guess that's that then.

       
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Posted on Dec 11, 2007 7:38 am by Samson in: | 13 comment(s) [Closed]
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Actually, I was thinking it was pretty impressive that the batteries in the voyager probes launched back in 1977 are still expected to last through 2025.. why can't they put batteries like that into our cars, laptops, cell phones, toys, etc?? *sigh*

For that matter, it's fairly remarkable that these probes they launched back in 1977 are still able to transmit usable data all the way back to us even now, from so far away.. I wonder what the time delay is on that data's arrival, it's not as though the data is being transmitted back to us at the speed of light or via subspace afterall..

       
I have a feeling the reason the batteries we have don't last as long as they could is because people like not being radiated to death. The probe batteries are radioactive. They're also using minimal power, maybe 20 watts to transmit.

The signals it sends back are standard radio transmissions, so they do in fact travel at the speed of light. Now, unless I got the math horribly wrong....

7.8 billion miles / 186,000 miles/sec = 41935 seconds for the transmission to reach Earth.
41935 / 60 = 699 minutes.
699 / 60 = 11.65 hours.

So it takes about half of one day for anything it sends out to reach us, which still makes it technically feasible for NASA to send command updates out to the probe - they just take the same half day to get back.

       
*shrug* We've already had folks accuse the cell phone companies of making the phones radioactive.. personally, I've been advocating nuclear power cells for cars for years. *shrug* ...but I suppose for cell phones and laptops we sould probably work on shielding first.

You really think 20 watts would be enough transmitting power to send a radio signal all the way back to earth from billions of miles away? The maximum legal CB power output level, in the U.S., is four watts for AM and 12 watts which is not enough to exceed 155.3 mile range, in accordance with FCC mandates.

Do all radiofrequency wavelengths actually travel at the speed of light?

A roughly 12 hour transmission delay each way sounds plausible enough, as long as we're using nice round numbers and assuming the transmission speed, etc.

       
       
I think you've probably got your wattage figures wrong, either that or everyone we knew back in the 80s who had CBs were all breaking the law on a daily basis. My dad had a CB radio in his car that by itself had a power output of 200 watts. With the amplifier he had installed, that boosted total output to 1200 watts. He could talk to anyone in the world. Using what was called "sideband" back then. I never understood the gory details of it. Our base unit was larger than a full PC tower is today, got at least as hot, and pumped out 4000 watts with amplification. We also had a 60 foot sigma antenna on the roof which the homeowner's association tried to force us to take down before my dad made some legal threats against them. It was all quite legal and FCC approved.

As far as I remember from physics class, all radio waves travel at or very near the speed of light, along with everything else in the EM spectrum. So I don't think there's a problem with our math. And yes, 20 watts would be more than enough to send a transmission 8 billion miles back home. There aren't any other radio sources in deep space with that kind of power behind them. The only reason low wattage output on Earth wouldn't survive is because you'd be drowned out by AM band stations transmitting in the 25K-50K watt range. NASA probably also uses a different frequency spread than civilians have access to.

       
Yup, not only was he breaking the law by "shooting skip" and using more wattage output than was legal, I'll be he also was breaking the law by not obtaining the legally required license and call sign issued by the FCC, unless instead of CB he was ham operating under license... If it was legal, your father wasn't using CB, he was using ham (amateur radio), and probably was licensed. Not the same thing. (Btw, the base unit you're describing is typical of an amatuer radio set-up as well, not a CB radio set-up.) While I also dislike using wiki as a reference, it's quick and easy and fairly accurate: http://en.wikipedia.org/wiki/Citizens'_band_radio of course, if wiki's not good enough for you, you can always visit www.fcc.gov and get the scoop on CB, Amatuer, Radio, Television, Cell Phones, etc.. (One stop shopping and all that)

Well, you might be right (or close enough) with regard to the transmission speed, physics classes were a long time ago. But no, there are plenty of other radio sources in deep space that are putting out far more power than that, otherwise radio telescopes wouldn't work and the whole field of radioastronomy would be a bust. Quasars alone would drown out a 20 watt transmission while still billions of miles from us, and we won't even get into the combined effect from them and pulsars and... But,yes, I'm sure they use a different frequency range than civilians do which only means that the FCC is more than happy to let them transmit with whatever wattage they need/want to behind them. I think you'd find the information about wattage as applied to transmitting power available at www.fcc.gov highly enlightening. For example, an FM class A license (the highest transmitting power one can obtain license for, at least as a private citizen) allows one to transmit at 6k watts, not 25-50k watts.

       
Interesting read on wikipedia. If all of his stuff was supposed to be restricted to 12 Watts ( we had SSB equipment ) then everyone in the neighborhood, and nearly all of the local truckers who rolled through that we talked to were all flagrant law breakers since we were all running equipment that ran into the hundreds of watts, and base units into the thousands. We tended to hang out mostly on channels 17, 19, 21, and 40. 40 tended to be pretty dead so we used it when we wanted less traffic to deal with. The equipment we ran was powerful enough to blanket the 49Mhz range with junk, which if you're up on your frequencies is the range channel 2 operated on in the TV bands. :)

As far as FM/AM wattages, if what you say is true then every radio station broadcasting in Southern California is breaking the law because they all advertise wattage values anywhere from 25K to 50K - with the 50KW guys bragging about it regularly on the air. Unless the government doesn't consider a broadcast corporation a "private citizen" of course.

You'd also be surprised just how super sensitive radio astronomy telescopes are. The really big ones like Arecibo, and the arrays like the one in New Mexico, are all capable of picking up radio signals in the milliwatt range. Barely enough to rise above cosmic background radiation. That's the whole point of equipment like that. It's built to be sensitive enough to pick up a walkie talkie on Mars, like we'd ever use one :)

       
Yes, but todays standards are a lot different from what they were years ago. and it is Samson's Grandfather we're talking about here.

FM Class A is a private Citizen license, not the same license a radio station gets.

       
No, it's my father we're talking about here. And myself to a limited extent since I wasn't really all that into the whole CB thing that much.

       
Sadly, yeah, back in the 70's most truckers and, even more so, most of the neighborhood kids were defintely flagrantly breaking the law when it came to licenses and call signs and often the power boosting done to their CB rigs, but everything you've said about your Dad's set-up so far really sounds like a ham operator's set-up. He undoubtably could also pick up (and contribute to) all the CB traffic, but he was really way beyond what the CB radios could do.

No, commercial radio stations aren't considered "private citizens" and without spending an inordinate amount of time/effort going through a slew of documents in various formats I couldn't find on the FCC's site what the current legal limits are for them, still, even the major radio stations that constantly brag about transmitting at "50,000 watts of power" still only reach about 80 miles under fairly ideal circumstances generally.. on the other hand, without the curvature of the Earth and the atmospheric interference, who knows how far those signals may reach once they leave our planetary shell?

I think I've still got a 2.5 watt walkie talkie in my car.. if I take that to Mars on my next trip there (like that's likely this lifetime), I should be able to chat away with the scientists in California and New Mexico then, eh? Oh, sure, they're be a good 7+ minute transmission delay each way, and they'd only have monitoring equipment initially if they even heard me via that... but I'm sure we'd be able to have a jolly good chat eventually.. oh wait, my walkie talkie radio wouldn't be able to pick up their responses anyway would it? You know, given that here, dirt-side, I can pick up maybe half a mile away if conditions are good... Hmm.. which raises the other end of the problem for those probes too... they need quite a set of antennas and radio equipment on-board if they're going to be transmitting two-way communication back to Earth a few billion miles or so...

       
My dad's stuff might well have been modified ham equipment, but who knows. He doesn't have it anymore and I didn't want it when he got rid of it. But I'm also rather certain that the mobile units he owned were also well beyond what CB radio is described as on Wikipedia. IT leads me to question Wikipedia's accuracy since even the mobile units we had were equipped with serious linear amplification.

FM stations transmitting around 50K watts only reach out about 120 miles or so because FM signals go straight out from the antenna and don't bounce off the ionosphere. So yes, the curve of the Earth is the limiting factor in why you can't hear LA radio stations in Phoenix even on the best of days. But if you're in the line of sight of the transmitter 300 light years away, then you'll have no trouble picking it up.

The same principle applies to NASA equipment. They're highly focused directional signals. If you swing even 1/2 of a degree off axis, you lose the signal entirely. That's how they can get away with using so little power over so much distance. I'd need to look it up to be sure, but I swear I've heard it quoted from several sources that the lag time to Mars for "2-way" communication is 40 minutes. I'm assuming that's 20 each way. But that also depends on the distance Mars is from the Earth at any given moment. Either way, a 2.5 watt transmitter would be more than enough. I don't think the rovers use much more than that for their own radio signals.

       
Indeed, Connor has made the connection. It's all in the antenna. The more directional your send/receive, the less power you need to get the same distance. Any sort of electro-magnetic wave system works this way. Woot for gain and signal beaming.

       
Call me crazy, but I thought I was the one who pointed out directional beaming :P

In any case it looks like the math is more or less right. Transmissions to Mars take anywhere from 3 to 19 minutes based on the distance between it and the Earth. So the round trip communication can be as little as 6 minutes.

The power requirements listed at http://www.uhf-satcom.com/dsnspace/ range from 12-65 watts depending on what the probe is doing. There's also some ERP figure that indicates hundreds of thousands of watts but I'm not sure how that's supposed to work.

       
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