8 ms·
Why it takes so long to get data back from New Horizons
- sytelus 11y agoSo looks like the reason is 4 kbps download link due to various reasons.
- mrb 11y agoI dislike that the article doesn't even clearly answer its own question. The probe only records 8GB of data to one of two 8GB memory bank (one primary, one backup). Then the data is compressed and sent at either 2 kbit/s or 4 kbit/s (if they manage to get the higher speed working). By this math it should take either 12 or 6 months. Actually even less since the data is compressed. So why will it take 16 months?? Gizmodo needs better writers.
- maaku 11y agoBecause redundancy in transmission? Also I would be very surprised if the the data is not already compressed.
- CamperBob2 11y agoOne possibility might be DSN receiver availability. There are only three sites in the network, and they have several other missions to support.
- aleph_naught 11y agothe DSN (deep space network) is a shared resource under high contention---it is effectively used by every deep space mission from Voyager to MSL(via MRO relay). missions generally have a dsn rep that negotiates dsn tracks based on need. high priority events like MSL's EDL or spacecraft safing events generally take priority. i haven't looked into the specifics but it is unlikely that new horizons gets 24hr coverage for 12 months straight.
- logicallee 11y agothis totally makes sense - you should work for comcast!
- simoncion 11y agoGiven Comcast's reputation, it sounds like you're being insulting. Was this your intention? If it was, you should actually read about the DSN. It (like most things pertaining to extraterrestrial operation) is pretty damn cool.
- jjtheblunt 11y agoI thought not meant to be insulting but hopeful comcast would then improve.
- logicallee 11y agoneither, it was just a joke :) I mean that this is a great way to explain 2 kbit/s or 4 kbit/s (makes sense in context) and I was just joking that that's about as bad as Comcast's service sometimes... :) (And so with explanations that great he should be their PR guy...) I guess it didn't go over too well, but no worries! You miss 100% of shots you don't take...
- waterlesscloud 11y agoYou can see what DSN is up to right now here- http://eyes.nasa.gov/dsn/dsn.html http://eyes.nasa.gov/dsn/dsn.html
- ArikBe 11y agoIn the press conference shortly after the flyby, one of the staff (I can't remember her name) said that the speed depends on the position of the probe relative to the ground antenna. So not only is transmission not 24 hrs, but when the probe appears to be on the horizon, transmission speeds are slower.
- hga 11y agoLately I've been checking out the real time reporting page for the DSN (http://eyes.nasa.gov/dsn/dsn.html http://eyes.nasa.gov/dsn/dsn.html), and frequently one of the three big dishes at Goldstone, Arizona, USA, Madrid, or Canberra is allocated to a satellite, but is not right then doing any communicating, presumably because it's waiting for the earth to turn enough to get line of sight. Right now New Horizons is downloading at 2.11 kbs to Canberra, the max with both amplifiers transmitting with opposite circular polarizations. For a bit of comparison, Rosetta is talking to Goldstone right now at 104.86 kbs, with a received power 3 orders of magnitude stronger. And Voyager 2, which doesn't have much to say, is still plugging away with a smaller Canberra antenna at 159 b/s, and a received power about that of New Horizons.
- rootbear 11y agoThe downlink is actually 1kbps, with 2kbps optional, by using both amplifiers with opposite circular polarizations. But they can't use that trick all the time. A nice article with more technical details is at http://www.planetary.org/blogs/emily-lakdawalla/2015/01300800-talking-to-pluto-is-hard.html http://www.planetary.org/blogs/emily-lakdawalla/2015/0130080...
- dang 11y agoOk, since the consensus in the thread is that the submitted article (http://gizmodo.com/why-itll-take-new-horizons-16-months-to-send-us-this-we-1717769317 http://gizmodo.com/why-itll-take-new-horizons-16-months-to-s...) wasn't very good, let's try switching to that one.
- mikeash 11y agoGreat article. And for anyone who didn't see it, to call this mode "optional" is putting it extremely mildly: to get enough power to run both amplifiers at once, they have to shut down the spacecraft's guidance system (!) and keep it pointed at Earth by using the thrusters to spin it up like a top. And this whole crazy scheme was developed after launch.
- deleted 11y ago[deleted]
- mgkimsal 11y agoisn't the fact that the transmitter is going further away during those 6 or 12 months adding to the time as well? or do the distances travelled not have much of an impact over that time frame?
- icw 11y agoForward error correction can add significant overhead. From the specification of the RF subsystem posted earlier (http://www.boulder.swri.edu/~tcase/NH%20RF%20Telecom%20Sys%20ID1369%20FINAL_Deboy.pdf http://www.boulder.swri.edu/~tcase/NH%20RF%20Telecom%20Sys%2...), 5/6 of the data returned is redundant.
- maaku 11y agoThe real reason, which hopefully gives some engineering insight, is why not? It's a flyby mission whose primary data collection occupied only a couple of days. Like any spacecraft design they had to choose between high bandwidth (more power == larger RTG == more weight) or large storage (more memory modules == more weight). There is some tradeoff between the two that yields the optimal science return, and one would expect it would involve months of transmission: storing things locally to transmit low power requires way more resources than a high gain antenna used for 1 week only.
- raverbashing 11y ago"and one would expect it would involve months of transmission: storing things locally to transmit low power requires way more resources than a high gain antenna used for 1 week only." You're contradicting yourself there. Memory today is cheap (even if we're talking about the radiation-hardened stuff, for "space mission" levels of cheap) Getting data for a long time takes a lot of resources (basically usage of Deep Space Network infrastructure) So, yeah, if they could they would download all the data more quickly, but you're right that the power budget wouldn't allow it (not sure how powerful it would have to be to get the data at, let's say 32kbps)
- acqq 11y ago> not sure how powerful it would have to be to get the data at, let's say 32kbps My back-on-the envelope calculation says the probe would have to emit with at least 6 times more power for that than it emits now (and I can be way off as I'm not taking into account all the competing noise at that distance). The RTG produces around 200 W for everything on the probe, and I estimate just the cost of the fuel for the current RTG alone in tens of millions. It's simply not worth optimizing for the fast burst when you expect the probe to be able to communicate for a few years more.
- Zancarius 11y ago> Memory today is cheap (even if we're talking about the radiation-hardened stuff, for "space mission" levels of cheap) New Horizons was launched in January 2006, but I suspect you know this and you're just being dramatic to make a point. :) The problem with "cheap" as a metric whose context is solely monetary is that it's ignoring maaku's fundamental argument: More storage is more weight; more transmitter capability is more weight. So it's not just the cost of the storage you're looking at but also the cost to get that into space. Launches are not cheap, and you have to account for every bit of spacecraft weight which leads to design tradeoffs. Sometimes you can have one or the other--not both. Interestingly, the New Horizons program has its roots dating back to the 1990s, at least according to Wikipedia, with cancellations due to projected costs exceeding levels NASA was willing to fund. But this is par for the course when dealing with long-term, long-distance missions. Everyone has an argument about what to put on board, but there's neither budget nor space to accommodate them all (sadly).
- nraynaud 11y agowhat speed is the spacecraft going? is the doppler effect an issue?
- Qantourisc 11y agoHardly, all you have to do is listen to a slightly lower or higher frequency. To be exact: 58536 km/h (spacecraft) divided by 1079252848.8km/h(speed of light) -> So that's frequency shift of 0.0054% (If I am wrong please correct me.)
- nraynaud 11y agoYeah, I might have been stupid. It has nothing to do with the carrier frequency (I was thinking low frequency vs high spacecraft speed, instead of propagation speed vs aircraft speed).
- rootbear 11y agoThe Doppler shift is tracked, as it tells them how fast (in radial terms) the spacecraft is going. Some years back, I watched the live coverage on NASA TV of the Cassini spacecraft going into orbit around Saturn. When they went into orbit, they didn't announce it with some Hollywood dialog like "Saturn orbit confirmed!" No, what they said was, "The Doppler curve has flattened!" and the room went wild! It was wonderful.
- david-given 11y ago...they actually forgot to account for Doppler shift when deploying the Huygens probe to Titan: as planned, the 5.5km/s velocity change as Huygens reentered Titan would have caused Cassini's receiver to lose lock on the signal (due to a timing change, rather than a frequency shift, IIRC). As Huygens had no storage and ran on batteries this would cause a total loss of mission. The problem was discovered after launch, and corrected for --- not by reprogramming the receiver, but by changing Cassini's flight profile to reduce the Doppler shift: http://www.thespacereview.com/article/306/1 http://www.thespacereview.com/article/306/1
- jafingi 11y agoIs there any error correction in the data stream? I mean, if we loose e.g. 1 minute of data, will it just be sent after the full data had been sent, or how do we ensure that we grt valid data over such a huge distance?
- tams 11y agoAll long range transmissions use forward error correcting codes which allow error detection and data recovery without retransmission (to a certain degree). See [1] (paragraph 2.5 on page 10) for a bit more on the kinds of FECs used. [1] http://deepspace.jpl.nasa.gov/dsndocs/810-005/208/208B.pdf http://deepspace.jpl.nasa.gov/dsndocs/810-005/208/208B.pdf
- jafingi 11y agoThanks! That's exciting reading.
- jkot 11y agoIt is actually VERY impressive if you account for distance, background noise etc.. It is transferring several gigabytes of data across solar system.
- Sharlin 11y agoAnd the fact that the transmitter uses less power than a light bulb.
- dharma1 11y agoLasers are the future. http://www.space.com/23350-laser-space-communications-incredible-technology.html http://www.space.com/23350-laser-space-communications-incred... Easier for shorter distances (I think Facebook is researching this for satellite networks?) however it becomes more challenging the deeper we go into space because the beam is narrow - but the bandwidth is fantastic.
- ginko 11y agoAt the distance from Pluto to Earth, Earth's diameter amounts to about 1.5 * 10^-4 degrees in angle. Precisely pointing a laser beam at a given target, even a large one, at that distance seems almost impossible. Especially since the space probe probably won't stay perfectly still but will always slightly tumble.
- BurningFrog 11y agoAs long as it can get feedback ("a little to the left") somehow, it seems doable to this software expert.
- icegreentea 11y agoIts a 5 hour time lag from Earth to Pluto - it's going to be one hell of a bad time doing feedback control on that link.
- mikeash 11y agoI'm guessing that the laser itself is wider than that. According to this fellow, a realistic beam width is 3.5 microradians: http://space.stackexchange.com/questions/2005/how-much-power-loss-does-the-llcd-suffer-in-transit/2006#2006 http://space.stackexchange.com/questions/2005/how-much-power... That's about 2e-4 degrees, wider than Earth as seen from Pluto.
- jhallenworld 11y agoIs the earth visible in an optical telescope from the spacecraft? This should be the basis for automatic tracking... (transmit the laser through the same telescope back to earth). The transmitter on earth could track on the "star" of the laser from the spacecraft. I've been trying to understand the noise temperature of erbium doped fiber amplifiers- not nearly as good as DSN masers.
- kayoone 11y agoA measly 25-30 years ago the first modems were sending data at 1Kbps, today we can send data through 7.5 billion kilometres of space at the same speed, i think that is pretty impressive.
- rootbear 11y agoThe data rate to Mars isn't shabby, at about 230kbps, roughly four times the speed of a fast dial-up connection. I used to point that out when the only Internet at my folks' house was dial up. Eventually, my iPhone cell connection beat their dial-up connection and I stopped complaining.
- kayoone 11y agoCool, do you have a source for the 230Kbps ? i love to read about stuff like that.
- karlshea 11y agoData rate to the Mars Reconnaissance Orbiter can be up to 6Mbit/sec: https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Telecommunications_system https://en.wikipedia.org/wiki/Mars_Reconnaissance_Orbiter#Te...
- jessriedel 11y agoWe could send a lot more data over that same distance if New Horizons had a large power supply. It was just limited by the plutonium supply available at launch.
- hliyan 11y ago"A 3-Billion-Mile Snapchat"? Is this what journalism has been reduced to now?
- drcross 11y agoWould someone care to comment on this hypothetical- If New Horizons was launch today instead of nine years ago, what equipment improvements would we likely to have the since that time?
- lmm 11y agoI don't think we'd see any qualitative differences at a high level. Just everything would be slightly better - higher resolution cameras and so on. Compare to Juno which was launched four years later.
- sophacles 11y agoDoes anyone know if the 2kbps/4kbps numbers are the transfer rate of actual data, or the link speed? I would presume that there are checksums or other error detection/correction schemes involved - does anyone know if that's a good assumption? If so what schemes are used and how much overhead they have?
- nullc 11y agohttp://www.uhf-satcom.com/amateurdsn/Paper-969.pdf http://www.uhf-satcom.com/amateurdsn/Paper-969.pdf < If you'd actually like to know more about the communications sub-systems of this spacecraft.