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It is written here that before the final descent Rosetta was programmed for "auto shutoff" after reaching the surface of the comet, to prevent the possibility of the further communications in the case of unlikely survival.

What is the reason for wasting time in programming such a behavior? If the probe is no longer required, why cannot it just be abandoned? And maybe it could take a couple of extra pictures from the surface of the comet before that, or at least finish the transmission of data collected during the descent? And if not, then not, since the chances are low.

While the team of tele-operators costs money, the probe has been flying for many years. It would be strange to hear that it is not possible to afford this team for the extra month or two.

This seems so strange that I even suspect that the "auto shutoff" may be a kind of urban legend. Could this be true?

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Nightrider
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    I am 99.9% sure that the answer is to prevent signal pollution, but I cannot find a source saying whether that is the only reason. – called2voyage Oct 04 '16 at 21:15

2 Answers2

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It is true.

As soon as Rosetta hits the surface, its main systems will be turned off, including the attitude and control systems, as well as the main transmitter, the latter in order to meet regulations aimed at avoiding interference on deep space network communications channels. The software that will enable this ‘passivation’ will be uploaded to the spacecraft a week prior to the planned end of mission, and it will be activated around the time of the collision course manoeuvre, approximately ten hours prior to impact. No automated re-activation will be possible after the systems have shutdown on impact. In any case, as soon as Rosetta hits the surface, its high-gain antenna will very likely no longer be pointing towards Earth, making any potential communications impossible.

From this Rosetta FAQ page. (Emphasis added.)

Mark Adler
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    This confirms but not yet explains very well why. Hence I upvote but not accept yet. – Nightrider Oct 05 '16 at 06:46
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    The 'why' is in the last sentence: 'making any potential communications impossible'. There's no point in keeping the spacecraft working if you can't extract data from it. – Hobbes Oct 05 '16 at 09:00
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    The 'why' is also in the first sentence: 'in order to meet regulations'. – OrangeDog Oct 05 '16 at 10:37
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    I find it meaningful that the quote states "no automated re-activation will be possible." That feels more like the perspective the OP wants to see. It suggests that, if we feel like it, we can upload new code to wake it up (presuming lucky antenna pointing), but unless we do so, it stays quiet. – Cort Ammon Oct 05 '16 at 18:03
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    @CortAmmon: I think it means that if someone (human or otherwise) were to visit the probe in person (and was able to find anything left of it) it might be possible to restart the systems by means of physical manipulation, but that otherwise the system will remain inert. – supercat Oct 05 '16 at 18:33
  • @supercat Exactly. I hope there's going to come a time when all these probes become museum pieces. I imagine they will be left in place as "living exhibits". The Voyagers and Pioneers may take a while to visit, of course. – SusanW Oct 05 '16 at 21:54
  • Relevant: http://blogs.esa.int/rosetta/2016/09/29/how-rosetta-gets-passivated/ . This starts with "ITU regulations require us to permanently switch off the craft's radio transmitter at end of mission." The only regulations I can find at http://www.itu.int regarding end of mission spacecraft passivation pertain to satellites in GEO. Perhaps it was an overzealous reading of regulations. – David Hammen Oct 06 '16 at 08:38
  • @SusanW: A very sophisiticated propulsion system is needed to bring back to earth any of those probes. There is no chance doing that by burning liquid fuel. Without an extreme reduction of the necessary start mass such a probe collection mission is impossible. Sending a new never returning giant probe at the same place is very cheap and easy compared to such a return mission. We might see some photos of Rosetta on the surface of the comet in the far future, but before we do that there is a lot of other places to send probes to. – Uwe Oct 24 '16 at 08:49
  • @Uwe yep absolutely agree ... I'm not talking about the near future. I'm sort of thinking about how we're like 15th century explorers, and how right now, I personally could technically visit just about anywhere they went, using my spare cash, by next week. If we're still using liquid fuel rockets in a thousand years time, I shall be very disappointed! :-) – SusanW Oct 24 '16 at 09:59
  • The sourced answer has less points? +1. Not anymore it doesn't! – Magic Octopus Urn Jun 21 '18 at 20:42
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At first it might seem a bit crazy to crash a still working satellite into the comet. You could say that if you wait long enough the comet will come around again providing enough energy for more research.

This does bring some risks however, to name 2 obvious ones, the satellite could slowly drift out of orbit or it could never wake up again due to the harsh conditions it is in. In both cases, you lost your valuable satellite and now have debris that can endanger other missions.

Because space debris already is a growing problem an international treaty was signed to reduce the amount of debris in space. This states that all missions should have a end of life plan. In this case a crash landing on the comet.

But what if the satellite crashes into the comet, yet still survives the crash? In that case, each time the satellite receives enough energy, it will start to transmit desperately trying to contact earth. Now instead of the useless satellite debris, there is a comet that sends a useless signal, which can in fact disrupt the signals from other missions.

So to make sure it is completely harmless, it is not only crashed, but also turned off, just in case it survives.

Martini
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    And it is turned off because the beam is very sensitive to direction; it's virtually guaranteed that after the landing it will never be able to locate Earth again. The phrase further communications is badly chosen - there can't be any. –  Oct 05 '16 at 10:19
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    The risk of the drifting probe causing any issue is 0, or at least statistically insignificant. It might be "obvious" but it can be completely ignored. – Antzi Oct 05 '16 at 10:26
  • The probe drifting without any influence is indeed close to 0. The comet however still has active pits, which spew out dust, which could push rosetta slightly out of its course. Which over time will be enough for Rosetta to drift. – Martini Oct 05 '16 at 10:46
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    @Martini The chance that any random course will ever cause a problem (occlude, hit, or even be seen again) is basically zero. Space is big, the probe is small. – Yakk Oct 05 '16 at 13:39
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    @ Yakk, that's true, after reading the response of Antzi again I see his objection was to the chance of causing issues, not on the actual drifting. Thanks, and @Antzi, you're right, the actual chance of hitting Rosetta is next to nothing. – Martini Oct 05 '16 at 13:54
  • @Yakk The problem is that if you send up enough probes, those numbers all start to cancel out, and suddenly old probes are causing problems. – Williham Totland Oct 05 '16 at 13:54
  • @WillihamTotland There are E37 m^3 within 10 AU of the sun. With a 4 m^3 cross-section at 30 km/second, a probe consumes E5 of it every second. So it takes E32 seconds for a probe to hit something placed at random on average, or E24 years. If we put up 100 probes for every human on Earth, that is E10 * E2 = E12. Using the Birthday paradox, if we have 100 probes within 10 AU of the Sun for every human on Earth, there would be a collision about once per year. (Napkin math). Exactly how many probes are you talking about? – Yakk Oct 05 '16 at 14:00
  • @Yakk The probes are not going to be evenly distributed through that space; they are going to be concentrated into lanes related to the movement of the planets and other solar system bodies. A significant portion will be clamped into cosmically speaking irrelevant distances from the ecliptic. Comet missions are less of a potential problem in that regard, to be sure, but even there you risk problems if some future astronomer by chance set their eyes on the same comet and there's junk flying around. – Williham Totland Oct 05 '16 at 14:09
  • @Antzi the risk becomes non negligible next time you want to study the same comet. – Davidmh Oct 05 '16 at 14:43
  • I think the "Sir Issac Newton is the deadliest son of a bitch in space!" speech from Mass Effect is relevant here. Notably this line: "Once you fire this Hunk of Metal, it keeps going til it hits something That can be a ship, or the planet behind that ship. It might go off into deep space and hit somebody else in ten thousand years...You're ruining someones day, somewhere and sometime." The probability of the probe causing issues for any given mission on any given year is minuscule, but probabilities have a way of adding up over time. Send a shutdown program costs nothing, so why not? – UIDAlexD Oct 05 '16 at 15:04
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    The question was not why did they land it, but rather why did they shut it off. (And whether they really did that.) The spacecraft could have left the comet, assuring that they would never see each other again, and then shut it off. That would have met their disposal needs. Collision is non-issue in deep space. Also they could have landed and not shut it off, so why they landed has nothing to do with the original question. In any case, they landed because they could get more science with really close-up pictures. And because it would be cool. Which it was. – Mark Adler Oct 05 '16 at 17:52
  • @Yakk, 67P/Churyumov–Gerasimenko is a short-period comet. It's entirely believable that a followup mission will be sent sometime in the next few decades, and you don't want it to need to repeatedly dodge Rosetta, or have Rosetta's transmitter interfere with its own. – Mark Oct 05 '16 at 22:38
  • I'm fairly convinced that the satellite will not remain in orbit by itself. As the comments is not a place to continue such a dissolution, I created a separate question: http://space.stackexchange.com/questions/18515/how-long-could-a-satellite-orbit-a-comet-without-any-control – Antzi Oct 06 '16 at 02:34
  • No "dodging" would ever be necessary. Rosetta could only be kept in the vicinity of the comet through constant, deliberate maneuvering. Once you turn it off, it either crashes, or its gone far far away forever. – Mark Adler Oct 06 '16 at 07:57