As follows from the mission timeline:

Launch - August 5, 2011
Deep Space Maneuvers - August/September 2012
Earth flyby gravity assist - October 2013
Jupiter arrival - July 2016
Spacecraft will orbit Jupiter for 20 months (37 orbits)
End of mission (deorbit into Jupiter) - February 2018

Why is it necessary to destroy the spacecraft? Would it hurt to keep it on the orbit indefinitely or re-purpose it for later use?

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Jul 6 '16 at 20:52
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    $\begingroup$ Because Dave Bowman and the monoliths specifically told us not to go near Europa. $\endgroup$ – Wad Cheber stands with Monica Jul 10 '16 at 6:52

In order to assure that it cannot crash into Europa or other possible ocean moons and potentially contaminate them with Earth organisms. Juno is qualified to survive the radiation environment up to the end of its mission. After that it could succumb to the radiation at any time and become uncontrollable. Planetary protection then requires the disposal of the spacecraft while it is still controllable.

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    $\begingroup$ I understand now, although non-carbon based life could exist on Jupiter too and perhaps DNA from dead micro-organisms on Juno could still interact with other life. But currently I don't know how Earth's life's DNA or living organisms would fare on Jupiter so won't make any assumption. $\endgroup$ – Creative Magic Jul 5 '16 at 6:15
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    $\begingroup$ The spacecraft will be quite thoroughly sterilized by its extremely high-speed entry into the Jovian atmosphere. $\endgroup$ – Mark Adler Jul 5 '16 at 7:55
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    $\begingroup$ When I said thoroughly sterilized, I meant thoroughly sterilized. All RNA, DNA, and proteins would be broken up by the intense heat. $\endgroup$ – Mark Adler Jul 5 '16 at 13:58
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    $\begingroup$ We don't risk that at all. As I said a few comments ago, as a result of the incredibly high speed entry into Jupiter's atmosphere, any hitchhiking organisms and biological molecules on Juno will be not only merely dead, but really most sincerely dead. The same could not be said for an impact on an icy moon, which would break up the spacecraft, but not sterilize it. $\endgroup$ – Mark Adler Jul 5 '16 at 16:06
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    $\begingroup$ I'm not getting across how hot everything will get. Over 1000C. All carbon-carbon bonds will be broken. There will no biological structures left. Period. $\endgroup$ – Mark Adler Jul 6 '16 at 13:56

Why is it necessary to destroy the spacecraft?

It's because life might well exist on some of Jupiter's moons.

Despite the best efforts to assemble the spacecraft in extremely clean conditions, and despite exposing the spacecraft to vacuum and to the Sun's ultraviolet and X-ray radiation, there's a chance that Earth-borne life remains intact on the satellite.

Suppose the dead spacecraft collides with one of Jupiter's moons, and in the process contaminates that moon with Earth-borne life. If that moon already bears life of its own, no matter how primitive, our dead and useless satellite may have spread the seeds that eventually kill that life.

To this end, thirteen years ago, the Galileo probe met the same fiery death that Juno will one day meet:

The spacecraft was purposely put on a collision course with Jupiter because the onboard propellant was nearly depleted and to eliminate any chance of an unwanted impact between the spacecraft and Jupiter's moon Europa, which Galileo discovered is likely to have a subsurface ocean.

The concept of planetary protection is ensconced in international treaty. Article IX of "Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies" (aka the Outer Space Treaty, to which the US and every other space-faring nation are parties) says that parties to the treaty

shall pursue studies of outer space, including the moon and other celestial bodies, and conduct exploration of them so as to avoid their harmful contamination and also adverse changes in the environment of the earth resulting from the introduction of extraterrestrial matter and, where necessary, shall adopt appropriate measures for this purpose.

NASA has an Office of Planetary Protection. Two of its key goals are

  • Preserving our ability to study other worlds as they exist in their natural states; and
  • Avoiding the biological contamination of explored environments that may obscure our ability to find life elsewhere – if it exists.

In addition the scientific concerns cited above, there are even stronger ethical concerns regarding contaminating other solar system bodies with Earth-borne life. To this end, the COSPAR (Committee on Space Research) Planetary Protection Policy subcommittee held a workshop in 2010 on "Ethical Considerations for Planetary Protection in Space Exploration." One of the key recommendations from this workshop was to bolster COSPAR's Planetary Protection Policy with a statement that

Inherent in the conduct of scientific, exploration, and other activities—whether by robotic or human missions—is the need to consider and appropriately protect potential extraterrestrial life.

This workshop was instigated in part by NASA, was chaired by NASA's previous planetary protection officer, and attended by NASA's current planetary protection officer. Additional attendees included representatives from the European Space Agency, the French space agency (CNES), and the German space agency (DLR).

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    $\begingroup$ Also note that the Galileo probe was sent to a fiery death 13 years ago for exactly the same reason: The spacecraft was purposely put on a collision course with Jupiter because the onboard propellant was nearly depleted and to eliminate any chance of an unwanted impact between the spacecraft and Jupiter's moon Europa, which Galileo discovered is likely to have a subsurface ocean. Source: nasa.gov/vision/universe/solarsystem/galileo_final.html. $\endgroup$ – David Hammen Jul 5 '16 at 12:51
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    $\begingroup$ Cleaned up most of the comments here. If you want to start a conversation about the need for planetary protection, then either ask a specific question related to it, or else discuss it in chat. The comment section isn't really suited for such conversations. $\endgroup$ – PearsonArtPhoto Jul 5 '16 at 13:12
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    $\begingroup$ @DavidHammen: Not only that, but even if there were no life there but some traces of life were later found that resembled Earth organisms, it would be impossible to know for certain whether those were Earth-like organisms that had been there previously (perhaps a bunch of asteroids entered the solar system with the seeds of life, and while some landed on Earth, others landed elsewhere) or whether those were simply contamination from the earlier mission. $\endgroup$ – supercat Jul 5 '16 at 14:24
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – called2voyage Jul 5 '16 at 15:21

You can't re-purpose Juno after its mission as:

  1. Its instruments are designed for a specific mission profile, if you sent it elsewhere it wouldn't be able to produce good science. Its solar panels won't produce enough power if you get much further away from the sun
  2. Juno is going to be exposed to a huge amount of radiation which will degrade its instruments, power systems, computers and support hardware. It's not going to be much good for anything else
  3. It doesn't have enough fuel to go anywhere. Even if it had instruments that would be useful in other places, and hadn't been bathed in radiation for so long it doesn't have enough delta-v to leave orbit and go somewhere else

There are other good reasons for destroying it other than preventing the contamination of Jupiter's moons:

  1. Even dead spacecraft cost money, if it's still out there someone has to try to keep track of it. It's irresponsible not to do so
  2. A dead spacecraft is a hazard. Sure, space is big, but having a piece of junk somewhere in the vicinity of a place you want to go is something you want to avoid
  3. The destruction of the probe may actually be an opportunity for science. We may possibly get some data as it nears the atmosphere, if not from the probe itself but from observations of its path
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    $\begingroup$ A good example of the importance of the dead spacecraft hazard is our own planet. If we had been more thorough in de-orbiting Earth's artificial satellites, our orbits would be less polluted with "space junk". $\endgroup$ – JS. Jul 5 '16 at 18:35
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    $\begingroup$ I think there currently is no possible way to monitor a dead spacecraft around Jupiter from earth. Once its transponders stop working, it's much too far to just bounce passive radar off of it. $\endgroup$ – uhoh Jul 7 '16 at 6:02
  • $\begingroup$ I agree @uhoh, but if its there you have to try. $\endgroup$ – GdD Jul 7 '16 at 6:43
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    $\begingroup$ No, if it's currently impossible, you really don't have to try - honest - unless you are feeling particularly Quixotic: https://www.youtube.com/watch?v=RfHnzYEHAow#t=55 "This is my quest, to follow that 'star', no matter how hopeless, no matter how far..." $\endgroup$ – uhoh Jul 7 '16 at 6:57
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    $\begingroup$ Actually this is interesting, so I've asked about detection limits. $\endgroup$ – uhoh Jul 7 '16 at 7:24

Juno starts its tour of Jupiter in a 53.5-day orbit. The spacecraft saves fuel by executing a burn that places it in a capture orbit with a 53.5-day orbit instead of going directly for the 14-day orbit that will occur during the mission's primary science collection period. The 14-day science orbit phase will begin after the final burn of the mission for Juno’s main engine on October 19.

(NASA; emphasis added)

After that probe will not have sufficient fuel to again burn main engine and will ultimately dive into Jupiter’s atmosphere where it will be crushed and vaporized.

It is also fact that mission is planned in such a way that probe will dive into Jupiter’s atmosphere because it can be possible that some Earth organisms may still be alive aboard Juno. Scientists and NASA officials don't want to take the chance that such microbes could contaminate Jupiter's ocean-harboring moon Europa — one of the solar system's best bets to host alien life.

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