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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?
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.
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.
(http://www.nasa.gov/vision/universe/solarsystem/galileo_final.html)
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.
(http://disarmament.un.org/treaties/t/outer_space/text)
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.
(https://planetaryprotection.arc.nasa.gov/about)
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).
You can't re-purpose Juno after its mission as:
There are other good reasons for destroying it other than preventing the contamination of Jupiter's moons:
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.
