Why do we keep destroying our own probes?

[daniel l.]

Galileo crashed into Jupiter. Cassini crashed into Saturn. Juno is set to be de-orbited into Jupiter as well.

Why do we keep sending these probes to their destruction?

I know that there is a risk of contaminating the moons: Europa, Ganymede, Titan, etc... but why not simply make a final burn at periapsis and send them out of the planetary system entirely? That would keep them away from the moons and provide opportunities for them to be eventually recovered for reasons of science and sentimentality.

Is there going to be nothing left for our descendants to one day salvage and place inside museums on the very worlds these probes once explored?

[mikegarrison]

Most of them don't carry enough fuel to have enough delta-v to leave the system entirely.

[PB666]

Also they often generate one last pulse of data gathering as the plummet.

[Just Jim]

Probes like Cassini gathered and transmitted back a wealth of info as they were diving into the planetary atmospheres, right up to their very last seconds. They didn't burn up in vain... far from it.

Opps, @PB666 beat me by about 2 seconds... 

Edited January 25, 2018 by Just Jim

[YNM]

We don't destroy our probes, the environment around (or ahead of) them did.

Edited January 25, 2018 by YNM

[HebaruSan]

The reasons are valid, but I can't help sharing the wish that we would accumulate long term observation capabilities instead of discarding them. Maybe probes will go through a "reusability" style transition sometime after the launch vehicles.

[ARS]

Most probes don't have enough Dv to leave the system, since probes usually carry a very limited fuel. Even without deorbiting, the orbital decay will still makes them plummet to the planet they orbited. That said, their last transmission of data provides a valuable science discovery about planet's characteristics

[Starman4308]

1 hour ago, HebaruSan said:

The reasons are valid, but I can't help sharing the wish that we would accumulate long term observation capabilities instead of discarding them. Maybe probes will go through a "reusability" style transition sometime after the launch vehicles.

I'm not sure that's necessarily practical. Launch vehicles are used for a short time and recovered, with refurbishment and repair fairly frequently.

Probes are out there often for decades. Their RTGs decay, their electronics and sensors decay; after a typical mission is complete after 10-20 years of active use, there's not really much that would be worth salvaging. There are cars here on Earth that aren't kept running that long.

[mikegarrison]

40 minutes ago, Starman4308 said:

I'm not sure that's necessarily practical. Launch vehicles are used for a short time and recovered, with refurbishment and repair fairly frequently.

Probes are out there often for decades. Their RTGs decay, their electronics and sensors decay; after a typical mission is complete after 10-20 years of active use, there's not really much that would be worth salvaging. There are cars here on Earth that aren't kept running that long.

Besides that, the probes were probably designed 10 years before they were launched. Then they are out there for 20 years. Now you're talking 30-year-old technology. Maybe OK if it's a fridge, but why would you pay all the money it takes to send a probe back into space and use it to send one that is 30 years out of date?

[Superfluous J]

One thing not mentioned is that we do not want to risk contaminating any ecosystems we don't yet know about. I can't imagine how we'd feel 100 years from now when we discover that Galileo had - after we abandoned it - crashed into Europa and some microscopic germs spread across the surface obliterating the life that was once there.

[Bill Phil]

Even if they could escape the system (possible with gravity assists, I supposs), there's still the chance that they could return and be recaptured by the planet, continuing the risk to the Moons.

[HebaruSan]

2 hours ago, mikegarrison said:

send a probe back into space 

Sorry, I didn't expect anyone would think I meant something that ridiculous. I just meant designing probes to stay where they are more permanently. You never know when it might be useful to be able to point a nearby telescope at something going on near Jupiter or Saturn, even with a 30-year old camera.

[Theysen]

Costs and planned mission times. You evaluate your primary mission goals beforehand the launch , purchase the needed resources and since its space - they only last that long. Your batteries are designed for your planned time, the solar arrays only provide guaranteed power for their rating, your Fuel system (e.g. if it’s pressure driven) might start to leak and so on. Every minute mission control can plan to use the probe longer than its original mission before its EOL date is taken with pleasure , but it is not expected. Since probes so far away might do something unforseeable after EoM time , they plan the end specifically to ensure above mentioned reasons. It’s not so much of a problem for a probe in LEO , except more junk if it fails before you can securely park or deorbit it. 

[kerbiloid]

We just don't want to overload the game engine.

[Hannu2]

6 hours ago, HebaruSan said:

Sorry, I didn't expect anyone would think I meant something that ridiculous. I just meant designing probes to stay where they are more permanently. You never know when it might be useful to be able to point a nearby telescope at something going on near Jupiter or Saturn, even with a 30-year old camera.

If probe loses all fuel it loses ability to point solar cells to sun, antennas to Earth or make maneuvers to keep orbit. It would die and hit in some body in relatively short time, because there are typically no stable orbits around planets with significant moons. In end of the mission probe has probably only few m/s left so it can not maneuver itself to more stable distant orbit or leave the system.

[Starman4308]

7 hours ago, HebaruSan said:

Sorry, I didn't expect anyone would think I meant something that ridiculous. I just meant designing probes to stay where they are more permanently. You never know when it might be useful to be able to point a nearby telescope at something going on near Jupiter or Saturn, even with a 30-year old camera.

The question is, what's going to give out first:

The power supply

The antenna

The hydrazine supply (to keep the antenna oriented at Earth)

The attitude thrusters (again, pointing the antenna at Earth)

The electronics

Funding for the mission crew

You don't want any of those to happen before you've put the probe into a disposal orbit to avoid even the slightest chance of contaminating possible life-bearing moons. Mission extensions such as the multiple reprieves Cassini got were because the equipment was still in good condition and there was still enough maneuvering fuel.

Juno isn't going to get that luxury: each time it passes by Jupiter, its electronics get more and more fried by Jupiter's immense magnetic field.

[DDE]

10 hours ago, HebaruSan said:

Sorry, I didn't expect anyone would think I meant something that ridiculous. I just meant designing probes to stay where they are more permanently. You never know when it might be useful to be able to point a nearby telescope at something going on near Jupiter or Saturn, even with a 30-year old camera.

There is no such thing as permanence. Disposal orbits only slightly alleviate the planetary protection hazard, which is the primary impetus for deliberate disposal.

[Starman4308]

28 minutes ago, DDE said:

There is no such thing as permanence. Disposal orbits only slightly alleviate the planetary protection hazard, which is the primary impetus for deliberate disposal.

A quick point of clarification: what did you mean by disposal orbit? I was including "orbit that ends inside a gas giant", and not just graveyard orbits that would theoretically be stable for a long time... but are hard to achieve sometimes without a lot of delta V.