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I'm not really sure what you mean, how else would they be able to take photos? The Voyagers and New Horizons spacecraft weren't in orbit, but they performed a fly-by.

100-50k above the surface but as close as possible enough to orbit

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In many cases they are flybys and need to be at exact distans to slingshot to next target or just to be able to stay in orbital range

Okay Juno took photos of Io some of which were from several hundred to a thousand kilometers away.

I assume that IO has less gravity well than Earth. There are objects even smaller than Io that have had photos taken from orbits as well.

I always wonder why they can’t orbit closer and get even greater surface detail.

I’m basing an assumption that an object with less mass (like an asteroid or a moon) can be orbited much closer than a satellite around the Earth before the gravity well becomes an issue.

Are these missions at their absolute closest orbits possible given the fuel reserves?

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Spacecraft like Juno have very specific orbits to follow that are planned very far in advance in order to maximize their mission time. If, say, Juno swings too close to Io, it might get a gravity assist that ejects it from Jupiter, in which case it’s no longer very useful is it.

Keep in mind that Io is not just hanging there in deep space with no other bodies around. It’s subject to intense gravity from Jupiter and even the other three Galilean moons that impact the shape and proximity of the orbits that can be plotted.

If Io had been Juno's target, they would probably have calculated its trajectory and fuel needs to achieve either an actual orbit around Io or, if that wasn't possible, a closer flyby. But Juno's main target was Jupiter itself, so all photos of Jupiter's satellites were taken from the most fuel-efficient orbit around Jupiter.

The closer you get to a sphere the less of its surface area you can see. If you want pictures as close to half the surface area you want, you need to go really far away. Infact, to see 50% of the surface area of any sphere, you need to go infinitely far away

Juno main mission wasn't taking pictures, it was magnetosphere analysis to try to understand Jupiter's internal structure. The orbits were set for that mission, not pictures.

Also, any orbit on Jupiter and Saturn has to be carefully analysed to minimise the impact of the radiation on the spacecraft.

That makes a lot of sense.

Take this with a pinch of salt because I don't remember when I heard it or read it. Juno originally wasn't going to have any cameras and NASA PR department demanded them to have something to show to the public because unless you are a scientist the telemetry data alone is not cool.

When the trade is between coverage and orbital altitude, coverage wins and closer is not necessarily better. It is also more dangerous to insert a spacecraft into such a close orbit, where it is also affected more by the perturbations induced by the body it is orbiting and thus more difficult to navigate. Look into the history of the MESSENGER mission and you'll see that once complete coverage of the surface was obtained the orbit was lowered progressively until impact was achieved at end of life, this seems to be the scenario you are asking for.

The closer to the object you are orbiting, the faster you need to go to stay in orbit. The faster you move, the more of an issue image blur is. Distant objects don't get much light. That means camera shutters need to stay open longer times, which also adds to blur. Yes, you would get better detail but it becomes useless because all the pixels are blurred.

A few possible answers:

1. If a spacecraft is orbiting a planet, each flyby of a moon is setting up the subsequent flybys. There are very precise conditions that they need to hit at each encounter for the whole trajectory to work.
2. For a spacecraft in orbit about a planet, it is costly to capture at one of the moons. Due to propellant constraints, if you capture into orbit at one of the moons you are generally staying there (e.g. getting better science at Io but sacrificing science at Ganymede, Callisto, Europa, and Jupiter itself).
3. Some moons are very difficult to have a spacecraft in orbit. Jupiter, for example, has a very hazardous radiation environment (especially for the electronics on board the spacecraft). Even if we had a dedicated Io mission, it might be better to have a series of flybys instead of orbiting the body because the spacecraft could live longer and return more data.
4. While being closer to a body generally will provide better resolution, it may not be required to meet the mission's science objectives. Also, the closer you approach a moon the faster you fly past it. You may actually get more data by having a lower resolution over a longer time span. It all comes down to what cameras and other instruments are on the spacecraft.
5. You have to keep in mind the scale of the solar system. Several hundred (or even thousand) kilometers away from a body is still pretty dang close. Even if you are 3,000 km away from Io, that's still less than 1/100 the distance from the Earth to the Moon. There's a lot of science you can do from there.
6. Due to navigational uncertainties, there is a limit to how close a spacecraft is allowed to approach each body. They like to be 110% sure that their 100+ million dollar spacecraft isn't going to become a new crater on Europa. Also, some moons (like Europa) may harbor life and have additional protections which can constrain your flyby distance.

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Mike Malin had proposed the Junocam instrument and been rejected. He will point out that JPL-run missions have a much higher than statistically expected predominance of JPL-developed payloads. However, Mike managed to convince enough folks to eventually get HQ to add Junocam to the payload suite anyway, partly by arguing that it would be a very effective way to engage the public in the mission.

Number 6 is a big factor in the limitations and I have more typically seen 200-300% sure as the navigator rule of thumb.

Hello u/Different_Muscle_116, your submission "How come space probes generally take photos of moons or asteroids from several hundred (or thousands) of kilometers away from the object they orbit?" has been removed from r/space because:

• Such questions should be asked in the "All space questions" thread stickied at the top of the sub.

Please read the rules in the sidebar and check r/space for duplicate submissions before posting. If you have any questions about this removal please message the r/space moderators. Thank you.

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