system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't we just keep probes in orbit around objects, instead of doing fly-bys?

We have Cassini-Huygens orbiting Saturn right now, couldn't we have done this with other probes? New Horizons is going to Pluto, why not just keep it there to do science for a while?
gpt
Two reasons, really: 

One is that in order to get to Pluto in a tolerable amount of time (~10 years) NH has to move very fast, using a gravitational slingshot. This means that if we wanted to slow it down to orbit Pluto, we'd have to expend a huge amount of propellent (which we don't have) in order to get rid of all this spare kinetic energy in time to go into orbit around Pluto. 

The second reasons is a little more technical. Pluto is tiny, with a very small gravity well into which we'd want to drop our spacecraft. In order to avoid overshooting, we'd have to start decelerating essentially immediately after slingshot to the planet, thus burning a lot more propellent and extending the duration of the mission substantially. 

In the end, it comes down to tradeoffs. Do you bump up the cost of the mission hugely for these more complicated propellent systems, or do you fire a leaner, cheaper, faster system and try to maximize what you can get as you pass by? In the end, the decision came down on the side of the latter with NH.