system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can the Sun's gravitational pull be strong enough to keep Pluto in orbit but not just pull Mercury into it?
gpt
The Sun's gravity is pulling Mercury, but the planet is traveling so fast that it can't fall fast enough to ever hit the Sun. Mercury's orbit is elliptical though, so it does vary in distance from the Sun. As it gets closer to the Sun the pull from the Sun's gravity does increase, but so does the orbital velocity of Mercury, which means it simply keeps on moving in its orbit. As the orbital radius increases and Mercury gets further away the strength of gravity decreases, but so does Mercury's orbital velocity. The end result is a stable orbit.

It is actually VERY difficult to make something crash into the Sun. If we were to suddenly reduce the Earth's orbital velocity by 20% we would simply move into a much more elliptical orbit. We would not fall into the Sun. If we wanted to send a space probe to the Sun we would have to completely counteract the orbital velocity it has from being launched from the Earth. This takes something like twice the fuel as it does to send a probe out of the Solar System!