We live in a village at the top of a very steep, mile deep, ice covered valley. Mercury is a village at the bottom of the valley.

Your house is high up the valley but you actually live thirty feet deep in a pit. To get to Mercury you have to build a car, lift it out the pit, then slide down the steep valley. The car has to be lightweight so that you can lift it out the pit: the engine and brakes are weak and do next to nothing.

So you build a feeble car — more of a four wheeled bicycle really, hoist it out the pit, and slide down the valley in it. By the time you whizz past Mercury you’re doing 100mph so you can’t stop. You go straight past and start sliding up the other side, but you do also put the brakes on for a bit. You’ll eventually slide back down to Mercury again at 90mph and put the brakes on again, and slow down a little more.

Do this ~10x and you’ll eventually stop.

Going in the other direction is easier. Earth is surrounded in a 100km soft squishy cushion of gas which means to all intents and purposes you can just fly straight at it.

The Wikipedia page has a nice description of the issues of entering Mercury's orbit:

https://en.wikipedia.org/wiki/Mercury_(planet)#Research_with...

It's going too fast and doesn't carry enough fuel to slow down directly into orbit.

By way of example, here is a video explaining what was needed to get the Messenger spacecraft into orbit around Mercury:

https://youtu.be/gBwspxfW10w

Here is an example of a rocket design/mission in Kerbal Space Program that can insert into orbit on the first pass:

https://youtu.be/lYwrbhzj694

Worth noting the Kerbol* solar system is significantly smaller and lighter than the real one, and has fewer eccentricities in orbit, axis of rotation for the various bodies, and so on. E.g. it takes about 3,300 m/s of 𝛥v to reach orbit around Kerbal, and LKO velocity is ~2200; for Earth, about 9,400, with LEO velocity of ~7300.

If you play KSP and would like a challenge much closer to real rocketry, check out the Realism Overhaul mod—and don't forget Principia. For an actual campaign based on the Apollo program, starting in 1951, check out RP-1 (Realistic Progression).

* Kerbol is the name of the star

I'm having enough difficulty reaching low Kerbin orbit. If anything, I'd like a mod that lets my son and I learn the basics of building and launching rockets with more of an auto-pilot flight mode. We are kern to explore more of the Kerbol system but are having great difficulty with basic missions.