You don't go direct, and the target is also moving. But the main limitation is "delta V", or the effective speed change you can achieve with a particular quantity of fuel.
The Mariner 10 trip did it in 5 months, which is reasonable. The article doesn't mention Mercury at all. A minimum-fuel trip to Mars is estimated at 3 years, I believe.
Wikipedia:
> Another reason why so few missions have targeted Mercury is that it is very difficult to obtain a satellite orbit around the planet on account of its proximity to the Sun, which causes the Sun’s gravitational field to pull on any satellite that would be set into Mercury's orbit. Furthermore, spacecraft naturally accelerate as they approach the greater gravitational pull of the Sun, but must slow down for orbit insertion, so this entails considerable fuel requirements. This is different with superior planets beyond Earth’s orbit where the satellite works against the pull of the Sun. Therefore, reaching an orbit around Mercury requires especially expensive rocketry. Mercury's lack of an atmosphere poses further challenges because it precludes aerobraking or the use of a parachute type device.[3] Thus a landing mission would have even more demanding fuel requirements.
The Mariner 10 trip did it in 5 months, which is reasonable. The article doesn't mention Mercury at all. A minimum-fuel trip to Mars is estimated at 3 years, I believe.
Wikipedia:
> Another reason why so few missions have targeted Mercury is that it is very difficult to obtain a satellite orbit around the planet on account of its proximity to the Sun, which causes the Sun’s gravitational field to pull on any satellite that would be set into Mercury's orbit. Furthermore, spacecraft naturally accelerate as they approach the greater gravitational pull of the Sun, but must slow down for orbit insertion, so this entails considerable fuel requirements. This is different with superior planets beyond Earth’s orbit where the satellite works against the pull of the Sun. Therefore, reaching an orbit around Mercury requires especially expensive rocketry. Mercury's lack of an atmosphere poses further challenges because it precludes aerobraking or the use of a parachute type device.[3] Thus a landing mission would have even more demanding fuel requirements.