Nuk
No it can't because it is too small and to close to the sun.
No, Mercury is too close to the sun and pointing the HST in that direction could possibly damage the sensitive instruments on board.
The space probe Voyager 2 passed close to Neptune in 1989 for purposes of observation. It also passed by its moons Nereid and Triton.
Mercury orbits very close to the Sun and looking near to the Sun could damage the telescope.
None.
The Hubble Space Telescope cannot see Mercury clearly because Mercury is too close to the Sun, making it difficult for Hubble to observe without damaging its instruments. Additionally, Mercury's small size and proximity to the bright glare of the Sun make it challenging to distinguish from the surrounding brightness.
By the space race
No space probe has ever landed on Mercury.
The Mariner10 space probe and the Mercury Messenger space probe.
They study mercury with high powered satellites from outer space. Of course if they got to close to it where it is so close to the sun it would fry in an instant. The closest planet they have been to the sun is venus and that satellite burnt up from venus's hot atmosphere. Anything closer than that would vaporize. The only planet or considered planetoid we have not taken a look at is Pluto where it is so far away and it is so small.
In a mercury barometer, the space above the mercury is a vacuum or a space filled with a small amount of inert gas like nitrogen. This space helps to create a more accurate measurement of atmospheric pressure by allowing the mercury to respond only to changes in atmospheric pressure, without interference from other gases.