* Closest in terms of distance: That would be the Sun's own atmosphere, specifically the photosphere, which is the visible surface of the sun.

* Closest in terms of spacecraft: That would be the Parker Solar Probe, which has flown through the sun's corona, the outermost layer of its atmosphere. It's getting closer with each orbit and is expected to reach a distance of just 6.2 million kilometers from the Sun's surface.

* Closest in terms of theoretical future missions: There are hypothetical missions proposed that would aim to fly even closer to the Sun than the Parker Solar Probe.

Let me know if you have any other questions!