[avz]

Technical detail: Having crossed all meridians is generally not considered the proper criterion for circumnavigation since this is trivially done near the poles. The definition I have come across include a loop that partitions Earth's surface into two parts of comparable area.

[Andrew_nenakhov]

Any two non-zero areas are comparable, but I don't think 1:10000000 will satisfy the definition. It is necessary to establish an acceptable ratio. 1:6 or something like that?

[lmm]

There's a tradition of having to pass through antipodal points, but that's probably too strict for a balloon.

[bhickey]

The requirement I'm familiar with is crossing all meridians and the equator.