If you can focus all of the solar light that hits an earth-acre space, into a much smaller space (say, a 3' square) on the ground, yes, it's gonna be hot and perhaps dangerously hot. But not weapons-grade dangerously hot.
But you can't. A perfect mirror can't focus a 1/2 degree wide source into anything smaller than 1/2 degree, as seen from the mirror.
That means the size of the image of the sun will be roughly 1/100 of the distance to the mirror. If the mirror is 300 feet away, it could focus the sun into a 3 foot circle, and you'd have to worry about it getting hot. But the sail is going to be hundreds of miles away, and won't add noticeably to the heat from direct sunlight.
There's no risk at all. The original post was a joke.
Slashdot Mirror
But you can't. A perfect mirror can't focus a 1/2 degree wide source into anything smaller than 1/2 degree, as seen from the mirror.
That means the size of the image of the sun will be roughly 1/100 of the distance to the mirror. If the mirror is 300 feet away, it could focus the sun into a 3 foot circle, and you'd have to worry about it getting hot. But the sail is going to be hundreds of miles away, and won't add noticeably to the heat from direct sunlight.
There's no risk at all. The original post was a joke.
The sun isn't a point source. It's about half a degree across, as seen from here.
Its image will be about half a degree across, as seen from the mirror.
If the mirror is 100 km up, that means the image of the sun will be around 1 km across.
It won't be very hot, unless the sail is much bigger than that.