From Clive Semmens
In his letter, Michael Phillips suggested a solar furnace could focus sunlight into a beam to power a plasma-engined rocket (28 May, p 31).
Unfortunately, solar furnaces cannot focus sunlight into a beam, they focus it into an image of the sun at a single focal distance. The sun subtends an angle of 0.5 degrees at the Earth, and the focused image subtends the same angle.
The rocket would rapidly reach a height where the diverging “beam” was much larger than it. This is why you need lasers: the beam remains narrow over a much greater distance.
Ely, Cambridgeshire, UK
