Astute observation, but the problem is easily fixable. Plants use particular wavelengths of light (IIRC mostly from the visual spectrum) for photosynthesis. If the proposed spacecrafts were to block mostly UV and infrared, letting most of visible spectrum through, it'd do plants no harm at all.
The relevant technology is already available and widely used. For example typical car's windshield consists of two or more layers of special UV/infrared absorbing foil sandwiched between glass panes for structural support.
In case of a spacecraft, all you'd have to do is spin the fuselage at low RPM to keep the foil extended.
EDIT:
my only contention with any sun-blocking spacecrafts is that they'd have to be carefully engineered to prevent use as a weapon for attacking ground & orbital targets with focused beams of light.
Astute observation, but the problem is easily fixable. Plants use particular wavelengths of light (IIRC mostly from the visual spectrum) for photosynthesis. If the proposed spacecrafts were to block mostly UV and infrared, letting most of visible spectrum through, it'd do plants no harm at all.
The relevant technology is already available and widely used. For example typical car's windshield consists of two or more layers of special UV/infrared absorbing foil sandwiched between glass panes for structural support.
In case of a spacecraft, all you'd have to do is spin the fuselage at low RPM to keep the foil extended.
EDIT: my only contention with any sun-blocking spacecrafts is that they'd have to be carefully engineered to prevent use as a weapon for attacking ground & orbital targets with focused beams of light.