An early hitch saw the mirror catch on a radio aerial. The plastic parasol, covered with aluminium, began to unfold in the early afternoon, but then jammed again, stopping the test for the day.
The mirror is designed to work like an artificial moon. It was meant to reflect a beam of sunlight about 8km (5 miles) across on to several regions in Russia and other former Soviet republics before reaching Germany and the Czech Republic. It would not have been visible in Britain.
It is designed as a prototype for much larger models that could illuminate northern parts of the Earth.
Russian Mission Control said the experiment might go ahead today if they could resolve the problem.
The Space Regatta Consortium, principally backed by the Russian company Energia, has funded the experiment. The designers suggested that a series of mirrors - or one giant mirror - could harness sunlight to overcome darkness and boost agriculture by lengthening the day.
However, that poses the huge problem of controlling the mirrors' angle while the Earth and the Sun are moving. Tiny variations would mean huge differences in what part of Earth was illuminated.
The experiment had triggered an avalanche of dramatic reports, many of which described the mirror as a "second moon" that would glare from the skies. However a spokeswoman called the idea "ridiculous rumours".
Astronomers were less happy. "This could get so bright that it's impossible to miss," said John Kelly Beatty, senior editor at Sky & Telescope magazine.
Astronomers believe that such bright light will seriously impede observations from ground-based telescopes.
David Williams, president of the Royal Astronomical Society in Britain and professor of astronomy at University College London, said: "A lot of money - taxpayers' money - has been spent on building new telescopes in remote locations. This could ruin all those plans."
Astronomers fear that theRussians financing the experiment will ignore their objections and push ahead with plans to put up more mirrors, all beaming sunlight on to points on Earth in the middle of the night.
"This situation is different from many big space projects which are funded by countries or internationally," said Professor Williams.