BRUSSELS — When an astronaut took nighttime pictures of Europe from the International Space Station this year, one nation stood out far below on the twinkling surface of the earth: Belgium.

It is the only country in Europe to keep nearly all of its 2.2 million streetlights on through the night, making it a world leader in light pollution, and easily identifiable even from space.

The phenomenon has been a source of ridicule and humor in Belgium for decades. But since the images were published in May, some have also begun to ask a simple but tough question: Why?