An increasing share of the signals from the universe are drowned in man-made noise on the last tine leg of their long odyssey. Modern city dwellers hardly know what stars look like, let alone the magnificent stellar panoply of a truly dark sky. The light that appears so bright is scattered back into telescopes and prevents observations of faint objects, be they faraway galaxies or small, near-Earth asteroids. The glare from poorly designed lighting fixtures can blind rather than help people see objects better. Worse, without darkness, nocturnal animals go astray or disappear, disturbing delicate ecosystems.
Radio astronomy faces and even more acute problem of interference from man-made radio transmissions.
If Neil Armstrong had taken one of today's cellular telephones along to the moon, it would have been one of the four brightest radio sources in the sky. The radio signals from remote galaxies are millions of times fainter than this. This worked well when we could observe only the local universe where everything is nearly at rest with respect to us. As technology improves, so that we can observe more distant galaxies and quasars, a wider range of radio frequency bands is needed. As more satellites are launched pieces collide, the amount of debris can only grow, and no cleanup method is known.