Meteors are most frequent after midnight, because late at night an observer is on the forward side of the earth in its motion around the sun and the field of view tends to sweep up more particles. An observer can only see meteors within 300 miles, but but the number of meteors visible somewhere on the surface of the earth is several hundred per day. Beginning in the 1940's, radar allowed astronomers to obtain echoes from meteors. The solid particles themselves are too small to detected, but they produce a column of electrically charged gas that reflects a radar beam. Radar, unlike photography, is as effective in daylight or in a cloudy sky as it is on a clear night.
Radar can also study meteors too small to be recorded photographically.
While astronomers more easily determine a visible meteoroid's precise direction in the sky with photographs, they measure its distance and velocities more accurately with radar. The velocity at which meteoroids enter the atmosphere varies from 7 miles per second to 44 miles per second. The first value corresponds to the velocity an object would acquire falling toward the earth under gravity. This velocity is the same velocity a rocket must have to escape the earth's gravitational pull. The escape velocity from the sun is 26 miles per hour, while the escape velocity of the earth around the sun is 18 miles per second.