About 100 tons of extraterrestrial material fall onto Earth every day, most of it dust. When extraterrestrial objects enter the Earths atmosphere at high velocity, their surface is strongly heated and the surrounding air molecules are ionized, forming a plasma trail at altitudes between 120 and 80 km. Thus, shooting stars form by producing a trail of light, which is actually ionized air, as they burn up in the atmosphere, whereas fireballs are caused by larger fragments of extraterrestrial material. In general, these short-time light phenomena are called meteors. Meteors are commonly seen with the naked eye, best visible during the night, and can be detected using video cameras, such as surveillance cameras or specific fisheye cameras. Only extraordinary large events can also be seen during daytime, as, for example, on February, 15th, 2013, when a 20 m diameter asteroid exploded over the Chelyabinsk area in Russia. Because plasma trails reflect electromagnetic waves at radio frequencies, meteors can be detected during the day and night and in all weather conditions, using a low-frequency radar.
Meteor radar station, Hall 5
Meteor radar station To be able to see and listen to meteor echoes, a transmitting and a receiving system are necessary. What is observed are basically radio signals that are reflected from the plasma trails of the meteors. To do so, the GRAVES French Space Surveillance Radar located near the city of Dijon (France) is used as a transmitter, and an antenna located on the roof of the Natural History Museum as receiver.
Basic concept of meteor detection. This technique can also be used to obtain statistical data on meteor numbers, their masses, their trajectories, and the manner in which ionization trails develop, break up, and decay.
Examples of meteor echoes These are typical radio signals of meteors as recorded with our system at the Museum (see below). We are looking at a frequency of 143.05 MHz, the frequency of operation of the transmitter that is used. On the left side is the time in day, hour, minute, and second of the recorded signal. The larger the extraterrestrial object is, the longer and brighter the actual meteor trail and reflected radio signal is. In blue is the background signal, whereas in the central part, in white, yellow-orange, to red, depending of the intensity, are meteor echoes.