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.
Top, x-axis, is the frequency; Left, y-axis, is the time when the detection was recorded; Right, y-axis, is the intensity of the signal. The more or less continuous lighter blue lines on the right which are due to interferences (i.e., not associated with meteors).
See and hear meteors live! Here meteors echoes detected with the system at the Museum are seen and heard in real time, with sounds like bumps, thumps, and chirps.
When to look for meteors? Meteors can be seen every day of the year, at any time of the day or night, but more so in the early morning, with a peak at 6 a.m., due to the rotation of the Earth and the direction in which it travels around the Sun.
Daily meteor frequency. The frequency at which meteors can be seen also varies during the year. Each year, at the same season, so called meteor showers occur. They form when a large number of small cometary or asteroidal particles enter the Earths atmosphere.
Major meteor showers.
Meteor camera Since August 2015, a specific fisheye camera is installed on the roof of the NHM building to detect and record meteors in the sky above and in the surrounding of Vienna. It is part of the French Fireball Recovery and InterPlanetary Observation Network (FRIPON), with the objectives being, to determine the source regions of the various types of meteorites, to be able to collect fresh (and rare) meteorites, and to perform scientific outreach. The only way for determining the source regions of meteorites (in most cases from the asteroid belt, but a few also come from Mars and the Moon) is to observe the falls with cameras from more than one location so that the pre-atmospheric orbits can be back-calculated. The camera on the roof of the NHM building is a first test installation, as about a dozen further cameras would be necessary to cover the entire territory of Austria. Geometric triangulation of a meteor trace on the sky from more than one location allows to determine the actual trajectory in the atmosphere, which can on the one side be back-calculated into outer space to derive the orbit of the object in space, and on the other side can be forward-calculated to estimate the falling location of any surviving meteorite. If a search on the ground is successful, a new meteorite might be recovered. The real time image (refreshed each 15 minutes) taken by the camera on the roof of the NHM building can be seen here: https://www.fripon.org/IMG/jpg/stations/RT_ATWI01.jpg