In a significant discovery for international space research, Polish scientists have found a way to trace the spots where meteorites fall to Earth.

Obtaining extraterrestrial material for research has long been one of the greatest dreams of astrophysicists and geologists trying to unravel the mystery of the solar system's origins. Meteorites falling to Earth are an invaluable source of information. The problem is to trace the spot where they fall.

The Apollo missions of the 1960s and '70s were a veritable feast for scientists, bringing back over 380 kilograms of lunar matter to Earth. Until recently this was about all the space matter available to researchers.

The situation was supposed to change thanks to three ambitious space missions. The first of these, Genesis, in 2001-2004, was meant to catch particles of the solar wind and send them to Earth in a special capsule landing with the help of a parachute. The enterprise was a fiasco because the parachute failed. The next project was Japan's Hayabusa mission, which was to have obtained and brought back samples of soil from the Itokawa planetoid. Unfortunately, the probe probably never managed to land and is now lost in space. The third mission was successful. The Stardust probe flew through the tail of the 81P/Wild 2 comet, caught many particles from it, and brought them to Earth in 2006.

These three missions clearly show that agencies such as NASA are willing to spend hundreds of millions of dollars to obtain matter from outer space.

Zooming in on meteors

Often, though, the simplest solutions can be the cheapest and the best. Every day several million particles of interplanetary dust fall into the Earth's atmosphere. A great many of them are the size of a grain of sand and are incinerated about 80-100 km above the Earth, resulting in the extraordinary phenomenon of "falling stars." Only very few of these grains of dust, weighing a few kilograms or more, can turn into bolides, also known as fireballs-meteors of substantial size that shine as brightly as the full moon-which then come down to the Earth's surface as meteorites.

This kind of space matter is an invaluable source of information on the beginnings of the solar system. The problem is that scientists miss many of these bright phenomena and thus lose valuable research material. Time is also against them. Most meteorites are in the stony class, which means they quickly become similar to ordinary stones due to erosion.

It is no wonder then that for many years scientists have been conducting laborious observations of bright phenomena in the sky and identifying them by taking photographs. The technique of these observations is no simple matter. To collect the largest possible amount of information, such as determining the orbit, the trajectory in the atmosphere and the place where a meteorite is likely to hit the ground, it has to be observed from at least two stations separated by several dozen kilometers.

The first person to apply this method on a greater scale was Fred L. Whipple, a well-known investigator of comets and meteorites. In 1939-1951, at Harvard University, he began a survey of the skies using Schmidt photo cameras. Whipple aimed not to determine the points of impact of meteorites, but to identify the orbits of the greatest possible number of these phenomena. His instruments were sensitive but had a relatively narrow field of vision. After the project ended in 1951, Whipple's work was continued in Czechoslovakia under the supervision of astronomers from the Ondrejov Observatory.

European fireball network

A breakthrough in the development of the Czech and European fireball network came in 1959 when, on April 7, Czechoslovak stations recorded the flight of a fireball with a brightness of 19 mag, or 250 times brighter than the full moon. A mag (from the Latin magnitudo) is a logarithmic scale used to describe the brightness of stars and other heavenly bodies. A brightness of 0 mag is equivalent to that of the brightest stars in the northern hemisphere; 6.5 mag is equivalent to the weakest stars visible to the naked eye; -4 mag corresponds to Venus, the brightest planet in the sky; and -13 mag is the brightness of the full moon.

Photographic observations allowed a fireball's orbit to be determined, and the place where it would potentially fall. This led to the rapid discovery of four meteorites that fell near the town of Pribram.

This case made European astronomers realize that it was worth investing in a fireball network covering the largest possible area. In 1963, the Czechoslovak stations were transformed into the beginnings of the European Fireball Network (EN). Five years later Germany joined the project, followed by the Netherlands in 1978. Today there are more than 30 photographic stations scattered across countries that include the Czech Republic, Germany, the Netherlands, Austria, Slovakia, Belgium and Switzerland.

Statistics from the best, Czech, network show that an average of 32 bolides are recorded above that country each year, of which one is capable of flying through the atmosphere and falling as a meteorite, and the point of impact can be accurately determined.

Poles at work

Given that Poland is four times larger than the Czech Republic, it is easy to calculate that the lack of a fireball network here means missing over 100 bright bolides and one to two potential meteorite impacts every year.

The main problem is money. The Czech network's fully automated photographic stations have top-of-the-range electronic optical devices. Apart from accurately mapping the route of the phenomenon in the sky, they can precisely determine its angular velocity, the time it appeared, plot the curve of brightness changes and record potential sound effects. One station costs tens of thousands of zlotys. There should be at least 30 of them in Poland to make a good network. But there is still insufficient funding available for this kind of research, for one thing because there is no tradition of investigating the smallest bodies of the solar system.

Luckily, professional astronomy has been drawing extensively from the work of enthusiasts for some time. One of the world's most active organizations of meteor observers is the Polish Comet and Meteor Workshop (PKiM).

In 2004, with the help of graduate and PhD students at the Astronomical Center of the Polish Academy of Sciences in Warsaw and the Warsaw University Astronomical Observatory as well as PKiM members, researchers began work to set up the Polish Fireball Network (PFN). Due to modest funding, they had to apply a completely different work system than their Czech counterparts. They were helped by state-of-the-art electronics including good and cheap cameras used in close-circuit television (CCTV) systems. The researchers also managed to obtain supplementary funding for the project from Siemens Building Technologies, a leading manufacturer of cameras and lenses.

Eye on the sky

Observations in Poland began in earnest in 2005. Today there are more than 10 stations operated by PKiM members, each equipped with two to three cameras with lenses that ensure an angle of vision of almost 70 degrees and record meteors with a brightness of up to two stellar magnitudes.

The number of observations is impressive; in 2005 all the cameras monitored the sky for over 10,000 hours, and in 2006 this was several thousand hours more.

One of the brightest phenomena observed to date was the "Krzeszowice fireball" that appeared on April 3, 2005, and was recorded by as many as five Polish observation sites. Its brightness was not much smaller than the full moon's. The fireball entered the atmosphere at 28.85 kps and began shining at an altitude of 98.3 km. It traveled 115 km and burned out 37.9 km above the ground. During its flight it split into several pieces. However, its speed was too great for it to make it through the atmosphere and end its life as a meteorite.

Meteor showers

Particularly intense fireball activity was observed in August 2005, when three bright phenomena were recorded during three nights. They seemed to come from the same location on the celestial sphere, which suggested a physical correlation and the existence of a new stream rich in large particles.

An unexpected explosion of activity of early Perseids, one of the most active meteor showers in the sky, was observed from July 17 to Aug. 25, 2005, with a peak on the night of Aug. 12. This shower was formed by Comet 109P/Swift-Tuttle. Under favorable conditions, the Perseids can be the source of about 100 "falling stars" per hour.

The meteor shower showed the power of video observations. The researchers' cameras are sensitive to near-infrared, which enables them to record objects even through thin clouds and also just after sunset and just before sunrise.

Without video technology, the researchers would have missed the explosion of the early Perseids observed on the morning of July 14, 2005. The cameras recorded more than 100, while usually there are just a few.

Over the past two years, Polish researchers have set up more than 10 fully automated stations to monitor the sky on every clear night. The first results are promising, yet the Polish Fireball Network is still waiting for its first fireball that would get through the layers of the atmosphere and land on Earth as a meteorite.

Arkadiusz Olech

The writer is an assistant professor at the Astronomical Center of the Polish Academy of Sciences in Warsaw and a former long-serving head of the Polish Comet and Meteor Workshop (PKiM).