Резюме. In the recent years meteorite hunting has turned to a well-known recreational activity and has become popular among children and the youth. Being such a tempting area of practice, meteorite prospecting draws attention and brings pleasure to the involved individuals thus predisposing them to get acquainted with the hidden secrets and underlying nature of meteorites and the methods and physical laws that make finding meteorites using machines possible. The current article tries to explain why meteorite hunting may attract students to physics and help them better understand certain phenomena of astrophysics and physics. The student may learn much about meteorite structure, chemistry, astrophysical origins, meteoroid entry into Earth atmosphere and its consequential mechanical dispersion, and finally and most importantly the physics of electromagnetism, on which are based the modern meteorite searching metal detectors.

Ключови думи: meteorite hunting, physics education

Introduction

In the near past meteorite prospectors were using their naked eyes to identify and gather meteorite pieces around the world. The only tool occasionally used was a simple magnet glued to a stick. Because most meteorites contain iron and nickel they tend to be attracted to magnets especially to rare earth magnets. The latter ones exhibit a very strong magnetic field. To distinguish a meteorite from a terrestrial iron bearing rock (a “hot rock”) the observer needs to have certain experience or should rely on laboratory verifi cation.

Nowadays, modern metal detectors came into play and searching for meteorites became a pleasant and involving experience. Handheld metal detectors are gaining interest among the young and elderly for their sophisticated machinery . But the most important aspect for the physics education endeavour is that the prospector and hence the user of the metal detector will inevitably get acquainted with the underlying physics laws that enable the metal to be detected and found by a metal detecting apparatus. Physics students may benefit strongly from such an experience through raising their interest in electromagnetism, astrophysics and other branches of physics related to the meteorite hunting enterprise.

Motivation

What motivates the author to start investigating physics education through meteorite hunting is his personal experience with the metal detector hobby and the observation on how metal detector prospecting attracts people and raises their interest in the underlying physics of the searching technologies.

Involving people in physics education was proven to be possible through nonstandard approaches such as using computers in education (Zabunov, 2004) and more precisely implementing physical phenomena by the means of computer-aided physics simulations (Zabunov, 2010). The student is further motivated by cutting-edge hightechnology utilization such as stereo 3D online e-leaning systems (Zabunov, 2012). Any modern and attractive technology raises students’ attention in the studied subject. Metal detectors are such a technology and meteorite hunting is an activity based on this technology. Through many experiments and observations, the author has acquired the confidence that people are strongly inspired by the metal prospecting occupation and specifically by the meteorite hunting avenue, and further, they are willing to understand how things are happening and what physics laws are used in the metal detector.

The author also wants to point out that meteorites of large scale have fallen in his country Bulgaria such as the Belogradchik meteorite (Toshev, 2009; 2013; 2014).

Meteorite types and taxonomy

About 94% of all meteorites are stony meteorites divided into two major groups - chondrites and achondrites (Bischoff & Geiger, 1995). The rest 6% are iron or stony-iron meteorites. For complete modern classifi cation of meteorites the reader may consult “1.05 Classification of Meteorites” (Krot et al., 2007). Near 86% of all meteorites are chondrites (cf, Meteoritical Bulletin Database1) ).

Chondrites contain round and small inclusions in the stony matrix that are called chondrules. Chondrules consist of mostly silicate minerals (Fig. 2). There are chondrites that contain traces of organic material, for example amino acids. Due to chondrites age of about 4.5 billion years it is speculated that they are part of the asteroid belt that did not merge into large space bodies. Near 8% of all meteorites are achondrites. They are stony meteorites that do not contain chondrules. There is a group of achondrites that comes from the Moon. These achondrites are similar to the rocks brought by Apollo and Luna programs. For another group of achondrites scientists are almost certain that they come from Mars.

Of the 6% non-stony meteorites about 5% are iron meteorites and 1% is stonyiron meteorites. Iron meteorites are composed of iron-nickel alloys (kamacite and taenite). The lar gest meteorite on Earth is an iron meteorite (Fig. 1). Stony-iron meteorites contain iron-nickel metal and silicate minerals. A typical representative of the stony-iron group is the pallasite (Fig. 3). Another major group of stony-iron meteorites is the mesosiderites.

Metal detectors as a technology for attractive physics education

Metal detectors exhibit a lot of physical phenomena to the user and in this manner they utilize physical properties of the ground and target objects in order to derive information about the sought material. Metal detectors use various physics laws such as acoustics, electromagnetism, ionizing radiation, etc. Most wide-spread metal detectors are those based on electromagnetism. These are the induction balance detector, the ground penetrating radar and the pulse induction detector. The most ubiquitous metal detector type is the induction balance metal detector. These devices have moderate weight, cost, and the lowest power consumption among all three types of electromagnetism-based detectors mentioned above.

The ever increasing capabilities of induction balance metal detectors make them practical winners in the meteorite hunting enterprise. They are also affordable to universities for educational purpose.

Metal detectors through the years

First metal detectors were implemented in searching for artillery shells and landmines. One of the fi rst metal detectors was the French “Alpha” (Fig. 4) developed by M. Guitton after WWI (Honoré, 1919).

Another early but with modern is the Polish Mine detector Mark I (Fig. 5). It was a metal detector for searching of landmines was developed during the ond World War. After Germany invaded Poland in 1939 and cupied France in 1940, work the detector was interrupted and it was restarted in the winter of 1941 – 1942. The inventor of this modern detector was Polish lieutenant Józef Kosacki.

The Polish detector was also of induction balance technology, having two coils in its search head positioned in induction balance. One coil was transmitting coil and the other was a receiving coil. It was operated by headphones (Fig. 5). The whole apparatus was 14 kg in weight (Croll, 1998). During the World War Two a few hundred thousand detectors were manufactured for the Allies.

Modern models of induction balance metal detectors

A famous pioneer in modern metal detector design is Gerhard Fisher . He invented a radio direction-finding system. His system was used for navigation. While performing tests, Fisher noticed errors in navigation data in areas rich in ore-bearing rocks. His further developments in this direction led to the 1925 patent grant for metal detector.

Today numerous manufacturers offer complex metal detectors to the market (Fig. 6).

Modern high-end metal detectors are controlled by a microprocessor . Further enhancements include multi-frequency transmission, digital signal processing, graphical visualization of the search data, etc.

Meteorite hunting with metal detectors and physics education

In order to involve young people into physics topics one could demonstrate physical phenomena through the means of metal detectors. It is a thrilling experience for everyone to hold in their hands a metal detector and to hunt for hidden objects. How about hunting meteorites?

Using an induction balance metal detector, the teacher may carry out education in the laboratory or in the open. The instructor would demonstrate the basic principals used by a metal detector. This could be carried out by hiding metal objects in one’s hands or clothes and detecting them using the metal detector. Further, the theoretical principals of electromagnetic metal detection may be disclosed.

The metal detection physical laws for ferromagnetic and non-ferromagnetic objects are different. An induction balance metal detector can distinguish between the two metal types. The ability of the metal detector to make difference between metal types is called discrimination. The ferromagnetic metals tend to magnetize under magnetic field while non-ferromagnetic ones do not. On the other hand any metal is conductive and conductivity yields eddy currents in the conductor. These two physical phenomena are separate. But in some objects they coexist. Upon magnetization using alternating magnetic field the magnetized material will produce secondary alternating magnetic fi eld. The secondary fi eld will have its phase lagging behind the transmitted fi eld and the resultant magnetic fi eld will also lag. We say that the phase shift is negative (Fig. 7). With the eddy currents phenomenon things are in reverse. The eddy currents are induced in the object by the alternating magnetic fi eld of the transmitting coil of the detector. These eddy currents create secondary alternating magnetic field with positive phase shift (Fig. 7). Thus conduction materials may be discriminated from ferromagnetic materials. Even salt water, which is an electrolyte, is spotted by the detector although it is not a metal. Further, a ferromagnetic metal that is grained and has pour conductivity is well recognized by the metal detector as ferromagnetic target.

Most meteorites contain iron, nickel and cobalt, which are ferromagnetic metals. Thus the meteorite hunter should use its metal detector to search for ferromagnetic targets. The meteorite hunter is stimulated to understand the physical laws just described, because this knowledge makes the hunter more successful in their search. Everyone would like to know more about the physics of metal detecting in order to gain the most of their detectors. Also, the whole process of adjusting the detector, the interpretation of its information on the display and the digging for a target is fun and predisposes the student to perceive physics education with pleasure, not with fear.

Conclusions

Generally, modern technologies predispose students positively to the taught material. Implementing computerized and sophisticated, but also personalized equipment in the tutoring process increases attention and involvement in students. The implementation of meteorite hunting using metal detectors as an attractive method of teaching physics at schools and universities presumes improved results. Because this method reveals unthought-of benefits to the teaching process, the author is confident to propose it for implementation and testing in the educational programme in Sofia University, Faculty of Physics.

NOTE

1. www.lpi.usra.edu)

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