FEATURE ARTICLE
V.S. RAMACHANDRAN & JAYANT GANGOPADHYAY
A fireball was observed in the skies of Kerala on 27th February 2015 spreading panic throughout the state. What was it?
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N the night of 27th February 2015, people in the town of Kochi noticed balls of fire streaking across the night sky. As the news spread like wildfire, more and more people rushed out of their houses to watch the bizarre spectacle. Very soon people across the state were gripped in a state of panic as the incidence of the huge ‘fireball’ streaking across the sky was also reported from Thrissur, Ernakulam, Palaklad, Kozhikode and Malappuram districts of Kerala. The fireball was reported to have struck an area around Karimalloor village SCIENCE REPORTER, MAY 2015
in Ernakulam. Based on the reports, members of the Kerala State Disaster Management Authority (SDMA) visited the area and found a scorched patch of land near a masjid at Karimalloor village. But this claim is still subject to further investigations. Nobody has reported the apparent path, which is the first and last sighting of the trail of the fireball with reference to background star constellations.
Varied Interpretations According to a scientist, Rajagopal Kamath, the observed ‘fireball’ could be “...a rocket or satellite debris. It also could be stony chondrite meteorites as in many places people have claimed that they have seen a bluish flame, which is peculiar to meteorites”. According to Kamath, there are following possibilities: 1. China’s Norad 4063 is believed to have
The fireball observed in Kerala on 27th February 2015 was a rare celestial display. By seriously studying the impact craters and the meteoroid samples it will be possible to get better insight into this space debris. 26
FEATURE ARTICLE About one in 1,200 observed meteors becomes brighter than -5 mag, while only one in 12,000 reaches -8 mag. Bright meteors are called fireballs. A large number of such samples collected from various locations of the Earth will definitely throw more light on the nature and composition of the meteor and the materials that form the building blocks of our solar system. re-entered into the earth’s atmosphere and since February 23 the disintegrating pieces of the satellite have been lighting up the night sky in many parts of the world. This is the most plausible reason. A satellite had fallen in Siberia recently and remnants of that satellite could have caused the streaks. 2. The fireball could be a phenomenon called ball lightning. 3. There is a possibility that the streaks could have been part of a meteor storm, Gamma Normids. Scientists had predicted the event from February 23 to 27. Some people also experienced a mild tremor during the same period. As per Sekhar L. Kuriakose, member of the State Disaster Management Authority, the possibility of an earthquake was ruled out. Further, many people heard some sound along with the celestial spectacle. The sound that people heard could have been due to sonic boom. A sonic boom is the sound associated with the shock waves created by an object travelling through the air faster than the speed of sound. Sonic booms generate enormous amounts of sound energy, sounding much like an explosion.
Fireballs – Origin & Nature There is debris in space. This debris may be the dust particle left out by a passing comet, or small pieces of rocks displaced from the asteroid belt between the planets Mars and Jupiter. As the Earth moves in its orbit, its gravity pulls some of the grazing debris towards itself. When the debris moves
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A fireball of -6 mag zenithal magnitude terminaƟng about 50 km above the Earth’s surface will appear as a meteor of -1 mag at an elevaƟon of 5 degrees above the horizon for an observer 600 km from the event. The zenithal magnitude for visual observaƟons can be calculated using the formula:
M = m + 5 log (sin h) where M is the zenithal magnitude, m the apparent magnitude and h the elevaƟon of the event above the horizon. through the atmosphere of the Earth, the frictional force between the atmospheric gases and the space debris causes the debris to get heated up. At one point the temperature is so high that the debris is set ablaze and becomes visible on the Earth at a height of around 100 kilometres. These are often called meteors, resembling streaks of light in the sky. But if the size of the debris is large, then the illumination from the ignition is larger thereby making it appear brighter. Scientists measure the brightness of the celestial objects as observed from Earth’s surface in terms of apparent magnitude represented as a numerical number and written as mag. The higher the number, the fainter is the object. The lower the number, brighter is the object. In this scale, the full moon has a mag of -13 and Sun has a mag of -27. Most meteors seen in the course of an observing session are faint ones. Only a small fraction exceeds magnitude Zero, which is caused by millimetre-sized meteoroids. About one in 1,200 observed meteors becomes brighter than -5 mag, while only one in 12,000 reaches -8 mag. Bright meteors are called fireballs. The definition of a fireball is somewhat arbitrary and in literature the required minimum magnitude varies between about -2 mag to -6 mag. According to the International Meteor Organisation Fireball Data Centre (FIDAC), meteors of at least apparent magnitude -3 mag are called fireballs. The apparent brightness decreases with the square of the distance between the object and the observer, and furthermore, the absorption of light is proportional to the optical path length. In the case of a near-horizon meteor, the distance to the observer is very large resulting in a strong reduction of the apparent brightness.
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Energy Transformation A meteoroid is the space debris when it passes through the atmosphere of the Earth. The energy transformation occurring during the atmospheric flight of a meteoroid strongly depends on its geocentric velocity. Normally, we would expect a faster meteor of a given size to appear brighter than a slower particle of comparable size. In the case of a larger meteoroid producing a bright fireball, we also have to consider the fact that the lower the velocity, the deeper the meteoroid can penetrate into the atmosphere before ablation ceases, making the energy transformation very effective in denser layers of the atmosphere and thus producing a brighter meteor. A meteor of the same size but of high velocity only causes an energy transformation at a higher altitude. It is clear that the velocity at entry into the Earth’s atmosphere is the lowest when a meteor comes from the antapex (where its relative velocity is equal to the difference of velocities between the particle and the Earth), while it is highest when it approaches from the apex. Around the vernal equinox (21st March) the antapex reaches its northernmost declination and is located nearest the zenith in the evening sky. Since this is a physical model, the relation in principle holds also for the southern hemisphere. Bright fireballs should be more frequent in the southern spring, when the antapex reaches its southernmost position. The real picture may differ somewhat from this because the distribution of meteoroids is not isotropic along the Earth’s orbit.
Fireballs & Meteorites Only a very few fireballs are connected with meteorite falls. A meteorite may survive its atmospheric flight and then SCIENCE REPORTER, MAY 2015
FEATURE ARTICLE
Left: Meteor break-up during atmospheric flight; Right: Kerala State Disaster Management Authority officials examining the site
Most meteors seen in the course of an observing session are faint ones. Only a small fraction exceeds magnitude Zero, which is caused by millimetre-sized meteoroids. at least part of the body decelerates from its entry velocity down to its free-fall terminal velocity, which is about 100 m/s. According to some models, less than 50% of the remaining mass will be ablated as soon as the velocity falls below 8 km/s. Another destructive process that may operate against the fall of a meteorite is fragmentation of the meteoroid. The peak pressures these bodies can withstand were found to be about 106 N/m². This is much less than the crusting stress of about 108 N/m² found in laboratory measurements. This may be due to past history of the meteorite. So, a very large fraction of meteoroids undergo breakup when entering the atmosphere. Astronomers mostly assume (based on some valid scientific reasons) that a successful passage through the atmosphere will occur if the object enters at less than 23 km/s and survives to reach a velocity of less than 8 km/s without serious disruption. After the transformation of its kinetic energy and its total deceleration, the surviving meteorite falls only pulled by the Earth’s gravitation without the emission of light. This period is known as dark flight. During this phase the wind’s SCIENCE REPORTER, MAY 2015
force and direction have an important influence, and the surviving body may drift away from its original course. The effect can be considerable because both the free-falling meteorite and the wind have comparable velocities. Furthermore, the dark flight trajectory is affected by the shape of the meteorite. For a meteorite falling from a height of 33.6 kilometres above ground, a meteorite needs about 3 to 4 minutes in free-fall, so there will normally be a delay of this duration between sightings of the end of the track and reports of impact noises being heard. If there is information about such impact noises, a systematic search for meteorites is recommended.
Identification of the Meteorite The identification of a meteorite is in many cases not indisputable. A freshlyfallen meteorite usually has a black and relatively smooth melted crust a few tenths of a millimetre thick. This crust may crumble away and be lost within a short time-scale, dependent on the material involved. Carbonaceous chondrites are especially susceptible to this effect. The interior may be very different, and unambiguous identification
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is only possible using analytical methods or etching a cut and polished plane with acid to find the so-called Widmannstätten patterns in iron meteorites. In the case of fresh meteorite falls, an isotope analysis is made as soon as possible after the event. During recovery searches for meteorites all meteorite-like material are collected. It is prudent to include a lot of terrestrial material than to miss a real meteorite. The fireball observed in Kerala on 27th February 2015 was a rare celestial display. By seriously studying the impact craters and the meteoroid samples it will be possible to get better insight into this space debris. A large number of such samples collected from various locations of the Earth will definitely throw more light on the nature and composition of the meteor and the materials that form the building blocks of our solar system. Mr V.S. Ramachandran is Senior Curator and Head of the Regional Science Centre & Planetarium, Jafferkhan Colony, Calicut 673006, Kerala; Email:
[email protected] Mr Jayant Gangopadhyay is Technical Officer, Regional Science Centre & Planetarium, Calicut, Kerala; Email:
[email protected]
