Tunguska comes down to earth

Few natural events this century have excited as much popular scientific interest as the gigantic explosion that rocked the Siberian taiga near the Tunguska river on the morning of 30 June 1908. Second only to the Loch Ness monster and Bigfoot in the popular science press, the Tunguska explosion has moved otherwise reputable scientists to propose explanations ranging from antimatter meteorites to mini black holes to near-critical fissionable masses. One theory popular in 1930s Russia attributed it to the explosion of a nuclear powered (!) spacecraft. And the UFO explanation seems prevalent in some quarters even today. However, serious scientific study has been converging on a more prosaic explanation wrapped up by Chyba and colleagues.

At its simplest, this explanation holds that the Tunguska explosion was caused by the fall of a large meteorite that broke up and deposited its energy in atmospheric blast waves before reaching the ground. The essentials of this picture were established by L.A. Kulik, who made the first on-site investigations of the explosion in the years 1927-30. Kulik attributed some boggy depressions near the explosion site to meteorite impacts, a proposal that was later discredited. No impact crater or large meteorite fragments were ever found at Tunguska.

Kulik's work was refined by E.L. Krinov, who proposed that the explosion was created by a comet. The idea of a cometary impactor was strongly supported by more recent work, although the density of the required comet is very small, 0,01-0,001 g cm-3. Compared to the density of roughly 0,6-1,0 g cm-3 reported for comet Halley. This would make the Tunguska object decidedly unusual. However, Chris Chyba, Paul Thomas and Kevin Zahnle now argue that a full consideration of the dynamics of a meteorite traversing the atmosphere shows that the Tunguska explosion is fully compatible with the entry of aroughly 30-m diameter meteorite of the common stony class.

H.J. Melosh, Monthly Nature

The 1908 Tunguska Explosion: Atmospheric Disruption of a Stony Asteroid

The explosion over Tunguska, Central Siberia, in 1908 released 10 to 20 megatons (high explosive equivalent) of energy at an altitude of about 10 km. This event represents a typical fate for stony asteroids tens of meters in radius entering the Earth's atmosphere at common hypersonic velocities. Comets and carbonaceous asteroids of the appropriate energy disrupt too high, whereas typical iron objects reach and crater the terrestrial surface.

Light Nights over Eurasia

Widespread "light nights" were observed over Eurasia for the first few nights after the Tunguska event. The Tungliska explosion may have lofted enough material high enough to account for the European "light nights".

Ch. F. Chyba, Paul J. Thomas & Kevin J. Zahnle, Monthly Nature

КОНТРОЛЬНЫЕ ЗАДАНИЯ

I. Перепишите предложения. Письменно переведите их на русский язык. Подчеркните глаголы в различных временах. Определите, в каком лице и числе стоит глагол.

II. Перепишите предложения. Письменно переведите их на русский язык. Определите видо-временную форму глагола в страдательном залоге.

III. Выразите числительные буквами и цифрами.

IV. Перепишите предложения. Определите степени сравнения прилагательных. Переведите предложения.

V. Прочтите текст. Перепишите его и письменно переведите.

VI. Выпишите все глаголы-сказуемые из текста и определите их видо-временные формы. Переведите предложения.

ВАРИАНТ 1

I. Перепишите предложения. Письменно переведите их на русский язык. Подчеркните глаголы в PRESENT SIMPLE. Определите, в каком лице и числе стоит глагол.

1)You’ll have some time to relax before dinner if you are tired after the journey.

2)I usually work in the roads department but this month I’m working in the tunnels department.

3)I read an interesting novel yesterday.

1)Houses are built very quickly now.

2)A new theatre will be built here next year.

3)America was discovered by Christopher Columbus.

III. Напишите числительные полностью.

1)the 1st

2)206

3)the 25th

4)1.985

5)130