speed of 15 km/sec. It exploded at an altitude of 8 km and
released energy equivalent to 20 million tons of TNT. Two
scientists discuss whether this object was a comet or an
asteroid.
Scientist 1
The object was a comet, a body made of ices (such as frozen water or methane) and dust. Most of this
cometary material is volatile (easily vaporized) and low in density. Friction in Earth’s atmosphere heated
the comet to a temperature at which it exploded, high above the ground. The majority of the ices and dust
were vaporized in the explosion, which explains why no crater was formed at the site and why no large,
identifiable fragments of the object were found. An asteroid would not have been completely destroyed.
Intact asteroid fragments that reached the ground would have created one or more craters upon impact and
left behind recoverable pieces. Evidence shows that the object decelerated rapidly before it exploded.
Because of their low density, comets are capable of such rapid deceleration, whereas high-density objects,
such as asteroids, are not.
Scientist 2
The object was a stony asteroid. As it entered Earth’s atmosphere, its high speed created a large air pressure
difference between the area just in front of the asteroid and the area just behind the asteroid. The large
pressure difference eventually exceeded the structural strength of the asteroid. The asteroid flattened,
decelerated rapidly due to the dramatic increase in its surface area, and fragmented before reaching the
ground. This fragmentation would have appeared like an explosion. Calculations show that a comet
between 10 m and 100 m in diameter would explode at an altitude much higher than 8 km, but a stony
asteroid of that size would fragment at or near an altitude of 8 km. Recovery of large asteroid fragments is
difficult due to the area’s boggy soil; however, small, glassy fragments were recovered and are believed to
be melted and resolidified pieces of the asteroid.
6. Which of the following phrases best describes the major point of difference between the 2 scientists’
hypotheses?
F. The location of the event
G. The speed the object was traveling
H. The density of Earth’s atmosphere
J. The type of object that entered Earth’s atmosphere
7. According to Scientist 2’s viewpoint, compared to the altitude at which a stony asteroid would have
exploded in Earth’s atmosphere, a comet of similar size would most likely have exploded at:
A. the same altitude.
B. a higher altitude.
C. a slightly lower altitude.
D. a much lower altitude.
8. Scientist 1’s viewpoint indicates that when the materials that compose most of a comet are sufficiently
heated, they change to:
F. solids.
G. gases.
H. liquids.
J. a vacuum.
9. Which of the following statements best describes how Scientist 2 would explain why no large,
identifiable fragments of the object have been recovered?
A. Any large, identifiable fragments that reached the ground have been removed from the area by erosion.
B. Any large, identifiable fragments were thrown hundreds of kilometers from the site.
C. No large, identifiable fragments of the object reached the ground.
D. No large, identifiable fragments of the object have been recovered due to the soil conditions in the
area.
