Near-Earth Objects 299
a collision course with Earth. Arthur C. Clarke used this
scenario in his 1994 bookThe Hammer of Godand Hol-
lywood movies followed. Consider the physics. Meteorites
fall to Earth, and there are impact craters on Earth and the
Moon. The largest lunar impact basins formed more than
3.3 billion years ago, and the largest impactors were swept
up or ejected soon after the solar system formed. Most ob-
jects colliding today with Earth and the Moon are small and
harmless. Fragments that are a meter to a centimeter in size
appear as bright bolides in the sky and can deliver mete-
orites to the ground, though these are essentially harmless.
There are two aspects of the collision hazard to be consid-
ered: the magnitude of the collision and their frequency in
time.
7.1 Magnitude
The primary physical parameter of concern is the energy
of the collision and particularly the energy transferred to
Earth. The controlling parameters are mass and velocity
according to the relation
E=^1 / 2 mv^2If a massive body were to collide with Earth, the energy
of impact would be proportional to its mass. Objects that
are 10s of kilometers to a kilometer in size can cause sig-
nificant damage to Earth as a whole by triggering changes
in global climate that will affect human systems such as
agriculture. Objects less than a kilometer in size still pose
a significant regional threat, having impact energies that
eclipse the world’s arsenal of nuclear weapons.
Energy is also proportional to the square of the velocity,
so a high-velocity object will have considerably higher im-
pact energy than relatively slow-moving objects. Most near-
Earth objects travel at similar orbital velocities to Earth
when nearby, about 30 km/s. But because their orbits are
often inclined or more eccentric than the Earth’s orbit,
there is still a measurable relative velocity. Objects in highly
eccentric and/or inclined orbits, such as comets, can have
tremendous impact energy.
Any object approaching the Earth is accelerated by the
Earth’s gravity. The minimum velocity of any object enter-
ing the Earth’s atmosphere is equal to the Earth’s escape
velocity, 11.8 km/s. So even relatively slow-moving NEOs
can have quite significant energy when they hit.
Assessment of the damage that a particular impact will
impart to Earth is based on how much energy any partic-
ular location can absorb and whether or not that location
can recover from an impact. Meteoroids enter Earth’s at-
mosphere with energies estimated in the 10^11 –10^15 Jinthe
1–50 m size range breaking up and burning up in Earth’s
atmosphere, leaving perhaps only scattered dust. On the
other hand, damage from meteorites has been documented
on various scales, from killing a dog in Egypt in 1911 to
punching holes in roofs, bruising a human thigh, and going
through the trunk of a car.
Craters are produced by impacts with energies on the
order of 4.2× 1016 J, or 10 megatons (MT) of TNT. [See
PlanetaryImpacts;Meteorites.] Impacts of greater
energies, by an order of magnitude or so, can impart re-
gional damage. Studies have shown that an impact of 4.2×
1017 J, or 100 MT, can destroy areas within a 25 km ra-
dius. Of further concern is that an impact into the ocean
might induce tsunamis that would destroy coastal areas.
The Cretaceous–Tertiary Event 65 million years ago has
been estimated at >4.2× 1023 J, or 100,000,000 MT!
Such large impacts occur very infrequently. But they do
occur.7.2 Frequency
A complete assessment of the situation requires knowledge
of the frequency of collisions by objects of different sizes.
Objects in the range of 100s of kilometers in diameter were
swept up and incorporated into the planets or dynamically
ejected as the solar system formed during the period called
the Late Heavy Bombardment. The lunar basins formed
during this time, which ended∼3.8 billion years ago. No
terrestrial collisions are expected from such large objects
today. [SeeTheMoon.]
An impact by an object less than 50 m in diameter with
energies<4.2× 1016 J(<10 MT) occurs about once ev-
ery 1000 years. An impact in Tunguska, Siberia in 1908 may
have been an NEO about this size. Interestingly, that object
did not make a crater because it probably was a weak (heav-
ily cracked) rocky body that broke apart in the atmosphere.
Only the shock wave of air from the∼12 MT explosion
reached the ground, felling thousands of square kilometers
of remote forest. The frequency of impacts increases expo-
nentially with decreasing size and energy. Conversely, for
larger objects the frequency decreases.
To assess the potential for any near-Earth object to col-
lide with the Earth, it is imperative to have an accurate as-
sessment of the numbers and locations of this population.
It is then important to know the nature of the orbit for each
object because that bears directly on the object’s velocity
relative to Earth in its motion around the Sun. The active
asteroid search programs are designed to inventory the pop-
ulation of objects that may impact Earth. Upon discovery
of a new NEO, its orbit is determined and its future orbital
evolution is projected by computer simulations. If there is
a potential threat of its impacting Earth, a call for follow-up
observations is made, and the threat is evaluated carefully.
The existence of potentially hazardous asteroids (PHAs) is
monitored closely, worldwide. Even though impacts are not
a likely occurrence, they remain a possibility.