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a massive bombardment by objects roaming through space, some of them enorm-
ous. What had become of these gargantuan bodies?
In the early 1970s, Hartmann and his collaborator Donald Davis had begun to
calculate the size distribution of the bodies in the last sweep-up of the solar sys-
tem, just before the final set of planets and moons emerged. They found that in
each region, one large body (the future planet) would have been present, along
withasmallerone,astillsmallerone,andsoondowntorelatively tinyobjectsthat
had so far avoided accretion and that, if they continued to escape, would become
the comets and asteroids. The second-largest body prowling near the proto-Earth
would have had a radius of 500 to 3,000 km; tens of bodies with radii larger than
100 km would have been nearby. The diameter of our Moon at its equator is about
3,500 km, Mars measures about 6,800 km, and Earth about 12,500 km. In other
words, near the early Earth, bodies the size of Mars and the Moon were racing
though space at tens of thousands of miles per hour. “The probability of the planet
interacting with a large body is much larger than has been considered,” Hartmann
and Davis wrote. 12 Since kinetic energy rises with the square of velocity, these
primitive giants held prodigious amounts of energy. If a planetesimal 1,200 km in
radius were to have collided with the Earth, just half its kinetic energy would have
been enough to launch two bodies the size of the Moon.
When Hartmann rose to present his results at the 1974 Cornell conference, he
speculated that if a body the size of Mars “hit the earth it might splash up the sort
of material you need to make the Moon and also get the right amount of kick in-
to the system.” 13 The “kick” referred to the combined angular momentum of the
Earth and the Moon, the nemesis of most other theories. The glancing blow that
Hartmannenvisionedwouldhavesentbothbodiesspinninglikegianttops,provid-
ing them with plenty of angular momentum.
LikethegreatRutherfordseventyyearsbefore,Hartmannfearedtheworstwhen
hespottedinhisaudienceamuchmoresenior,highlyrespectedauthority.A.G.W.
Cameron of Harvard was “a large, imposing man, who talks in a slow, pontifical
fashion that might indeed strike fear into the heart of a young astronomer,” wrote a
reporter. Unlike Lord Kelvin, Cameron was wide awake. But to Hartmann's relief,
Camerontoldtheaudience,“We'reworkingonthesameidea,andwe'recomingto
the same conclusions.” 14 Hartmann and Davis had been trying to work out the size
distribution of the next-to-last set of primordial bodies. In contrast, Cameron and
his colleague William Ward had been trying to explain the angular momentum of
the solar system starting from the collision of planetesimals. The two approaches
started from different places but were leading to the same destination: a theory of
giant impact.
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