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The asteroid Vesta was visited by
the Dawn spacecraft from 2011–12.
Its orbit lies within that of Ceres,
and it is the brightest asteroid as
seen from Earth.The Celestial Police kept up the
search and, in March 1802, Olbers
discovered a second body like
Ceres located at the same distance
from the sun, calling it Pallas. In
1804, Karl Harding found a third,
named Juno, while it was Olbers
again who spotted the fourth,
Vesta, in 1807. All these bodies
were later shown to be smaller
than Ceres—Vesta and Pallas
were slightly more than 300 miles
(500 km) wide and Juno was
half that size.
Asteroid belt
The Celestial Police called their
discoveries minor planets, but
William Herschel chose another
name—asteroid, which means
starlike. Herschel reasoned that,
unlike true planets, these small
objects had no discernible features,
or at least none that could be made
out with the telescopes of the day,
so would be indistinguishable from
starlight were it not for the fact that
they moved. Perhaps still smarting
from his failure to name the planet
he had found 20 years earlier,
Herschel qualified his suggestion
by reserving for himself “the liberty
of changing that name, if another,
more expressive of their nature,
should occur.”
Nothing more expressive did
occur, and after the Celestial Police
was disbanded in 1815, a steady
trickle of asteroid discoveries
continued. By 1868, their number
stood at 100; by 1985, it was 3,000.
The advent of digital photography
and image analysis has now
boosted the number of recorded
asteroids to more than 50,000,
spread around the 28-Bode-unit gap.
Olbers and Herschel had discussed
the possibility that the asteroids
were the remains of a planet that
once orbited in the gap before
being smashed by an astronomical
cataclysm. Today, it is thought
that the gravitational disruption
of nearby Jupiter prevented the
asteroids from accreting into a
planet in the first place, as similar
disks had done elsewhere in the
primordial solar system.
Under constant influence from
the cumulative gravity of other
asteroids, about 80 percent of
known asteroids have unstable
orbits. The 13,000 or so bodiesURANUS TO NEPTUNE
that come particularly close
to Earth—the Near Earth Asteroids
(NEAs)—are monitored in the
hope of predicting and preventing
devastating future impacts.Trojans
There are also asteroids known
as trojans, which travel in the
same orbits as planets, gathering
far from their host in gravitationally
stable “libration points.” Most of
these are in the Jupiter system,
where they form two clusters: the
“Trojan Camp” and “Greek Camp.”
Mars and Neptune have trojans,
and the first Earth trojan was
discovered in 2011.
In 2006, the International
Astronomical Union gave Ceres
the status of dwarf planet, the only
one in the asteroid belt. At the
same time, Pluto was reclassified
as a dwarf planet. The orbits of
neither Neptune nor Pluto match
the predictions of Bode’s law.
Despite the fact that it was
instrumental in the discovery of
Ceres, Bode’s law is now viewed
as a mathematical coincidence,
and not a key to unlocking the
formation of the solar system. ■The evening of the third,
my suspicion was converted
into certainty, being assured
it was not a fixed star.
I waited till the evening of
the fourth, when I had the
satisfaction to see it had
moved at the same rate as
on the preceding days.
Giuseppe PiazziThey resemble small stars
so much as hardly to be
distinguished from them.
From this, their asteroidal
appearance, if I take my name,
and call them asteroids.
William Herschel