Sky & Telescope - March 2024 - 74

BEGINNER'S SPACE by Monica Young
How Are Asteroids Named?
THERE ARE MILLIONS of asteroids in
the solar system. Some are as small
as an ostrich, while others outspan
Iceland; however, most first appear
as faint blips caught moving among
the background stars in a sequence
of photographs. So it may come as no
surprise that an asteroid's path to recognition
is long and, ahem, rocky.
What Is an Asteroid?
Functionally, an asteroid is a small,
rocky body that orbits the Sun. They're
essentially leftovers of the planet formation
that occurred in the solar system
4.6 billion years ago. Most of them
have changed little in the intervening
eons, though they can be altered by
impacts and solar heating.
Technically, " asteroid " is somewhat
trickier to define since, surprisingly,
the International Astronomical Union
(IAU) has never offered an exact definition.
Instead, in the same resolution
in which the organization set out a new
understanding of " planet " in 2006, it
also lumped together asteroids, comets,
and other objects as " small solar
system bodies. " That designation covers
anything that is not a planet, dwarf
planet, or natural satellite.
However, since the IAU defines
meteoroids as being smaller than
1 meter (3 feet), then anything larger
could logically be called an asteroid.
The smallest asteroid known to date,
2022 WJ1, is barely 1 meter across.
Most known asteroids orbit the Sun
in what's called the main belt, which
lies between the orbits of Mars and
Jupiter, 2.2 to 3.2 astronomical units
(a.u.) from the Sun. That is, their
orbits are roughly 2 to 3 times wider
than Earth's. Thousands of asteroids,
known as Trojans, share Jupiter's orbit,
at an average distance from the Sun
IAU's rules for proposed
asteroid names:
* 16 characters or less in length
* preferably one word
* pronounceable (in some language)
* not offensive or political
* must differ from existing name by
at least one letter
of 5.2 a.u. Others are renegades, with
elongated or unusual orbits. Small
objects much farther from the Sun,
near and beyond Pluto's orbit, have icy
rather than rocky compositions. But
all of these also receive designations
that follow the rules for asteroids.
A Brief History of Asteroid
Discoveries
Astronomers spotted the first asteroids
in the 19th century. On New Year's
Day of 1801, Giuseppe Piazzi was cataloging
the faint stars of Taurus when
he came across 1 Ceres (now classified
as a dwarf planet). A year later,
Wilhelm Olbers found 2 Pallas; he
later proposed that the asteroids had
once been part of a planet between the
orbits of Mars and Jupiter. The next
few years saw the announcements of 3
Juno and 4 Vesta.
Then, despite energetic efforts,
came four decades of nothing. But the
lull was only temporary: The next discovery
occurred in 1845, and by 1868,
observers had visually discovered some
100 asteroids. Driven by the advent of
photography, by the late 20th century
HISTORY OF A NAME Wilhelm Olbers discovered
Vesta in 1807 from Bremen, Germany,
while searching for more objects like Ceres
and Pallas. But Olbers didn't name Vesta;
he gave that honor to Carl Gauss, whose
calculations of the object's orbit had led to its
confirmation. Gauss named the asteroid for
the Roman goddess of the hearth and home.
74 MARCH 2024 * SKY & TELESCOPE
NASA / JPL / MPS / DLR / IDA / DANIEL MACHÁCˇ EK
Sky & Telescope - March 2024

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