Sky & Telescope - March 2024 - 75

ASTEROID MOON: NASA / JPL; MAIN ASTEROID BELT
DIAGRAM: BEATRIZ INGLESSIS / S&T
discoveries climbed into the hundreds
each year. Now, it's in the thousands.
Of course, the influx of new finds
meant that astronomers needed to find
a way to categorize them. Initially, the
discoverers named the asteroids, typically
after female Greco-Roman deities
such as the aforementioned Ceres, Pallas,
Juno, and Vesta. By the 1850s, the
editors of the Astronomische Nachrichten
had begun assigning numbers to
new discoveries. As the number of new
asteroids multiplied, modifications had
to be made, making these early efforts
at organization increasingly complex
and sometimes clumsy.
A New Way to Name
Starting in 1892, a new scheme
emerged, and the IAU still uses it
today. Here's how it goes: First, there
is the discovery, which entails at least
two nights of observations of a new
object. The observer submits these to
the IAU Minor Planet Center, which
assigns a provisional designation
according to a specific format: It begins
with the year of discovery, followed by
a letter of the alphabet (except I and Z)
to show the half-month of discovery;
then, another letter (except I) shows
the order of the discovery within that
half-month. If there are more than
25 discoveries within a half-month,
numbers are added to show how many
times that letter has been repeated.
If that's clear as mud, here's an
example: Let's take discoveries made
between September 16th and 30th in
2021, which receive an initial letter
" S. " The first few would be designated
2021 SA, 2021 SB, 2021 SC, etc. Once
the discoveries in that time period
reached 25 (2021 SZ), the sequence
would roll over with an additional
number (2021 SA1).
This designation is a placeholder
used to identify an object while additional
observations are conducted to
nail down its orbit. Astronomers need
to know the object's position to predict
where to find it in the future. Sometimes
figuring out the orbit of one
object ends up linking it to observations
of another one. Once the orbit is
determined, the Minor Planet Center
assigns a sequential number. As of
press time, those numbers (starting
with 1 Ceres) are up to 640000!
Identification doesn't end with
numbers. Once an asteroid's orbit is
well known, the discoverer may offer a
name to the IAU's Working Group for
Small Bodies Nomenclature. Its rules
(at left) are simple and allow a lot of
room for creativity. (Note: An asteroid's
name cannot be bought!)
To date, 24,534 asteroids have
received names, some for influential
people, such as 5102 Benfranklin,
284996 Rosaparks, and
316201 Malala; others for ancient
gods, like 433 Eros and 99942 Apophis.
Some are playful, like 13579
Allodd and 24680 Alleven. Still others,
like 13681 Monty Python and
88705 Potato, combine cultural recognition
with a touch of whimsy.
ASTEROID MOONS Even the moons of
asteroids may receive names. Discovered in
1994 after the flyby of NASA's Galileo mission,
the moon of 243 Ida was later named Dactyl,
after the mythological creatures that lived on
Mount Ida on the island of Crete.
Not every discoverer gets around
to naming their asteroid, though.
The IAU limits a discoverer's naming
privileges to 10 years after the
asteroid receives a number. After
that time, IAU's Working Group may
assign names from a pool of submissions.
With the uptick in discoveries
in recent decades, more than 600,000
asteroids could still receive names. ¢
Trojans
Venus
Mars
Mercury
Earth
Asteroid belt
Jupiter
Trojans
MAIN BELT A million or so asteroids reside in the main belt between the orbits of Mars and
Jupiter; thousands of additional asteroids belong to the gravitationally stable regions on either
side of Jupiter in its orbit around the Sun. (This diagram is based off of data from the Minor
Planet Center, published in 2006.)
sk yand tele scope .o r g * MARCH 2024 75

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