THE SOUTHWORTH PLANETARIUM
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Founded January 1970
Julian Date: 2458870.16
2019-2020: LXXXVIII
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THE DAILY ASTRONOMER
Tuesday, January 21, 2020
Here There Be Trojans
It's our fault. truth be told.
When we attempt to describe the solar system, we naturally mention the Sun,
its retinue of attendant worlds, and the myriad moons that revolve around
those words, save Mercury and Venus. Perhaps we'll also give a nod to
the main asteroid belt located between the orbits of Mars and Jupiter.
Given enough time, we'll include the Kuiper Belt, the disc from which short
period comets originate and also, perhaps, the Oort Cloud, that vast
rarefied realm well beyond the planets that store the long period comet
nuclei. (Short period less than or equal to 125 years; long period
includes the rest.) Given even more time, we'd likely just repeat the
information already provided and might discuss the hypothetical planet many
astronomers believe lurks well outside Pluto's orbit, but far closer than
the inner Oort Cloud objects. Given even more time, we'd probably touch
upon the Presidential campaign because none of us can ever get enough,
carp pathetically about the ten foot snow shrouds that we seem never to
expect this time of year, and end with "any questions."
Not a single word. Not a single syllable about the Trojans: asteroids,
that is.
The Trojan asteroids are akin to those hidden, but enchanting bistros that
tourists happen upon accidentally as they search in vain for Angela
Lansnbury's house. The Trojans refer to the two groups of asteroids
clustered 60 degrees ahead and 60 degrees behind Jupiter in its orbit.
As Jupiter revolves around the Sun, so do the Trojans. They maintain the
same distance from the planet because both group librate around Lagrangian
Equilibrium Stability Points, or L points. Each body in orbit around
another will, in theory, have five such points at which the gravitational
forces of the parent and orbiting body balance. The Trojans are
clustered around either the L4 or the L5 points, the most stable of the
five points. Ever since German astronomer Max Wolf discovered the 566
Achilles, the first known Trojan, in 1906, more than 7,000 Trojan asteroids
have been found. They are divided into Trojans and Greeks, the two
antagonists in the Trojan War. The Greek camp moves ahead of Jupiter
and the Trojan camp trails behind*
[image: slide_9.jpg]
Even though fewer than 10,000 Trojan asteroids are now known, more than one
million likely exist between both the L4 and L5 regions. They might
be as numerous as those in the main asteroid belt. Moreover, like the
asteroid belt members, the Trojans are divisible into "families," asteroids
that share a common origin through the fragmentation of a larger progenitor
asteroid. While the Trojan asteroids are more difficult to study
owing to their greater distance and the comparatively scant information
yielded by spectral analysis, a new space mission called LUCY is due to
launch in 2021. This mission will study six Trojan asteroids and one main
belt asteroid. This mission will allow astronomers to examine Trojan
asteroids for the first time.
Trojans are of mysterious origin. It was once believed that they might
have been Kuiper Belt objects originally. This notion has fallen out of
vogue after observations have shown the Jupiter Trojans exhibit stark color
differences from the Kuiper Belt members. They could have been captured
in the early stages of solar system development, when the planets were
believed to have been closer together before they migrated to their current
stable orbits.
While we will continue to largely ignore the Trojan asteroids in favor of
the planets, moons, comets and main belt asteroids, we will likely hear
much more about them later in the decade when the LUCY probe takes a closer
look at a few of them while most assuredly discovering many more
*The discovery of asteroid 624 Hektor and 617 Patroclus predated the
adoption of this convention, so a Greek asteroid loiters in the Trojan camp
and a Trojan asteroid has infiltrated the Greek camp.
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