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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*
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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