Sky and Telescope - June 2016 - 18

Trojan Asteroids

Jupiter

Sun

Hill
sphere
L5

CAMP FOLLOWERS
To date, observers have
identiﬁed nearly 6,300
objects in the two "camps"
that accompany Jupiter.
Curiously, the leading
"Greek" group is roughly
50% more populous than
the trailing "Trojan" group.
The dashed circle around
Jupiter shows its Hill
sphere, which is the limit
of stability of any of the
planet's moons.
S&T: LEAH TISCIONE;
SOURCE: SCOTT SHEPPARD (3)

2 astronomical units

UNEQUAL
DISTRIBUTION
Jupiter and Neptune have
captured many stable
Trojan asteroids. But
Saturn and Uranus have
none, likely because their
Lagrangian zones are
subject to the gravitational
perturbations of their massive planetary neighbors
and thus not stable.

Neptune

Uranus

Sun

Jupiter

Saturn

Neptune

Sun

NEPTUNE'S RETINUE
Although only 12 Neptune
Trojans are known, the
planet's Lagrangian regions
are likely even more
densely populated than
Jupiter's. The L5 region
appears sparser because
this region is currently
aligned with the galactic
plane, making it harder to
detect objects there.

A LU N A R C O N N E C T I O N ?
If a very large object struck our planet early in the solar system's history
to form the Moon, could it have been an Earth Trojan? Geochemists
think such an impactor would have needed a composition quite similar
to Earth's and a not-too-fast encounter velocity. An object that initially
shared Earth's orbit could satisfy both of these constraints.

June 2016 sky & telescope

Picking Planetary Partners
Max Wolf discovered 588 Achilles, the ﬁrst asteroid
known to share Jupiter's orbit, in 1906. Observers have
since found about 6,300 Jupiter Trojans - so many,
in fact, that the entire population is likely only slightly
smaller than that of the main asteroid belt and includes
almost a half million bodies larger than 1 km in diameter.
Jupiter's Trojans are named after mythological characters from the Trojan War. The leading (L4) region is the
"Greek camp," and objects there are named after participants on the Greek side of the war. Those in the trailing
(L5) region are named after participants from the "Trojan
camp" of the conﬂict. Each group has one object named
for a combatant from the opposing side: 617 Patroclus is in
the L5 swarm, and 624 Hektor is in the L4 swarm.
Saturn and Uranus have no known stable Trojans
(though at least two of Saturn's moons have them). This
is because the Lagrangian zones of Saturn and Uranus
are highly perturbed by the gravity of the other giant
planets that sandwich these worlds' orbits.
Unlike the "embedded" locations of Saturn and Uranus, Neptune is a "bookend" giant planet, as is Jupiter,
making its Lagrangian regions more stable. Numerical
simulations suggest 20% to 50% of Neptune's original
Trojan population should have survived for the age of the
solar system after any signiﬁcant planetary movement.
Observers discovered the ﬁrst object in Neptune's L4 location in 2001 and the ﬁrst one at L5 in 2008. The known
Neptune Trojan population, currently at 12, is low only
because these bodies are much harder to observe at their
extreme distance from the Sun. We've started to name
Neptune's Trojans after the Amazons, an all-female warrior tribe that sided with the Trojans during the war.
As for the terrestrial planets, Earth lacks any known
Trojans from primordial times, probably because gravitational perturbations from Venus, Mars, and Jupiter
would have destabilized their orbits over the age of the
solar system. However, our planet does have one known
Trojan in its L 4 region, a small body called 2010 TK 7.
This was likely an asteroid in an Earth-crossing orbit
that just happened to pass near Earth's L 4 region a few
thousand years ago and became temporarily trapped
there. Chances are good that 2010 TK 7 will likely escape
its conﬁnement within another few thousand years.
Mars also has a few known Trojans at both its L4
and L5 points. Those locations appear to be dynamically
stable over the age of the solar system, so these objects
could be primordial and related in some way to the Jupiter and Neptune Trojans.

Origin of the Trojans
In theory, the Trojans could have formed in place and
stayed put since the ﬁrst stages of planet formation.
After all, the Trojan regions would have been relatively
stable early on, and objects could have accumulated there

Sky and Telescope - June 2016

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