Sky & Telescope - February 2022 - 17

MARC W. BUIE ET AL.
largest member of the only known collisional family (Eurybates),
and one ultra-slow rotator (Leucus).
The Lucy mission plan looks a lot like a string of New
Horizons flybys. Because the flybys are so fast and the bodies
so small, Lucy won't completely map each world, but it will
obtain enough information to address questions of mass,
shape, volume, density, surface composition, interior structure,
and impact-cratering history. Its instruments draw
on heritage from both New Horizons and OSIRIS-REX and
comprise multiple cameras as well as a thermal infrared
spectrometer.
Science observations will begin a year before each flyby, as
the Lucy Long-Range Reconnaissance Imager (L'LORRI) studies
how each asteroid's brightness changes with time from
angles not achievable from Earth. The viewing angle also will
place each body against a totally different field of stars than
observers will be seeing from Earth, which will improve the
precision of orbit predictions and therefore improve the precision
of Lucy's navigation. As the spacecraft approaches, it will
also use L'LORRI to search for satellites. We already know
that one of the Lucy targets, Eurybates, has a moon, and there
could be others.
All of the optical instruments will map as much of the
surface of each world as they can see throughout the flyby.
The instruments are clustered together on a movable pointing
platform so that Lucy can keep its huge, decagonal solar panel
arrays pointed at the Sun throughout most of each encounter.
For targets that spin relatively quickly, Lucy will be able to
map the full globe (or full potato, considering that they are
not round). But some - especially Leucus, with its 445-hour
day - will only show one sunlit hemisphere to Lucy when the
spacecraft is close enough to map its landforms.
After Lucy, we will have visited enough of these (icy?)
worlds for astronomers to responsibly infer what virtually all
the Trojans are like, as well as the many asteroids of the outer
part of the main belt that share the Trojans' qualities. That,
in turn, will hopefully tell researchers where the objects came
from and suggest how they arrived at their current locations.
Psyche: A Metal Asteroid?
Psyche is the largest of the M-type asteroids, measuring 290
by 245 by 170 kilometers across (about 180 by 150 by 105
miles). When the Psyche mission was first proposed to NASA,
astronomers thought the asteroid could be more than 80%
metal. Since then, observers have measured Psyche's mass
and dimensions better, which has resulted, paradoxically, in
less certainty about what it's made of.
Mass and volume estimates yield an approximate density,
and Psyche's density is probably (but not certainly) just a
little higher than that of the very densest rocky components
of meteorites. Depending on what kinds of rock and metal
make up Psyche and on how porous it is, the asteroid may be
much less metallic than expected, about 30% to 60%.
No matter the answer to the composition puzzle, it's hard
to explain how worlds as dense as Psyche formed. We know
for sure that there is metal-rich material among the asteroids,
because approximately 5% of all meteorite falls are made of
a mix of iron and nickel. Scientists are relatively confident in
their understanding of how these meteorites formed: If you
begin with a differentiated world and shatter it in a collision
with another asteroid, then some fragments will come
from the core and be mostly or all metal. The problem with
explaining Psyche's formation by the same mechanism is how
to reassemble a big enough pile of core fragments to build
an asteroid 200 km across without also incorporating lots of
rock fragments. Perhaps lots of hit-and-run impacts spalled
off chunks of mantle, leaving an intact core behind? It's a
10 kilometers
Get Involved
Teams of amateur astronomers are already contributing
to the Lucy mission. Volunteers are assisting mission
astronomers by providing careful observations of how
long stars wink out (or don't) as Trojans pass in front of
them as seen from various locations. The stellar occultations
have already yielded improved orbit ephemerides
and size and shape estimates for Lucy's targets - for
instance, thanks to amateurs, we now know that slow
rotator Leucus is surprisingly elongated and likely heavily
cratered, while Patroclus' binary partner Menoetius
seems to have a huge impact crater that has gouged a
void out of its south pole. Such information makes flyby
planning simpler and more efficient.
The Lucy team welcomes observers from around the
world. Read more at https://is.gd/lucyoccultation.
11351 LEUCUS Based on more than a dozen teams' observations
(chords), this plot is a projection of the shadow the Trojan
Leucus created when passing in front of an 11th-magnitude
star in late 2019. The dots mark sites that saw the star wink out;
blue lines are observers who saw no occultation, clarifying the
asteroid's extent. Combined with other occultations, these results
indicate that Leucus has an oblong, irregular shape.
sk yand tele scope .o r g * FEBRUARY 2022 17
https://www.is.gd/lucyoccultation
Sky & Telescope - February 2022

Table of Contents for the Digital Edition of Sky & Telescope - February 2022
