Sky & Telescope - February 2022 - 13

PETER RUBIN / NASA / JPL-CALTECH / ASU
C
ountless tiny worlds travel among the stately
orbits of the solar system's planets. These
worlds populate the asteroid belt, congregate
in clumps along planets' orbits, and make excursions
to just about every region, with occasionally
striking results. Some formed where we find
them now; others migrated from elsewhere. And
although they might look like boring planetary
debris, they are our best connections to the earliest
history of the solar system.
Since 1990, a dozen missions have visited asteroids,
comets, and Kuiper Belt worlds. In combination
with observations from optical and radio
astronomy, data from these missions have helped
us understand both how the planets formed and
how the giants herded the non-planets into their
present-day locations (S&T: Mar. 2021, p. 22).
But the scientific theories we've devised based
on these discoveries are only useful insofar as their
predictions stand up to more observations. Two
new missions will soon test the theories that grew
from the last three decades of work, by exploring
parts of the solar system we haven't visited yet:
The Lucy mission will complete a flyby tour of at
least seven asteroids caught along two parts of
Jupiter's orbit, while Psyche will head to its eponymous
asteroid, 16 Psyche, located in the main belt
between Mars and Jupiter.
Both spacecraft are from NASA's Discovery
program, each developed and launched quickly
with a relatively small budget of " only " $450 million.
Both feature enormous solar panels, some of
the biggest ever sent beyond Earth. And both face
several-year cruises and planet flybys before they
reach their science targets. Together, Lucy and
Psyche will test whether existing theories of planet
formation and migration can explain their target
asteroids' unique properties and current locations.
The Story So Far
In The Beginning, all was gas and dust. Then, gas
began to condense onto dust grains, which helped
the grains stick together upon collision. At the
center of the swarm, the proto-Sun ignited and
blasted out intense radiation, transmuting stable
atoms into the radioactive isotope aluminum-26.
This isotope was then trapped in condensing grains
rich in calcium and aluminum. Scientists have
analyzed such ceramic-like grains carried to Earth
16 PSYCHE This artist's illustration shows one option for
how Psyche might look. Scientists think the asteroid is a
blend of rock and metal, which might be mixed together (as
shown here) or in discrete sections.
in meteorites and determined that these solarsystem-forming
events took place 4.568 billion
years ago.
The solar nebula's gas dragged on the tiny dust
particles as they orbited the prenatal Sun, moving
them into crossing paths. The resulting collisions
were slow, the particles nudging together gently
enough that they didn't fly apart. Planetesimals
grew from such gentle processes of pebble-collecting
very quickly, doubling in size every thousand
or so years. Arrokoth, the tiny Kuiper Belt world
visited by New Horizons in 2019, bears signs of this
construction process (S&T: Feb. 2020, p. 34).
Close to the young Sun, the planetesimals were
mostly rock. Farther away, where colder temperatures
prevailed, both rock and ice could condense,
and planetesimals grew larger, faster. Within half
a million years, uncountable tiny particles had
merged into fewer, but still numerous, worlds
hundreds to thousands of kilometers in diameter.
They were loose piles of dust, gravel, fluffy ice, and
empty space; their self-gravity could hold the particles
together but couldn't pack them tightly.
Meanwhile, the radioactive Al-26 was rapidly
decaying into magnesium-26, generating heat.
Bigger worlds trapped that heat, warming up. In
some of them, ices turned to liquids or gas. Gases
escaped but liquids flowed, collapsing or filling
pore spaces. The liquids chemically reacted with
rocks, making new minerals and sometimes even
creating short-lived underground oceans of salty
liquid. Where worlds became sufficiently hot, the
rock began to melt, and still-solid metal grains fell
down toward their centers. This process, called
differentiation, produced onion-layered worlds with
metal cores enclosed in mantles of rock (and, if far
enough from the Sun, ice), with or without a top
layer of never-melted, cold material.
The Al-26 heat wave ended early in solar system
history, except inside the largest worlds. Nowadays,
nearly every small body in the solar system is too
cold to retain interior liquids, but they preserve
their layered structure.
Al-26 wasn't the only thing to dramatically
change these small worlds. When the giant planets
grew big enough, their orbits began to shift, sending
the giants plowing through the planetesimals
around them and scattering the small worlds
across the solar system to where we find them
today. The details and timing of this re-sorting
should be imprinted in the worlds' characteristics
- which we've only seen from afar.
(continued on page 16)
sk yand tele scope .o r g * FEBRUARY 2022 13
Sky & Telescope - February 2022

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