Sky & Telescope - March 2021 - 24

Solar System Rubble

strong theories have emerged that may work
together, or separately, to explain how to
When astronomers spotted 1 Ceres and its
SPARSE
empty the belt.
fellow large asteroids in the early 19th cenNEIGHBORHOOD
A related conundrum that theorists had to
tury, they quickly came to believe they had
The average
also solve was the small Mars problem. Accordfound the remains of a torn-up planet. For
distance between
ing to classical planetary formation scenarios,
several decades, they argued about how the
two asteroids is
Earth and Mars should be roughly the same
destroyed planet, Phaeton, had met its end.
about 966,000 km
size. Instead, the Red Planet has less than a
Like the supposed planet, the theory of
(600,000 mi). That's
sixth the mass of our own world.
Phaeton eventually met its demise. As scien2.5 times greater
" The low mass of the belt is linked to the
tists discovered more asteroids, they realthan the distance
low mass of Mars, " says André Izidoro (Rice
ized that the asteroid belt had never been a
between Earth and
University, Texas). Because Mars sits so close
planet. Instead of a destroyed planet, perhaps
the Moon.
to the asteroid belt, the same process that
the belt was a failed one, kept from coming
swept away the planet's initial reservoir of
together by Jupiter.
building material would also have removed
In this picture, the belt started off as
part of the asteroid belt.
much as a thousand times more massive than it is today.
In 2011, researchers proposed the Grand Tack model as a
Primordial worlds in the region crashed together, grinding
solution to the small Mars mystery. The giant planets formed
one another down to create smaller asteroids and asteroid
faster than the smaller terrestrial worlds, their increased
families. As they interacted with other protoplanets, gas
mass allowing them to collect even more material in a case
giants, and one another, these asteroids found themselves
of the rich getting richer. According to the model, as Jupiter
ejected or destroyed. Some were hurled from the solar system
and Saturn gathered material from the dusty gas disk, they
by Jupiter's dominating gravity, while others were sent plumopened up gaps and launched spiral waves. These waves'
meting into the Sun. Only a fraction of the original asteroids
born in the belt would have survived the chaotic first few tens gravitational influence then moved the two giant planets
inward toward the rocky region of the solar system, shepherdof millions of years of planet formation and evolution.
ing icy material along with them.
But subsequent simulations of planet formation revealed
But it wasn't smooth sailing to the inner solar system. As
that shedding that much mass was more challenging than
the two worlds edged closer to the Sun, Jupiter encountered
astronomers originally suspected. In the last 15 years, two
the original, more massive asteroid belt. Caught up in the
giant planet's gravitational pull, the early asteroids found
T ODD SHAPES Among the few asteroids we've imaged up close are
themselves flung helter-skelter around the solar system.
(left to right) 951 Gaspra, 243 Ida, and 433 Eros. Ida (the largest here) is
Many were likely ejected, while others were cast into differless than 60 km long. The Galileo spacecraft flew past Gaspra and Ida;
ent regions. At the same time, material that would have fed
Eros was the first asteroid orbited and landed on, by NEAR Shoemaker.

24

MARCH 2 021 * SK Y & TELESCOPE

N ASA / JPL (2)

The Dance of Planets



Sky & Telescope - March 2021

Table of Contents for the Digital Edition of Sky & Telescope - March 2021

Contents
Sky & Telescope - March 2021 - Cover1
Sky & Telescope - March 2021 - Cover2
Sky & Telescope - March 2021 - 1
Sky & Telescope - March 2021 - Contents
Sky & Telescope - March 2021 - 3
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