Sky & Telescope - February 2020 - 36

Seeing Double

q IDA AND DACTYL This mosaic shows asteroid 243 Ida and its moon,
Dactyl, as seen by the Galileo spacecraft en route to Jupiter. The moon's
discovery shocked much of the planetary science community when announced in 1994.

FEBRUARY 2 02 0 * SK Y & TELESCOPE

The YORP effect can spin up a rubblepile asteroid enough for the centrifugal
force at its equator to exceed the body's
gravitational attraction, and pieces can
driƏt away.
trans-Neptunian objects, and six comets are contact binaries,
including Arrokoth.
That adds up to quite a diverse bestiary. Binaries are everywhere and come in varied forms. Some pairs are touching;
others have widely separated orbits. Some pairs are equal size;
others differ widely in size. Some rubble piles spin off chunks
that orbit the main body for a while, then slowly return to
merge with it. Astronomers think their differences reﬂect
where they formed and where they have wandered since.
To sort through the bestiary, we will start with near-Earth
binaries and move outward through the solar system to Arrokoth and other objects in icy orbits beyond Neptune.

Near-Earth Binaries and the YORP Effect
Radar has discovered about three-quarters of the multimember near-Earth asteroids, says Patrick Taylor (Lunar and
Planetary Institute). Maximum size is a few kilometers. He
says that one-sixth of near-Earth asteroids wider than 200
meters are binaries; another one-sixth are contact binaries
like Castalia and Arrokoth, with two similar-size lobes touching each other and rotating jointly.
The separate members of near-Earth binaries generally
orbit only a few kilometers from each other, or a few times
the size of the primary. The bigger objects tend to spin fast,
with "days" lasting only 2.2 to 4.5 hours. The smaller components typically are 4% to 58% the sizes of the primaries.
Near-Earth orbits are chaotic, and objects normally stay in
them for only around 10 million years before colliding with
a planet or the Sun or being ejected out of the region. Most
near-Earth objects are "rubble piles," accumulations of material held together loosely by gravity, which makes them vulnerable to three processes that can break them apart before
they are lost. Collisions can knock pieces out of the rubble
pile, which then drift back to form binaries with their parent
bodies. Close gravitational encounters with planets can tear
apart both asteroids and comets, as happened when Comet
D/Shoemaker-Levy 9 passed near Jupiter on July 7, 1992.
The third process relies on far weaker forces, but over
time it is far more effective because it is powered by sunlight. Light carries momentum that it can transfer to objects
when reﬂected or absorbed and reradiated, and the induced
torque can change the spin of an irregularly shaped object.
The process is called the YORP effect, and depending on the
object's rotational orientation it can either speed up or slow
down its rotation. "It's very weak, a measurable force but not
a huge force," says Daniel Scheeres (University of Colorado,

N ASA / JPL / DA NIEL M ACH ÁCˇEK / CC BY-SA 3.0

The ﬁrst convincing evidence of a pair of asteroids orbiting
each other, however, was a byproduct of NASA's Galileo mission to Jupiter. The spacecraft had to pass through the main
asteroid belt, so planners arranged ﬂybys of two asteroids. The
second, on August 28, 1993, revealed that the 60-km oblong
asteroid 243 Ida was orbited by a 1.5-km moon, later dubbed
Dactyl. The images of little Dactyl moving along with the
much larger Ida were ﬁnally enough to convince Brian Marsden, the long-time director of the Minor Planet Center at the
Harvard-Smithsonian Center for Astrophysics, that binary
asteroids existed.
More discoveries followed. Some have involved radar,
others were made using ground-based telescopes with adaptive optics or the Hubble Space Telescope, which can resolve
separate bodies if their orbits take them far enough from each
other. Often the best data come from spacecraft exploring
objects up close, like Galileo's visit to Ida and New Horizons' ﬂybys of Pluto and Arrokoth. The Rosetta mission to
famous "rubber duck" Comet 67P/Churyumov-Gerasimenko,
for example, showed us spectacular close-ups of a contact
binary nucleus (S&T: May 2017, p. 14). But extremely precise
measurements from far away of how an object's brightness
changes over time have identiﬁed more than half of known
binaries.
As of November 2019, astronomers had identiﬁed 375
asteroids and trans-Neptunian objects with at least one
companion: 359 have one, 15 have two, and the record holder,
Pluto, has ﬁve. These objects are spread throughout the solar
system: 72 near-Earth asteroids, 28 Mars crossers, 169 objects
in the main asteroid belt (one of them a comet), ﬁve Jupiter
Trojans, and 101 beyond Neptune. An additional 67 inner
solar system and main-belt asteroids, two Jupiter Trojans, 14

Sky & Telescope - February 2020

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