Sky and Telescope - September 2015 - 31

with most unlikely to come close to Earth again for
decades or even centuries.
Such small objects oﬀer planetary scientists one of
their best near-term hopes of bringing back a whole
chunk of the pristine early solar system. What makes
them attractive as targets is that, unlike a random NEO
zipping quickly past Earth, these mini-moons are not
only orbiting nearby but also traveling at much lower
relative velocities than other asteroids. (In fact, for a time
astronomers thought that slow-moving 2006 RH120 was
actually a large chunk of spacecraft debris.)
This means that future rendezvous missions would
require relatively modest rocket propulsion to reach
any given object and to return with samples of it - or
perhaps to bring back the entire space rock. To get to a
main-belt asteroid requires more powerful rockets than
to get to some NEOs. And mini-moons oﬀer the best
economy of all, because they're relatively close by. One
scheme involves having a spacecraft "parked" in geosyn-

Unexpected Visitor

Path of 2006 RH120
Jan 3,
2007 Mar 25
June 14

Bits of the Early Solar System
The starting point for all mini-moons is almost certainly
the main asteroid belt, explains dynamicist William
Bottke (Southwest Research Institute). Basking in sunlight, these main-belt objects heat up and then re-radiate
that energy, creating subtle forces that cause them to
move slightly and their orbits to evolve gradually. These
gentle nudges, combined with the long-term gravitational
inﬂuences of Jupiter and Mars, cause a small subset of
asteroids to migrate inward to establish Earthlike orbits.
However, only a tiny fraction are moving in just the
right direction and slowly enough to be captured. "Some
objects may stay a year, some may stay 20 years," says
Bottke. "They rattle around the Hill sphere with all sorts
of trajectories until they ﬁnd a way out. But it's a steadystate population; for every one that leaves, one enters.
And although there's no restriction on size, over the next
100 years, we might only have one 20-meter mini-moon."
Once freed from the conﬁnement of Earth's gravity,
they continue on nearly Earthlike heliocentric orbits,

Earth
Sept 11,
2006

Sept1,1,
Sept
2007
2007

Moon

Apr 1,
2006

Top: Discovered by accident, the tiny asteroid 2006 RH120
was initially mistaken for a piece of space debris as it
drifted slowly amid background stars. Peter Birtwhistle
stacked 180 CCD images taken through his 0.4-m telescope to record the 19th-magnitude blip. Above: The little
asteroid hovered near Earth for more than a year before
slipping back into interplanetary space.

S&T: GREGG DINDERMAN

It turns out that NEOs can only rarely become minimoons. Those that do typically take up residency after
passing through one of two gravitational "sweet spots"
known as Lagrangian points. L1 is situated about 1½
million kilometers (1 million miles) away from Earth in
the sunward direction, while L2 is at the mirror image
location in the direction opposite the Sun. Any two-body
system (Earth and Sun in this case) has ﬁve such points
at which objects can remain in a stable conﬁguration
relative to the two bodies for long time periods.
To become mini-moons, objects must pass through L1
or L2 slowly - moving only 1 to 2 km per second (about
2,000 to 4,000 mph) with respect to Earth. (In contrast,
uncaptured NEOs often zip by at tens of kilometers
per second.) Then they "fall" into the Hill sphere, the
dynamical netherland where our planet's gravitational
inﬂuence exceeds that of the Sun. Earth's Hill sphere
is about 1.5 million km in radius, some four times the
Earth-Moon distance.
Granvik and his team estimate that, at any given
time, there are probably about 100 mini-moons larger
than 20 centimeters (8 inches) in diameter, a dozen at
least 0.5 m across, and one or two larger than 1 m. An
object the size of 2006 RH120, at least 3 m across, might
be captured only once every decade. Objects of 100 m or
more might show up only once in 100,000 years.
"Mini-moons typically lie from one to roughly 10 lunar
distances on crazy trajectories around Earth," said Robert
Jedicke (University of Hawai'i), one of Granvik's coauthors. This distinguishes them from "pseudo moons" that
sometimes come close to Earth and even share our planet's heliocentric orbital plane but are never gravitationally
snared. In contrast, mini-moons are bona ﬁde bound
satellites - at least during the time they're captured.

Sk yandTelescope.com September 2015

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Sky and Telescope - September 2015

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