Sky and Telescope - September 2015 - 33

Earth

Capture Probability for TCOs
1.0°

2006 RH120
A

0.5°

0.0°
0.08

0.06

q =11 a.u.
u

Q =1
=1 a.u.
u

0.04
0.02

2006 RH120

Eccentricity

B
A

Inclination

1.5°
E

S&T: GREGG DINDERMAN, SOURCE: ROBERT JEDICKE

smithereens a few million years after it formed," says
geochemist Ed Scott (University of Hawai'i). "Some of its
centimeter-sized samples contain totally diﬀerent kinds
of meteorites from very diﬀerent main-belt asteroids. We
still don't understand how it acquired them."
The hope is that small mini-moons might also have
similarly interesting compositions. In fact, ﬁnding them
on a regular basis would allow astronomers to spot
and track potential Earth-impacting mini-moons well
in advance. Were one predicted to slam into our atmosphere, researchers could set up a system to photograph,
take spectra of, and measure a true bolide.
"With a month's notice, you could have people all
lined up to observe the object coming through the
atmosphere," says planetary scientist Clark Chapman
(Southwest Research Institute). "You would know where
it will hit, then have people ﬁnd the fragments. The
whole body of meteor spectra that sits in the literature
could be reinterpreted with much more conﬁdence. It
would be TC3 ten times over."
Some researchers, including Granvik, would like to use
mini-moons to determine whether small asteroids have
interiors that are more akin to rubble piles than monolithic pieces of space rock. This would have consequences
for strategies to mitigate Earth impactors, as well as for
future robotic and human missions to small asteroids.
But there's also more fundamental science at stake.
"The way to understand how the solar system formed,"
says Jedicke, "is to bring back chunks of it that haven't
been processed by tumbling through Earth's atmosphere
or by surface weathering." The end result would provide
what Jedicke terms a veritable set of "Rosetta stones"
from which to unravel our solar system's earliest history.
The ﬁrst step is simply to ﬁnd them. "Right now it's
just theoretical, not a viable population," Jedicke admits.
But the hunt is on - he's recently secured observing time
on Japan's 8.2-m Subaru Telescope and its sensitive, wideﬁeld detector called Hyper Suprime-Cam. Statistically,

0.0

0.92

0.94

0.96

0.98

1.00

1.02

1.04

1.06

1.08

Semimajor axis (astronomical units)
10-8

10-7

10-6

10-5

10-4

Capture probability
In order to become captured, an asteroid must have an orbit very
similar to Earth's, both in terms of its inclination (top panel) and
eccentricity (lower panel). Color coding shows the probability of
capture, with yellow denoting most likely, and the black lines indicate
orbits with aphelia (Q) and perihelia (q) of 1 astronomical unit. Paired
letters identify known asteroids whose orbits make them susceptible
to capture, though none have been. Also shown is the pre-encounter
orbit of asteroid 2006 RH120, the only known mini-moon to date.

Jedicke ﬁgures he's got a 40% chance of ﬁnding a minimoon during a single night and as high as 90% if he can
squeeze in ﬁve nights of observations spread over a few
months. "I've devoted a lot of time to understanding these
objects," he says, and if the Subaru eﬀort succeeds, he'll
ﬁnally have a real mini-moon or two to work with. ✦
Science journalist Bruce Dorminey is the author of Distant
Wanderers: The Search for Planets Beyond the Solar
System and a Forbes.com tech columnist. Follow him on
Facebook, Twitter, and Google+.

Moon
K. TERAMURA / UNIV. OF HAWAI'I

Simulated Mini-Moon Trajectory
The orbital gyrations of a simulated mini-moon begin as the
object approaches Earth from the right (yellow line), makes several irregular loops throughout the Earth-Moon system (orange)
and ﬁnally escapes (dark orange). Distances are shown to scale,
but the sizes of the Earth and Moon are not.

Sk yandTelescope.com September 2015

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Sky and Telescope - September 2015

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