Sky and Telescope - October 2016 - 10

News Notes

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LIGO / T. PYLE

PHYSICS I LIGO Detects 2nd Black Hole Collision

This artist's concept shows gravitational waves radiating out from the merging black holes.

The Laser Interferometer Gravitational-Wave Observatory (LIGO) has
detected another burst of spacetime
ripples created by the violent merger of
two stellar-mass black holes.
LIGO's ﬁrst discovery, detected last
September and announced in February
(S&T: May 2016, p. 10), was an astounding
achievement for science. The signal radiated outward from two surprisingly heavy
black holes, each about 30 solar masses.

As such the chirp was loud but short: it
lasted only 0.2 second and allowed scientists to glimpse the last 10 cycles of the
black holes' orbits before their collision.
This second signal, GW151226, came
from two lesser black holes, roughly 7.5
and 14.2 times the mass of the Sun. The
chirp was weak, barely visible above
the noise, but lasted a full second and
enabled researchers to witness the last
55 orbits of the black holes' inspiral. The

SOLAR SYSTEM I Earth's Newfound Quasi-Moon
Our fair planet has a tiny companion, an
asteroid that plays leapfrog with us in our
annual journey around the Sun.
The asteroid, 2016 HO3, is between 40
and 100 meters in size. Discovered as a
24th-magnitude blip on April 27, 2016, by
the Pan-STARRS 1 survey based on Haleakala, Hawai'i, 2016 HO3 occupies an
orbit very much like Earth's. Calculations
suggest that, although it evaded detection
until this year, 2016 HO3 has hung out
in Earth's vicinity for a century or so and
will stay for centuries to come.
10

October 2016 sky & telescope

2016 HO3's orbit takes it alternately sunward and ahead of Earth for six months at
a time, then slightly farther from the Sun
and behind us. Its orbit is tilted relative to
the ecliptic plane, resulting in a corkscrew
twist over several decades. Too distant to
be considered a true second moon, 2016
HO3's journey brings it as close as 38
times the Earth-Moon distance (9.1 million
miles or 0.1 astronomical unit) from our
planet, and as far as 100 times the EarthMoon distance.
"Since 2016 HO3 loops around our

signal reached LIGO on December 26,
2015, and was announced June 15th at the
American Astronomical Society summer
meeting in San Diego and in the June
17th Physical Review Letters.
The two black holes had been circling
each other for eons before they ﬁnally
crashed together 1.4 billion years ago. The
bang was so furious - with an energy
release equivalent to the entire mass of
the Sun - that it sent shudders across
the universe, distorting the length of
LIGO's arms by 10 million-billionths of
a centimeter. "That's like changing the
distance between the Earth and the Sun
by a fraction of an atomic diameter," says
Gabriela Gonzalez (Louisiana State University), chair of the LIGO collaboration.
Given the new signal, scientists think
the ﬁnal black hole had a mass of approximately 20 solar masses.
LIGO's next observing run will last six
months and increase LIGO's sensitivity
by 15% to 25%. Given that the last, fourmonth run yielded two mergers, LIGO
Executive Director David Reitze expects
that the added sensitivity will unearth six
to eight mergers over this fall's run. LIGO
could potentially spot dozens of these
events each year.
■ SHANNON HALL

planet, but never ventures very far away
as we go around the Sun, we refer to it
as a quasi-satellite of Earth," says Paul
Chodas (NASA Jet Propulsion Lab). "In
eﬀect, this small asteroid is caught in a
little dance with Earth."
Chodas explains that this body doesn't
qualify as a "mini-moon" (S&T: Sept.
2015, p. 30) because our planet's gravity
never truly captures it. "Captured objects
are true satellites of Earth and will loop
around with periods as large as a few
months," he says. "Quasi-satellites loop
around Earth with periods of almost
exactly 1 year."

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Sky and Telescope - October 2016

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