Sky & Telescope - January 2019 - 27

"Gradually we are breaking down those barriers and learning how to successfully work together and push science forward," says Michael Kramer (Max Planck Institute for Radio
Astronomy, Germany).
And the IPTA will only get better with the recent addition
of three new instruments: the Five-hundred-meter Aperture
Spherical Radio Telescope (FAST) in China, the MeerKAT
Radio Telescope in South Africa, and the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) array in Canada.

Looking Ahead
In their quest to advance the ﬁeld, scientists have devised
additional methods for detecting gravitational waves.
Astronomers will look for signs of gravitational waves in data
taken by the European Space Agency's (ESA's) Gaia satellite.
From its perch in space 1.5 million km beyond Earth, Gaia is
making extremely precise measurements of the positions and
motions of about 1 billion stars. Subtle shifts over many years
will indicate that Earth is bobbing on passing gravitational
waves, changing its position with respect to the stars.
"The drawback here is that Gaia's data set will only be
as long as the mission, which is expected to be at most
10 years," Mingarelli says. "This limits the detection capabilities to binaries with gravitational-wave periods of 5 years or
less, which is very restrictive." A pair of billion-solar-mass
black holes with this period has "only" another 200,000 years
or so before it merges, she explains; those detectable with
pulsar timing arrays, on the other hand, will be circling each
other for another 25 million years. Statistically speaking,

At first glance, gravitational-wave
detection would seem to have very little
to do with planetary science. But when
it comes to precise timing of radio
pulses from millisecond pulsars, the
more stationary the reference point, the
easier. That's why all the pulsar timing
arrays (PTAs) use the solar system's
center of mass (the barycenter) rather
than Earth, which orbits the barycenter
at a speed of about 30 km per second
(67,000 mph).
The barycenter is always located
inside or near the Sun, but it moves
around as the planets orbit our star. For
years, the PTAs used an ephemeris (the

200,000 years is not much of a window to catch the binary's
nanohertz gravitational waves.
And looking further aﬁeld, ESA is planning to launch the
Laser Interferometer Space Antenna (LISA) in the 2030s.
LISA will consist of three spacecraft orbiting the Sun in
an equilateral-triangle formation, each craft separated by
2.5 million kilometers. LISA is speciﬁcally tuned to catch the
spacetime ripples from merging black holes with masses of
roughly 10,000 to 10 million solar masses. ESA's recent LISA
Pathﬁnder mission exceeded its performance goals, proving
LISA's technological feasibility.
Taken together, these projects - ground-based interferometers, pulsar timing arrays, Gaia, and LISA - promise to
usher in a revolutionary era of gravitational-wave astronomy. By hearing gravitational rumbles across a broad spectrum, scientists will piece together a story of the universe's
most extreme objects, in a way that they could not obtain by
any other means.

¢ Senior Contributing Editor ROBERT NAEYE was S&T's editor
in chief from 2008 to 2014.

table of coordinates, etc., for celestial
bodies) calculated by JPL based on
the positions, velocities, and masses of
the planets. But the PTA teams came
to realize that the JPL ephemeris is not
precise enough for pulsar timing, where
changes to the barycenter of a few
hundred meters in light travel time add
up to hundreds of nanoseconds. Such
errors can partially mimic the effect
of low-frequency gravitational waves
passing through our solar system, making it a limiting factor in the teams' ability to detect the stochastic background.
PTA radio astronomers are now
working closely with JPL to refine

its ephemeris, and the pulsar data
are actually helping to improve our
knowledge of the barycenter. "We have
been able to figure out a way to mostly
deal with the errors in the solar system
ephemeris, albeit at a small cost to
our gravitational-wave sensitivity,"
says NANOGrav team member Scott
Ransom (NRAO). The Juno mission will
help further, he adds, by nailing down
the orbit and mass of Jupiter. Because
it's so massive, the giant planet has a
big effect on the location of the solar
system's center of mass, including in
its indirect gravitational effects on the
other outer planets.

t BARYCENTER The Sun and planets technically orbit their mutual center of mass, called the solar system barycenter. The barycenter's
location moves as the planets follow their elliptical orbits around the Sun. Sometimes it's inside the Sun (diameter marked by yellow circle),
other times (including now) it lies outside the photosphere. The positive x-axis points in the direction of the vernal equinox.

sk yandtele scope.com * JA N UA RY 2 019

http://www.skyandtelescope.com

Sky & Telescope - January 2019

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