Sky & Telescope - March 2019 - 25

Path of spin
axis over 4 days

Path of
viewing
directions
over 4 days

45°

Ecliptic
Sun

R AY THEON CO MPA N Y

But how does AGIS ﬁgure out the distance to stars? Like
any telescope, Gaia - through AGIS - can track parallax:
the tiny apparent looping motion made by a star once a year
that's caused by the spacecraft's moving viewpoint around
the Sun (see page 26). However, the looping motions of stars
in the same part of the sky will be synchronized, making
it impossible to decipher their true distance - only their
parallax relative to one another can be calculated. Gaia gets
around this problem by having two ﬁelds of view separated
by a wide angle. "If stars are widely separated on the sky,
their looping motions will be out of sync, causing a variation in their angular separation over the year," explains
Lindegren. "That variation contains the additional information needed to determine the size of the loops and hence the
absolute parallaxes."
By spinning in such a way that both ﬁelds pass over the
same stars within a short time, and by registering the stars'
positions multiple times to disentangle a star's real and apparent
motion, AGIS has all the data it
needs to churn out the locations,
drift across the sky, and distance of
all stars at the same time, transforming the night sky jigsaw puzzle
into a living 3D projection of the
known universe.
u MIRROR SPINNING Artist's concept
of Raytheon's rotating space telescope.
Depending on mission requirements, the
entire mirror and platform could rotate together (as shown here), or the mirror could
spin on its own.

Skinny, Light, and Cheap
Turning to the future - please excuse the pun - spinning
could hold the key to far more powerful space telescopes.
Technology company Raytheon has been in discussions
with NASA researchers on developing its Rotating Synthetic
Aperture (RSA) concept. "As a professor of physics said of this
technology, 'If Hubble sees toddler galaxies and the James
Webb Space Telescope will see baby galaxies, RSA will see
embryos,'" explains Raytheon Space Systems Vice President
Wallis Laughrey. "But the best ﬁt for the technology by far
and away is direct imaging of exoplanets."
Curved like a canoe, the primary aperture of the telescope would spin like a wind turbine while taking a series of
images. These pictures would then be combined using complex algorithms to produce a crisp ﬁnal scene.
Although it would have reduced light-gathering power, a
skinny, 20-meter-long RSA could provide the resolution of a
full 20-m telescope at the cost and
mass of an 8-m scope. This is crucial, because the 6.5-m aperture of
JWST is near the limit of what can
ﬁt into current rocket nose cones.
With NASA's recent cost cap of
between $3 billion and $5 billion
on future space telescopes, ideas
like RSA will be essential for pushing the boundaries of space-based
astronomy and delivering new
insights into our dynamic universe.

¢ BENJAMIN SKUSE is a science
writer based in Somerset, UK.
sk yandtele scope.com * M A RCH 2 019

http://www.skyandtelescope.com

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019