Sky & Telescope - March 2019 - 22

Twirling Telescopes
t THE CRAB This composite of the Crab Nebula combines data from
across the electromagnetic spectrum: radio waves (red), infrared (yellow),
optical (green), ultraviolet (blue), and X-ray (purple). Fortuitous AGILE
observations helped reveal that, much to astronomers' amazement, the
Crab produces gamma-ray flares. Why remains unclear.

To See and Survey

WMAP at L2

u WMAP'S STRATEGY Orbiting at L2,
WMAP gyrated in three ways (left). The
spacecraft spun around its axis about
every half minute, and this axis wobbled
around in a circle every hour, tracing its
two lines of sight through a complex
crisscrossed pattern (center). The craft
continued this bobbing motion as it revolved around the Sun, enabling it to scan
the whole sky (right).

M A RCH 2 019 * SK Y & TELESCOPE

North Ecliptic Pole
Spin rate:
0.464 rpm

B-side
line of sight

+90°

+45°

-45°

1.5 × 106 km
Earth

1.5 × 108 km
South Ecliptic Pole
Sun

-90°

WM A P: N ASA / WM A P SCIENCE TE A M; CR A B NEBUL A:
X-R AY: N ASA / CXC / SAO, OPTICA L: N ASA / STSCI,
INFR A RED: N ASA / JPL / CA LTECH, R A DIO: NSF / NR AO /
V L A , ULTR AVIOLE T: ESA / X M M-NE W TON

the Sun, while the rest of the craft started spinning very
slowly (one revolution every 7 minutes) at right angles to the
direction of the Sun.
"It created a lot of debate," recalls AGILE Principal Investigator Marco Tavani (National Institute for Astrophysics,
Rome). "Some engineers said, 'It's ﬁnished, it's the end of
AGILE,' but others said, 'No, it's the exact opposite'."
Although the telescope couldn't look as deeply as it could
before, every 7 minutes it covered 80% of the sky. This took the
blinders off the satellite and led to the team's greatest discovery. "We had looked at the Crab Nebula many times during
the ﬁrst two years for calibration purposes, because it is, or was
supposed to be, a stable 'boring' source," explains Tavani. But
forced to look again in 2010 due to
being in a spin, the team noticed the
Precession rate: 1rph
Crab Nebula was far from a stable
22.5° half-angle
source: It was emitting gamma-ray
ﬂares. "The result was almost heretical, creating tremendous excitement
in the community," he says. "We
A-side
would have never discovered it if it
line of sight
were not for this spinning mode."

More commonly, spinning is less a happy accident and more
an integral part of how the space telescope observes the sky.
Just when NASA's Parker Solar Probe was setting off on its
journey to "touch the Sun," scientists ﬁnally lost contact with
another distinguished solar observatory called the Reuven
Ramaty High Energy Solar Spectroscopic Imager (RHESSI).
However, during its 16 years of operation RHESSI made a
number of discoveries about solar ﬂares and other aspects
of solar physics and astrophysics. And none of it would have
been possible without spinning.
RHESSI's aim was to make movies of solar ﬂares in X-rays
and gamma rays in order to understand solar-ﬂare physics.
But imaging X-rays - particularly high-energy or "hard"
X-rays - is difﬁcult. This is because they don't easily reﬂect
off mirrors due to their sub-atom-scale wavelengths, which
endow them with the ability to penetrate deeply into matter.
To get around this, a pair of grids known as a collimator
was placed in front of each of nine detectors. On their own,
these detectors couldn't create an image; they solely picked up
the energy and arrival time of each detected photon. But by
allowing the spacecraft to spin, the area of the detector visible
through both grids from the Sun's perspective changed with
time. When the solar source of the X-rays was slightly offcenter, the X-ray photons were modulated, which the detectors registered as a variation in photon intensity with time.
This difference encoded the location and size of the X-ray
sources. Computers on the ground then decrypted this information to construct a series of snapshots of the Sun's ﬂares.

Sky & Telescope - March 2019

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