My first Magazine Sky & Telescope - 03.2019

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 fl ares. Why remains unclear. To See and Survey WMAP at L 2 u WMAP’S STRATEGY Orbiting at L 2 , 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 re- volved around the Sun, enabling it to scan the whole sky (right). 22 M A RCH 2 019 • SK Y & TELESCOPE 1.5 × 10 6 North Ecliptic Pole Spin rate: 0.464 rpm B-side line of sight +90° +45° –45° km Earth 1.5 × 10 8 km South Ecliptic Pole Sun –90° NR 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 Inves- tigator 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 discov- ery. “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 hereti- cal, 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 ﬁ nally 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 ﬂ ares 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 ﬂ ares in X-rays and gamma rays in order to understand solar-ﬂ are physics. But imaging X-rays — particularly high-energy or “hard” X-rays — is difﬁ cult. This is because they don’t easily reﬂ ect 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 off- center, the X-ray photons were modulated, which the detec- tors 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 infor- mation to construct a series of snapshots of the Sun’s ﬂ ares.

My first Magazine Sky & Telescope - 03.2019 | Page 24