My first Magazine Sky & Telescope - 03.2019

Many all-sky surveys also spin to map the known universe. For example, the two most recent cosmic microwave back- ground (CMB) space observatories, NASA’s Wilkinson Micro- wave Anisotropy Probe (WMAP, 2001–10) and the European Space Agency’s (ESA’s) Planck (2009–13), each deployed very different scanning strategies to interrogate the faint heat left over from the Big Bang. Their insights have since been used to determine the proportions of the universe’s fundamental constituents and establish the standard model of cosmology prevalent today. WMAP, orbiting about 1.5 million km from Earth’s nightside at what is known as the L 2 point (see page 24), had two telescopes, one to either side of its spin axis. Each leaned about 70° away from the axis, which always pointed away from the Sun. Not only did the spacecraft rapidly spin on its axis once every 2.2 minutes, but it also slowly precessed, or wobbled on that spin axis like a spinning top, every hour. The scanning patterns WMAP’s dance produced crisscrossed one another like the lines in a Spirograph drawing. It then moved this Spirograph donut around the sky as it orbited the Sun. To picture all of this movement together is difﬁcult, but not impossible. One way is to imagine a bobblehead doll (preces- sion) placed in a spinning hamster’s wheel (spacecraft spin) set on a merry-go-round (orbit). Why go to the trouble of gyrating your telescope in such a complex way when you could cover the whole sky by point- ing and staring at different regions, and then piece them together? Former Planck Survey and Archive Scientist Xavier Dupac (ESA) has an answer: “One of the problems with these missions is that there is noise from what we call systematic effects.” This could be any non-random error, like a tempera- ture ﬂuctuation in the spacecraft’s electronics, or perhaps an error in calibrating the instrument. “You can’t really average them out with statistics,” he says. Precessing is the only way to reduce this type of unwanted noise. Precession A-side line of sight 6 Months Earth E t th RHESSI PLANCK Given this advantage, it might be surprising that WMAP’s successor, Planck, followed a simpler path. Rotating once per minute with its instruments slightly off-axis and its base always pointed at the Sun, the spacecraft traced large rings on the sky that moved with the orbit about 1° per day to slowly build a picture of the sky. The beneﬁ t to Planck was that with minimal change in its angle to the Sun it could keep its instruments in cool shadow, helping to maintain a steady temperature — important when measuring tiny varia- tions in the CMB. Without precession though, the team behind Planck knew it would have to tackle systematic effects and would also leave a large chunk of sky unobserved above and below the Sun as viewed from 3 Months Planck’s orbit. Hence, the ground team used the telescope’s thrusters to tilt the craft’s spin axis a little each hour to create a circu- Spin axis lar bobbing motion that moved the axis by up to 7.5° above and below the ecliptic over the course of six months. Going back to B-side line of sight WMAP’s merry-go-round analogy, every- thing was the same with Planck but for the bobblehead doll, which traced much smaller 1 Day circles with its oversized head in jerky, dis- crete movements. When, Not Where Sun n More recently, ESA’s Gaia mission has taken spinning to a new level, capturing the entire sky, like the CMB scopes, and providing data that would be impossible to catch any other way, like RHESSI. Gaia is another sky mapper sk yandtele scope.com • M A RCH 2 019 23

My first Magazine Sky & Telescope - 03.2019 | Page 25