http://www.skyandtelescope.com.au 15the 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, 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
(precession) 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
pointing 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
atemperaturefluctuationinthespacecraft’selectronics,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.
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
alwayspointedattheSun,thespacecrafttracedlargerings
on the sky that moved with the orbit about 1° per day toRHESSIslowly build a picture of the sky. The benefit 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
variations 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 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 circular
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 WMAP’s
merry-go-round analogy, everything was the
same with Planck but for the bobblehead doll,
which traced much smaller circles with its
oversized head in jerky, discrete movements.When, not where
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
like WMAP and Planck. But its mission is ton1 Day3 Months6 MonthsSpin axisPrecession
A-side
line of sightB-side line of sightSunnSunEarthtPLANCKRHESSI: NASA / RHESSI TEAM / SPECTRUM ASTRO, INC.; PLANCK: ESA / AOES MEDIALAB