Planck Spacecraft Shut Down

But don't pull out the tissues — this end is a happy one.

The Planck mission has ended. On October 23rd (which incidentally would have been my grandma’s 100th birthday), ESA’s microwave-perusing spacecraft shut down forever.

This isn’t a woeful tale of fizzled-out funding or failed flywheels: Planck has reached its natural end after an electronic life well lived. The spacecraft produced exquisite maps of the temperature blotches in the cosmic microwave background (CMB), the ancient radiation permeating space that is our earliest view of the infant universe. It also revealed a great deal about vistas closer to home; among these achievements is cementing the existence of the long-debated, gargantuan bubbles ballooning from the Milky Way’s core.

Planck’s CMB map upheld to glorious precision the simple cosmological model that describes the universe using only six numbers. According to its first 15.5 months of observations, the universe is 13.798 billion years old, dark energy makes up 69.2% of the universe’s mass-energy content, and the universe’s current expansion rate — described by the Hubble constant, the ratio of a galaxy’s recession velocity, or redshift, to its distance — is 67.80 km/s per megaparsec. This Hubble value is significantly lower than that calculated from supernovae (73.8 km/s per Mpc). The discrepancy has intrigued astronomers, but as of yet there’s no solution.

Planck launched May 14, 2009 with the infrared Herschel Space Observatory, which also shut down earlier this year. It observed at an angular resolution roughly twice as fine as that of its predecessor, WMAP, and refined that mission’s cosmology results. The spacecraft’s High and Low Frequency Instruments worked together to finish five full-sky surveys before the HFI ran out of coolant on January 14, 2012 — within a week of its predicted end, notes project scientist Charles Lawrence (JPL).

Originally slated to run for 15 months, Planck received two extensions thanks to its flawless performance, continuing to observe with LFI until the mission team started shutting everything down a couple of weeks ago. While LFI’s hydrogen-absorption cooler might have kept working a wee longer, the extra bit of data wouldn’t have helped researchers enough to justify another extension, Lawrence says. As it is, the team received two “bonus” months: officially the mission should have ended in August, but the orbital mechanics guys picked October as the best to push the craft from the L2 Lagrangian point, the gravitational balancing point on the opposite side of Earth from the Sun.

Although we often talk about Lagrangian points as stable places in the Earth-Sun gravitational sandbox, they’re only semi-stable. Space at L2 is shaped like a saddle, with one stirrup side pointing right at Earth and the other directly away. A spacecraft can move forward and backward in the saddle and not fall out, but if it moves sideways too much (that is, directly toward or away from Earth), it’ll slide out. If it slides out on the Earth-facing side, it’ll head toward the planet — and we don’t really need a rogue spacecraft drifting toward us. Engineers therefore fire the thrusters just enough to push defunct missions out the saddle’s far side, sending them into an Earth-trailing orbit that will bring them back to Earth’s vicinity in about 300 years.

Despite the craft’s demise, there’s a bevy of beautiful data to explore. All told, the team has 29 months of HFI data and 50 months of LFI data in hand, data that Lawrence raves are “fantastic.” The researchers plan to release the next set of observations in mid-2014, including some of the long-anticipated polarization data. These data will not include what are called primordial B-modes, the spiral-like polarization fingerprint left behind by the theoretical epoch of inflation, which ostensibly happened about 10 nano-nano-nano-nanosecond after the Big Bang. The polarization data are a beast to analyze, so the team doesn’t expect to release anything related to B-modes until at least 2015. The 2014 release will deal with another pattern called E-modes, which reveal information about the density fluctuations that gave rise to the CMB’s mottled look.

In the end, Lawrence sums Planck up in three words: “A brilliant success.”