The Sound of a Cosmic Touchdown

Listen to Philae's historic "thud" on Comet 67P, recently released by the European Space Agency.

German Aerospace Center, DLR

If a spacecraft lands on a comet 330 million miles away, and no one is around to hear it, does it make a sound?

Yes, it does. And now we have a recording of it.

Last week the European Space Agency released the first ever sounds of the Philae probe touching down on Comet 67P. It’s a two-second “thud” made during Philae’s first of three bouncy landings. The clip, though short, reveals intricate details about Comet 67P’s icy and dusty surface.

“Philae’s feet first penetrated a soft surface layer—possibly a dust layer—several centimeters thick until they hit a hard surface—probably a sintered ice-dust layer—a few milliseconds later,” said Klaus Seidensticker, an astronomer from the German Aerospace Center’s Institute of Planetary Research in an ESA blog post.

But wait—in the vacuum of space there are no molecules for sound waves to travel through. So how exactly did Philae's landing make a noise?

The ESA said that when the probe touched down, the contact made between the soles of its feet and the comet’s surface created vibrations that traveled up the lander’s legs. Acoustic sensors on Philae’s three feet, called CASSE, or Cometary Acoustic Surface Sounding Experiment, picked up the vibrations and transmitted the sounds to the Rosetta spacecraft orbiting above it. CASSE is one of three parts that make up Philae’s Surface Electrical, Seismic and Acoustic Monitoring Experiment, or SESAME.

The other information that astronomers can glean from the recording is that Philae did not come back in contact with the comet immediately after landing. The scientists already knew this, but the finding provided further evidence that Philae drifted for a while before bouncing into a still unknown location.

Philae's touchdown is the second unique sound to come out of the Rosetta mission. Before the orbiting spacecraft launched Philae onto the comet, Rosetta made a recording of a "song" emitting from Comet 67P. The soothing sounds come from oscillations in the comet's magnetic field, according to the ESA. The sounds, which occur at 40-50 millihertz, are far too low for humans to hear, so the ESA increased the frequencies by a factor of 10,000 in this recording.

Despite all the new noises to emerge from the Rosetta mission, ESA scientists have not heard a peep out of the Philae since it powered down on November 14. As of Friday, the agency's scientific sleuths said that they have narrowed down the location of the probe's final resting spot to a 1,150-foot long strip of the comet. The clues for that conclusion came from Philae's CONSERT, or the Comet Nucleus Sounding Experiment by Radio wave Transmission, according to the ESA.

The ESA takes step closer to figuring out Philae's final landing site. (ESA/Rosetta/Philae/CONSERT)

The ESA said that it is trying to verify whether this stretch of comet holds Philae or if the lander is located in a different spot nearby. There is also still hope for ESA astronomers that the comet will move close enough to the Sun in the future to revive the solar-powered spacecraft so they can make contact with it again.