Landing on a comet

The AusIMM Future Mining Conference kicked off tonight with a presentation by Professor Monica Grady about the Rosetta (accompanied by the Philae lander) mission to comet 67P/Churyumov-Gerasimenko (which I’ve done a post on here). If you don’t know Monica, here she is celebrating Philae’s successful landing on 67P. Unfortunately for her, she hasn’t realised that it promptly bounced off, but more on that later.

Professor Monica Grady demonstrating gravity assists.
Professor Monica Grady demonstrating gravity assists.

“Why go to a comet?” asks Professor Grady at the start of her talk. Comets contain carbon and water, the building blocks of habitable worlds. Quite possibly, a good deal of our carbon and water here on Earth came from comets. The more we understand about comets the more we understand about our own origins.

In 1986 three probes were sent to 1P/Halley – Vega 1, Vega 2 and Giotto. This was the first comet we got up close and personal with, but we didn’t attempt landing. Then in 2006 the Stardust mission visited 81P/Wild 2, but again, no landing. Stardust did, however, collect dust from the comet’s tail and return it to Earth, allowing us our first glimpse at cometary material. However, due to the capture mechanism, carbon was difficult to capture and so we couldn’t analyse it. Finally, after 10 years in space and using gravity assists from Earth and Mars, Rosetta reached 67P.

Due to the nature of space mission design, the on-board instruments need to be finalised several years in advance so they can all be properly integrated together. As a result, the instruments being used now to study 67P were designed in the late 1990’s / early 2000’s, which by today’s standards of technology is ancient!

Comet 67P/Churyumov-Gerasimenko
Comet 67P/Churyumov-Gerasimenko. Image from

For the purposes of planning Philae’s landing, scientists and engineers assumed that 67P was roughly spherical and had an average comet density. In 2014, we got our first close up picture of 67P. Definitely not spherical. This meant we severely underestimated the gravitational pull of the comet. But that’s ok, the engineers said, that’s why Philae has harpoons to tether to the surface!

10 potential landing sites were selected. Care was taken to pick a spot that wasn’t too sunny (so the equipment wouldn’t fry from the intense heat of the sun), not too dark (so the solar panels could charge), not too steep and not too rocky (so Philae wouldn’t fall over). How did they go? Well, at least the equipment didn’t end up frying.

The below picture is the last image taken of Philae as it left the Rosetta craft. With much excitement and anticipation, the leadership team, 11 principal investigators and the media waited in a conference room for the fateful landing.

The last image taken of Philae as it leaves Rosetta.
The last image taken of Philae as it leaves Rosetta.

They waited 7 hours. Entertainment was provided by promotional videos such as this one (featuring ‘Littlefinger’), which I’m told got a little tired after the third time.

The investigators knew Philae bounced immediately. For a split second, they started to receive results from the comet surface. As soon as celebration erupted from the conference room, the data feed stopped. Not a good sign.

This cartoon from ESA provides a (somewhat simplified) explanation of what happened.

I could go into greater detail about an incredibly detailed presentation, but I need to be up in 7 hours to register. Who starts a conference at 8 am?

Until tomorrow.

Rosetta and 67P

67P/Churyumov-Gerasimenko, named after its founders by the same names, is rapidly approaching its closest point in orbit to the Sun. At almost 38 km/s to be precise.

Comet 67P/Churyumov-Gerasimenko
Comet 67P/Churyumov-Gerasimenko. Image from

Last November, the Rosetta spacecraft’s lander, Philae, became the man-made object to perform a soft landing on a comet.

And what a landing it was! Philae was unsuccessful at anchoring itself to the surface of the comet with its landing harpoons and bounced twice before coming to a halt in a dark zone. This was a problem as the lander couldn’t charge its batteries as well as planned using solar panels, and went into hibernation 3 days after touchdown.

Although Philae made contact at a very low speed, the low gravity on the comet (around one ten-thousandth that of Earth), meant that a small bounce was disastrous.

One proposed theory for the greater than expected ‘bounce factor’ is that the surface of 67P was elastic, with a hard crust under a metre thick overlying an elastic material (S. Ernst pers. comms.). This made me think of the recent announcement that the mysterious ‘craters’ on the surface are created when porous rocky material which has lost its water-ice due to outgassing. Eventually this porous rock can no longer hold its own weight, even in the low gravity of the comet (suggesting high porosity indeed… and a very high current or previous water content for the comet overall!), and it collapses, creating a sinkhole-like feature.

Perhaps the proposed ‘spongy material’ causing Philae to bounce is the same porous rock that is causing these sinkholes?

If so, is the whole surface of the comet poised on the brink of collapse with high porosity? Or did Philae get unlucky and land on a soon to be sinkhole?

These are the questions that excite me about space science. Part of my PhD will involve developing new and novel ways to test various models for the structure of asteroids and comets. Currently I’m looking at seismics and ground penetrating radar, but endless possibilities abound!

Until next time.

Edit: This article suggesting that 67P could be home to microbial life refers to an organic rich crust which is being constantly replenished. The presence of this crust and the replenishment of water (by outgassing from the deeper ice?) would support the above hypothesis!

Second edit: The Skeptics Guide to the Universe has stated that the original scientific research the above article was based on doesn’t actually make any claim to the existence of life on the comet. It just goes to show that one should always read the original science before commenting, as science journalism does get it wrong from time to time!