VOICEOVER: In 2014 a small craft called Philae landed on the surface of comet 67P/
Churyumov–Gerasimenko in a mission destined to make history.
Unfortunately things didn’t quite go as planned.
LAURENCE O’ROURKE: When it touched down, it was supposed to be held by harpoons on the surface,
but it didn't, these harpoons did not work. And so it bounced.
VO: Laurence O’Rourke led the team that located the wayward lander,
which had ended up stuck on its side underneath a cliff.
This awkward position meant that some of the intended experiments didn’t work.
LAURENCE O’ROURKE: For example, the drill. We know now that the drill didn't penetrate the surface,
primarily because it was too high above the surface itself.
VO: The Philae lander provided the first direct measurements of the comet’s surface.
But because of the crash landing it couldn’t complete its full mission
and reveal the secrets of the comet’s icy interior.
Or so everyone thought.
Now Laurence and his team have turned the crash to their advantage.
By pouring through thousands of photos of the comet’s surface with forensic precision
and reinterpreting data from Philae’s other sensors,
Laurence and his team have been able to reconstruct what happened the day Philae crashed,
and in-so doing got the first ever glimpse inside a comet.
Inside a comet: Philae’s final secret
LAURENCE: Philae was lost for about two years, well, I led the lander search campaign
to try to find an image of Philae on the surface of the comet.
But very close to the end of the mission, there was two images that came down, which struck me as being very, very unusual.
They showed what looked like ice that had been sliced open,
it was actually if somebody had got a chainsaw or something that actually sliced through the ice.
VO: Laurence suspected that on its accidental journey across the comet, Philae had touched
down on this particular boulder, scraping aside the hard, dusty surface to reveal the bright ice underneath.
By combining thousands of photos into a 3D model of the comet’s surface
the team managed to pinpoint the exact location of the touchdown site,
just 30m from Philae’s final resting place.
LAURENCE: You can see on the left hand side the touchdown point which shows
the crevice between the two boulders. Now, the image has been enhanced, primarily
because we want to see Philae lander, which is hidden in the distance underneath the cliff.
VO: Careful examination of photos of the boulder gave Laurence
some clues as to how exactly the lander had impacted the ice.
LAURENCE: The ice at the end of the crevice
shows an impression of Philae in its surface. Philae had actually compressed the ice there.
And when we look in from another image, we could measure that this depth was 25 centimetres.
VO: This 25cm measurement would turn out to be a vital piece of information.
LAURENCE: Now when you compress something and you put your foot for example in snow,
then you gradually notice it gets harder and harder. And you can actually tell
with the time it takes you to compress it, it tells you the strength of that snow.
VO: Laurence realised that they could use this concept to calculate the strength of
the ice that Philae crashed into, but they needed one more piece of information:
How long it took Philae to compress that 25cm.
And that information came from an unexpected source.
LAURENCE: So the Rosetta spacecraft has a number of different magnetometers, magnetic instruments,
on board which are there to measure the magnetic field around the comet. And the Philae lander had
its own magnetometer. But because Philae was crossing the surface of the comet,
then the magnetic data it was, it was generating was changing - primarily because
of the orientation of Philae. It was rotating. It was flipping, it was changing direction.
VO: The magnetometer was acting a bit like a gyroscope - detecting every time Philae moved
or changed direction. This meant Laurence and his team could pinpoint the exact moment
that the lander crashed into the boulder and compressed the ice.
LAURENCE: So this graph contains the data from the magnetometer instrument on the Philae Lander
And when we look at this area in particular, you can see
spikes. And this duration of this spike, is three seconds
and we know therefore that it took three seconds to compress the ice.
VO: Calculating the compressive strength revealed
that the interior of the boulder was incredibly soft.
LAURENCE: The ice is softer than freshly fallen snow,
it's softer than the the foam that you find on the seashore when you're walking on the seashore,
it's softer than the froth that you find on top of your cup, of your cappuccino.
VO: This incredibly soft interior had been hidden inside a hard outer crust.
Philae’s crash landing meant that its drill couldn’t sample
the inner material - but the lander’s haphazard journey across the comet
provided an unexpected peek below the surface...
LAURENCE: Philae has in fact excavated the, the ice,
it's actually quite broken the the top covering of the dust which was covering it
and exposed something that was that was hidden inside for billions of years.
And so you're seeing really the the interior of a comet
that has not been exposed since its formation.
VO: When Philae crashed landed six years ago, it looked like some parts of its
mission would never be completed. Now, thanks to some ingenious detective work,
Laurence and his team have uncovered the first ever glimpse
inside a comet,
and helped the Philae lander make one final contribution to cometary science.