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Home / Science / Bad Astronomy | Lander Philae’s final secret reveals the amazingly fluffy comet

Bad Astronomy | Lander Philae’s final secret reveals the amazingly fluffy comet



In 2014, a dramatic but tough journey took place in space more than 500 million km from Earth.

The Rosetta spacecraft took more than a decade to reach comet 67P / Churyumov-Gerasimenko (commonly known as 67P), and three months after it arrived – after extensive surface mapping – it deployed The small lander is named Philae.

Philae will land on the comet’s surface and examine its surroundings, send data back to Rosetta and from there back to Earth. But something went wrong. The javelins that were supposed to fix it to the surface in virtually nonexistent gravity could not fire, and instead of attaching to the comet, Philae hit it and bounced back.

Despite the low collision speed (under 40 cm / s) at this first down position, it still bounced high, going up a kilometer from the surface (and hitting a quick blow to the edge of a ridge) before falling back. Then it impacts one Monday time, much slower, this time taking two minutes to turn it back on a little bit, then finally making a third ground touch and finally about 30 meters away.

The first landing site is relatively well known and its last resting place has been found after nearly two years of searching.

But where is that second touchdown site?

After years of searching, scientists finally found it, and it was amazing they did: It revealed that in the second collision, Philae wiped out some of the surface matter of the comet reveals a shiny ̵

1; and extremely fine hair – rock underneath.

The first piece of the puzzle was to create an orbit for Philae after it first made an impact on a comet in an area called Agilkia, helped by knowing the last position in the region. Another is the Abydos*.

Along that orbit, just a few meters from where Philae lay, are two strongly reflective patches of ice that were not there before landing, implying that it was the landing craft that was involved in exposing them.

Assuming it happened and given the time when the lander would reach that point, the scientists turned to an unlikely source of information: the lander’s magnetometer, called the ROMAP.

This is a device that measures the magnetic field around the comet, used to understand the environment and interaction of comets with the solar wind. It jutted out of the lander at the end of a half meter long boom. But it was not the comet’s magnetism that helped find the lander so much: It was the surface magnetometer that performed the trick.

When the magnetometer physically impacts the comet’s surface, the data it records show the lander’s acceleration (the change in velocity on impact) and its attitude (the spacecraft’s orientation). . When all is in hand, scientists can finally get the story of what happened to poor Philae at this second collision site.

When it falls for the second time, it actually creates four separate points of contact with the surface. First, it collided and slid down the side of a rock, spinning like a windmill, and with its ground feet disturbed the dust that had been piled up. It went through a 2.5-meter-long crack in the rock, hit the wall of the crevice, and compressed the ice there. It pops up, touches a protrusion, then hits the surface again before bouncing onto the third (and final) touch down position 30 meters away. The whole process takes about two minutes.

Interestingly, when viewed in the right direction, the second touch down looks like a skull face!

The skull’s right eye (we see it to the left) is actually the last impact from Philae before bouncing to its final stop (other features already exist).

That right eye is where Philae compresses the surface, breaks a wall of dust, takes three seconds to compress it about 25 centimeters, then pops out, leaving bright ice.

That’s great, but there’s science there too! Given the mass and speed of the lander, the fact that it compresses ice a lot means ice extremely brittle, is basically the strength of the foaming milk in the cappuccino or the foam on the waves.

Incredible. We have long known that the physical composition of the comet’s surface is fragile, and the structure below the surface can have lots of gaps (like the empty space between oranges in a barrel), reduces overall density. But this is quite good evidence that comets are extremely soft. It would be really difficult to hold part of it in your hand without crushing it. The porosity of the rock is about 75%, that is they are mostly holes.

The only reason it could do so is that the comet’s gravity is so weak. It is only a few kilometers across, and low mass. During Earth’s gravity, it will likely collapse.

This also explains timing. When Philae got closer to the second spot, it was moving about 20 cm / sec below the surface and 10 cm / sec along the surface. The effects slowed it down, including as it compressed the dusty ice, so that when it eventually left second place it only moved 1 cm / s upwards and 9 cm / s along the surface. . If the surface was tougher, the lander wouldn’t slow down that much, but its moving energy would be dissipated by the fragile surface compression.

Please note, the exit rate at that point on the comet is just a little over 40 cm / s! If you try to walk along the surface, you will fly away. In other words, if Philae moves much faster when it first pops up, it won’t come back. If the surface cannot be compressed, it will likely bounce again and again before stopping.

To be fair, the amphibious mission did not go rather As planned. However, even then, some great science about the structure of the comet has still been obtained, information may not be known if the landing goes as planned! Science is funny that way sometimes.

So Philae sat. Around the comet’s horizon, the mother ship Rosetta also stopped, falling to a single point on September 30, 2014, not long after Philae was first found. I think, it’s fitting that the spacecraft stay with the goal they’re pursuing for so long. They had a pretty good ride, and I’m glad that now we know what adventure it was like.


*I know, the Stargate fans. I think it’s great too.


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