How does one find success?
Kamal Lodaya, Bengaluru
Old readers of JM will remember the European Rosetta spacecraft. It reached the nucleus of Comet Churyumov-Gerasimenko in 2014 and travelled with it as it went close to the Sun.
On November 12 that year, it released a lander called Philae which hit the comet's surface. The hooks to hold it to the comet's surface failed and it bounced. If Philae had bounced at twice the speed it did, it would have escaped the comet and gone into space. See the BOX on Escape velocity in article X on page Y.
It did not; it flew slowly over the comet. Then it hit a steep incline on one side of the comet, tumbled a bit and landed sideways in a rocky shaded area. Philae tried to drill the surface of the rock. Its drill broke. Its batteries ran out in less than a day. Without sunlight they could not be recharged.
You might think, what a disaster! We send a spacecraft to a comet, it takes 10 years. The photos from the orbiter are nice and we learn a lot. But Philae... the whole idea of landing didn't quite work, did it?
That's not quite the way to think. Had we ever tried landing on a comet before? No. Did we know what the comet surface was made of? No. Yet we managed to get some science out of the lander. It landed, bounced, hit something, then crashed in a dark place. We have pictures of the site where it bounced. We have pictures of the dark place it ended up in.
ESA scientists asked: what about the place it hit inbetween? After poring over extreme blowups of the picture using which the Philae lander was discovered in its final landing place, the scientists figured out the place where it had tumbled.
From the clock timing Philae, they calculated it approached this second site at a velocity of 20 cm/sec down and 10 cm/sec parallel to the surface. (These are called the components of its velocity.) If you convert to our usual speeds that is less than half a kilometre per hour.

BOX on Vectors
A vector is a convenient way of combining certain common values. For instance, you may want to push a book lying on a table. This requires a horizontal force. But you can still push it at an angle, let us say 45 degrees downwards. This is still a force, but a slanting one. Part of the force still pushes the book horizontally. The rest of the force does no work. So, you can think of the force as being composed of two components: one horizontal, and the other vertically downward, at ninety degrees to the "useful" one. Both are forces, but in different directions, and so can be combined into one object called a vector.
The slanting force that you applied is therefore a vector pointing at 45 degrees and has some value or magnitude. All vectors have direction and magnitude; see figure. All vectors can be composed from two components that are perpendicular to each other. For instance, the force of 10 N in the figure, acting in a South-Easterly direction (at 45 degrees to the horizontal), can be thought of as two forces, one acting eastward and the other southward.
END OF BOX
When Philae moved off after scraping the second site, its velocity components were 1 cm/sec up and 9 cm/sec parallel to the surface.
That whole second tumbling seems to have taken 4 minutes. Why? The impact slowed it, obviously. Hitting what kind of surface would slow it like that? If the surface were rigid, like that of its first hit, Philae would have bounced. It didn't.
Searching in the photos showed that the place Philae had impacted second had exposed, dusty ice. Its velocity decreased because its impact compressed that ice.
How dense would that ice be to get compressed like that? Given the lander's mass and velocity and that it compressed the ice that much, the scientists calculated this density. The answer was surprising: extremely fluffy, like froth in a sea wave. Or froth in a cup of coffee, if you prefer. It would be very hard to hold the ice in your hand, you would crush it.
We cannot have such ice on Earth. It is there on comet Chury because its gravity is very weak. Working on that data years after the mission gave us these results. Science requires a lot of patience.