Tuesday, November 11, 2014

A Four Billion Mile Road Trip to Grandma’s

A comet is a celestial body that is trapped in orbit
around something large. When close to the sun they
have tails. The tail is material from the frozen comet
that is released from the comet when the sun’s energy
heats it. But the tail doesn’t follow the comet. It
always points away from the sun – blown by the solar
wind. Comet 67P/C-G’s tail will be 100,000 miles long
when it is closest to the Sun next year.
On November 12, 2014, man will perhaps accomplish something unprecedented. That is, with a ton of good planning and more than a little luck. The landing craft, Philae, will set down on a comet. I hope you realize how amazing that is.

The comet, named 67P/Churyumov–Gerasimenko, or 67P/C-G for short, has a period of 6.6 years. Every 2/3 of a decade or so, it travels in from deep space to make a trip around the sun. For this particular trip, there was a probe waiting on it, the Rosetta orbiter. After going into orbit around 67P/C-G, Rosetta is now set to release Philae to drop down and land on the comet's surface.

But how did Rosetta meet the comet? It took a lot of doing. Comet 67P/C-G is traveling at a steady rate of 24,600 mph (39,589 kph). That means it travels from NYC to Boston in 30 seconds. Let’s see the planned NYC-Boston maglev train match that!

When we send rockets into space to meet the International space station, they only have to travel 17,000 mph (27,358 kph). The ability to reach this higher speed, just to get into the orbit of the comet could be achieved two ways.

One - you could put all the fuel on board the orbiter and thrust yourself to the proper speed. But that would require a space ship the size of a football field. You just can’t get all that fuel into space for a price anyone could pay. So that’s out.

Second - you could use the power of gravity. By launching Rosetta out into space and then having it swoop by a planet, it can use the gravity of the planet to pull it to a higher speed and then curve around the planet and get shot out the other side. Every time you do this, you gain some speed.

Rosetta achieved its amazing pace by three separate Earth gravity assists and one Mars gravity assist. It has traveled over 3.8 billion miles just to gain enough speed so it can sneak up behind the comet. And how long did this take? Rosetta/Philae were launched in 2004! And my kids have a hard time planning for a paper due in two weeks.

You can see the gravity assists that Rosetta used to catch
up with 67P. Also notice that Rosetta went into
hibernation for three years. It’s instruments run on solar
power, but it was 500 million miles from the sun. Out
there you only get about 4% of the sunlight we get here on Earth.
They’ve come a long way at ESA (European Space Agency) and NASA, the two agencies running the mission. Compare Rosetta’s progress to that of Voyager 1. True, Voyager has traveled farther (4.4 billion miles, 7.1 billion km), but it was launched in 1976!

After all this chasing, now Philae is ready to separate from Rosetta and land on 67P/C-G. This is where the luck comes in. Once Philae leaves Rosetta, there is no controlling it’s path. The plan, based on thousands of photographs taken of 67P/C-G’s surface by Rosetta while in orbit, is to land on Philae somewhere flat and sunny.

Flat is obvious, the comet’s surface is mostly irregular, with huge boulders and deep crevices; finding a good parking spot on 67P is harder than the week before Christmas at the mall. Flat is also important so Philae can get a good grip. Gravity is so low on 67P that there is a fear that Philae will just bounce off the surface, even though it will be moving at a pace similar to a slow walk.

The ESA had a naming contest for the landing spot for
Philae, shown above. The winner – Aglilkia. Suggested by
150 people, Agilkia is an island in the Nile River where
several temples were moved when the Nile Valley was
flooded by the Aswan dams. The Island they were moved
from – Philae. The numbers show the diameters of the
boulders to give it some scale.
To help with this, Philae has several hooks it will deploy to stab into the surface of the comet – hopefully. We don’t know how hard the surface is, or if the lander will come down in a place where there isn’t a rock in the way. See what I mean by luck?

As far as sunny is concerned, the sun is needed to charge the instruments on Philae. If they land in the shade, she’s only going to be functional for about 60 hours. With a good sunny spot, she might work for up to 6 months.

The question you’re now asking is, “Work to do what?” Why spend all this money and time to land on a comet? In response, I ask if you care where your parents came from, or their parents. Go back far enough and you have to wonder where life on Earth came from. Did life start here on its own or was it brought here?

Philae may help answer these questions. Comets are rock, ice, and who knows what else. Water is needed for all life that we know about, so did comets bring water to Earth? Is there something alive in the ice, or are the building blocks for life present on that comet? Philae has nine instruments to help answer these questions.

If Philae finds organic molecules, then we better start preparing a list of questions for our neighbors, because that finding will almost assure us that they’re out there. And it may mean more -chirality in organic molecules is important here on Earth.

Amino acids in Earth based life are all left-handed (called
L). Their mirror image is the D-amino acid, but they don’t
work the same way in proteins. D- amino acids would
make a protein fold differently, and the way a protein
folds determines its function. If there are amino acids of
67P/C-G, will they be L- or D-?
Many organic molecules have a handedness in how they are constructed. Our amino acids are all left-handed (see this post), while our sugars are right-handed (see this post). If the organic compounds on 67P/C-G are the same, then life elsewhere is going to look a lot like we do.

If they are opposite handed, then Star Trek got it wrong with all those humanoid aliens, and we may be for some real surprises in the future.

A 2012 paper explains how one of the instruments on Philae is designed just for chirality question. It will determine the handedness of any organic molecules found on 67P/C-G. After all, it is important to find Grandma, and 67P/C-G might be her house, or at least her hulking old sedan with the 35 gallon gas tank and steering wheel as big as a hubcap.

Comets are remnants from when the solar system was young; they are where we were, and we need to know them in order to get a better idea of where we should be going. I personally am very excited to find out if we are alone in the universe – that would be so sad. Isn't that important enough for a 3.8 billion mile road trip?

Contributed by Mark E. Lasbury, MS, MSEd, PhD
As Many Exceptions As Rules

Evans, A., Meinert, C., Giri, C., Goesmann, F., & Meierhenrich, U. (2012). Chirality, photochemistry and the detection of amino acids in interstellar ice analogues and comets Chemical Society Reviews, 41 (16) DOI: 10.1039/c2cs35051c

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