Landing on a Comet

by Prasad Ganti

Rosetta Mission Timeline Credit: ESA

Rosetta Mission Timeline.
Credit: ESA. Click to enlarge.

For the first time a manmade probe landed on a comet. Riding on a spaceship called Rosetta, the probe Philae was launched by ESA (European Space Agency) about a decade ago. Rosetta and Philae traveled four billion miles to chase down the comet. The last two years were spent in hibernation by powering down all the systems to conserve power. Rosetta woke up as it neared the comet, went into on an orbit around the comet and finally released Philae to land on the comet 67P. Earlier probes landed on planets and asteroids, but this was the first landing on a comet.

On Oct. 7, 2014, Comet 67P/C-G is framed by one of Rosetta’s solar wings, which is 46 feet long. A stream of gas and dust extends from an active area of the comet’s neck, about 10 miles away. Credit: ESA

On Oct. 7, 2014, Comet 67P/C-G is framed by one of Rosetta’s solar wings, which is 46 feet long. A stream of gas and dust extends from an active area of the comet’s neck, about 10 miles away.
Credit: ESA. Click to enlarge.

ESA named Rosetta for the Rosetta stone that the French found while conquering Egypt. It had three versions of a decree in ancient Egyptian hieroglyphs, Demotic script, and Ancient Greek. French scholar Jean Champollion studied and deciphered hieroglyphics from the Rosetta stone. He also got help from an obelisk found on an island called Philae. Philae was later submerged when a dam was built. Now experts can read all the writings on the walls of Egyptian temples and monuments, like we read English. The naming of the spacecraft and the probe after these ancient Egyptian artifacts was in the hope of studying the comet like we now understand the hieroglyphics.

Comets are the remnants from the creation of our solar system. Consisting of ice, rock and dust, these objects are very interesting to study. Periodic comets originate in the outer fringes of the solar system marked by the Kuiper belt and the Oort cloud and have a highly elongated orbit around the sun. The time to orbit around the Sun can take anywhere from a decade to a few hundred years. In fact, scientists speculate that a comet brought water and organic material to earth before life took shape here.

In addition to comets having elongated orbits, space travel is not so straightforward. It is not as simple as taking I-80 to drive from New York to San Francisco. Both the home and destination are moving platforms in the vacuüm of the space. Fuel being a very scarce commodity, it requires extra orbits around planets and Sun to get a slingshot-like gravity assist to reach the destination.

Comet 67P/Churyumov-Gerasimenko   Credit: ESA

Comet 67P/Churyumov-Gerasimenko Credit: ESA

Comet 67P has more complexities. It is just a few miles in size with a highly irregular shape. Its gravity is very weak almost to the point of nonexistence. Landing a probe on Mars is like throwing a basketball from Los Angeles to a basket in New York. Landing Philae is like throwing a basketball from Los Angeles to a basket located halfway around the world in Shanghai. And this million pointer long shot did work!

Philae was supposed to use a harpoon to land and get firmly entrenched into the surface. Because of the weak gravity, it was expected to bounce on the surface. To arrest the bounce, designers built a thruster on the top. The thruster was not functioning, but controllers released Philae anyway. It dropped on the surface of the comet, and landed at a place that was partly in shade. As a result the solar panels did not get adequate light to recharge the battery. Low power and an unstable perch did not prevent Philae from making observations and sending back the data.

In this risky space game, we should view the glass as half full, not as half empty. Congratulations to ESA on the wonderful achievement benefiting humanity!

Space missions usually have a fly-by as a first step; then orbiting, then landing and then returning with some samples. Will a returning spacecraft from a comet be the next step?

This entry was posted in December 2014, Sidereal Times and tagged , , , . Bookmark the permalink.

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