Landing on a Comet!

01
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Rosetta's Philae Lander Had Two Possible Targets

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The selection of landing sites on Comet 67P and the final two choices. ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

Scientists from European Space Agency made history on August 6, 2014, when controllers slipped the Rosetta spacecraft into an orbit around the oddly shaped Comet 67P/Churyumov-Gerasimenko. Even before its arrival, the spacecraft was sending images of the comet back to scientists, so they could figure out the best place to land a small probe called Philae (pronounced "FEE-lay"). It was supposed to settle gently onto the comet's dark crust in November and return the first images from the surface as well as make onsite measurements of the comet's ices and dirt.

In particular, the scientists who built and programmed Philae want to find out what elements the comet's surface is made of, how much of each element (such as water ice, carbon dioxide ice (if any), carbon-based materials (in the dust and dirt component of the comet), and if possible, measure any magnetic field in the area of the comet, and any plasma (excited gases) that exist near the surface. 

As you might imagine, landing a spacecraft on an object moving through space can be tricky. Now, consider that the comet's nucleus is oddly shaped — it has been described as a "rubber duckie" — and is tumbling through space as it orbits the Sun. As various parts of the surface are heated by the Sun, the surface warms and then cools as they move into shadow. In addition, the comet "outgasses" as it gets close to the Sun, which means that it sends out sprays of frozen gases from beneath the surface. As the material in those sprays interacts with the solar wind, molecules of gas interact with the solar wind's magnetic field and that forms a comet's plasma tail. So, far from being an easy landing site, Comet 67P presents a challenge worthy of the most talented spacecraft driver.

After looking over the comet for a few weeks, the scientists decided on a site they named Agilkia (uh-GILL-kee-uh).  

02
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Philae's First Choice for Landing

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A closeup of the "J" primary landing site on the surface of Comet 67P. ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA

This is Agilkia, the best choice for landing on the tumbling, odd-shaped Comet 67P nucleus. It has some large boulders, but shows some smoother areas that could be very supportable for the Philae lander to do its work. To help make their decision about this site versus other sites, scientists made 3D models of the nucleus, taken from mapping data made by the Rosetta spacecraft. During site selection meetings, they passed the models around to get a "feel" for what the lander might experience. The scientists also had consider the speed of the comet and the speed of the lander, the gravitational tug of the comet, the surface characteristics of the nucleus, and the amount of light the site will get during Philae's mission.

03
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Philae's Bouncy Landing

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This sequence of images from the high-resolution OSIRIS camera onboard Rosetta shows the Philae lander as it made its first touchdown on the surface of Comet 67P, and then bounced twice more before coming to a rest near a "cliff" on the comet. The exact point of its final touchdown is still being determined. Courtesy: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Philae did actually touch down on the comet, on November 12, 2014. However, the landing wasn't as smooth as planners hoped. It turns out the lander actually bounced twice before coming to rest at the base of a cliff, and partially shaded from the sunlight it needed to recharge its batteries. The lander was able to take images and data for about a day and a half and return it to Earth via Rosetta before going into hibernation mode. The teams hope that the lander will wake up again when the comet is closer to the Sun, which could provide more sunlight for the solar panels and battery system to power the instruments on again. However, the mission is considered successful at this point because all the primary science data collection occurred. The Rosetta mission orbiter continues to study the cometary nucleus and return data to Earth.

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Petersen, Carolyn Collins. "Landing on a Comet!" ThoughtCo, Aug. 23, 2016, thoughtco.com/landing-on-a-comet-3072106. Petersen, Carolyn Collins. (2016, August 23). Landing on a Comet! Retrieved from https://www.thoughtco.com/landing-on-a-comet-3072106 Petersen, Carolyn Collins. "Landing on a Comet!" ThoughtCo. https://www.thoughtco.com/landing-on-a-comet-3072106 (accessed December 12, 2017).