How to Land on a Comet
 
Nov 3, 2014: Generally speaking, space missions fall into one of three
categories:  difficult, more difficult, and ridiculously difficult.
 
Flybys are difficult.  A spaceship travels hundreds of millions of miles
through the dark void of space, pinpoints a distant planet or moon, and
flies past it at 20 to 30 thousand mph, snapping pictures furiously during
an achingly brief encounter.
 
Going into orbit is more difficult. Instead of flying past its target, the
approaching spaceship brakes, changing its velocity by just the right
amount to circle the planet.  One wrong move and the spacecraft bounces off
the atmosphere, becoming an unintended meteor.
 
Landing is ridiculously difficult.  Just play NASA's "Seven Minutes of
Terror" video. Watching Curiosity parachute, retrorocket, and
sky-crane its way to the surface of Mars rarely fails to produce
goosebumps. Since the Space Age began, the space agencies of Earth have
succeeded in landing on only six bodies: Venus, Mars, the Moon, Titan, and
asteroids 433 Eros and Itokawa.
 
https://www.youtube.com/watch?v=D5iyZTXiX78&feature=youtu.be
 
A new ScienceCast video previews the first-ever landing on a comet. Play it
In a move that could set a new standard for difficulty, the European Space
Agency is about to add a seventh member to the list. On Nov. 12th ESA's
Rosetta spacecraft will drop a lander named "Philae" onto the
surface of Comet 67P/Churyumov-Gerasimenko.
 
"How hard is this landing?" asks Art Chmielewski, the US Rosetta
Project Manager at JPL. "Consider this: The comet will be moving 40
times faster than a speeding bullet, spinning, shooting out gas and
welcoming Rosetta on the surface with boulders, cracks, scarps and possibly
meters of dust!"
 
Rosetta will drop Philae from a height of 22 km as the comet rotates freely
below. No active steering will take place during the slow descent.
 
"Unlike previous landings, where reconnaissance had been done
beforehand--at Mars, for instance, we mapped the planet well in
advance-Rosetta just started learning about its target a couple of months
ago," explains Claudia Alexander, Project Scientist for the U.S.
Rosetta Project. "This introduces much more risk."
 
Rosetta arrived at 67P on August 6, 2014.  What it found was shocking.  The
comet's nucleus is strangely shaped, (one observer has likened it to a
"freak-show mushroom") dominated by a pair of mile-wide
"knobs" joined by a boulder-strewn "neck." Picking a
landing site would not be easy.
 
http://tinyurl.com/ls29vxv
 
Click to learn more about landing site J. Credit: ESA. Rosetta spent more
than a month surveying the comet before engineers and scientists gathered
in France to make their decision.
 
"None of the candidate landing sites met all of the operational
criteria at the 100% level," says Stephan Ulamec, Philae Lander
Manager at the German Aerospace Center (DLR), "but Site J is clearly
the best solution."
 
Site J is a relatively flat, boulder-free location on the smaller of the
comet's knobs.  It gets plenty of sunlight for the lander's solar panels
and has good line-of-sight visibility for communications with Rosetta
orbiting overhead.
 
The descent will take about 7 hours, a drawn-out process that could be
enlivened by unpredictable jets of gas emerging from the comet's core.
 
You thought 7 minutes of terror was bad? "This will be Seven Hours of
Terror," says Alexander.
 
If all goes well, Philae will touch down at walking pace and deploy
harpoons to fasten itself to the crusty surface.  A suite of 10 sensors on
the lander, including a drill for sample collection and an acoustic sounder
to probe the comet's sub-surface structure, can then begin an unprecedented
study of a comet at point-blank range.
 
"A comet is unlike any other planetary body that we've attempted to
land on," says Alexander. "Getting Philae down successfully will
be an incredible achievement for humankind!"
 
Try your hand at landing a spacecraft on a comet with NASA Space Place's
Comet Quest: http://spaceplace.nasa.gov/comet-quest/
 
Credits:
Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:
Science{at}NASA
 
More information:
 
JPL Rosetta Mission Site - http://rosetta.jpl.nasa.gov/
 
ESA/Rosetta Mission Site - http://rosetta.esa.int
 
 
Regards,
 
Roger

--- D'Bridge 3.99