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@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   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)