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(Click for frequent words.) 74 rover 70 Mars rovers 70 rovers Spirit 67 twin rovers 66 lander 65 Martian surface 64 NASA rovers 64 robotic rovers 64 spacecraft 63 Opportunity rovers 62 Mars rovers Spirit 62 Mars rover 62 Red Planet 62 Mars Exploration 61 rover Opportunity 60 Sojourner rover 60 Mars landers 60 robotic explorer 60 Martian rovers 59 wheeled rovers 59 Mars Pathfinder 58 Spirit rover 58 robot rovers 58 NASA rovers Spirit 58 twin rovers Spirit 57 Martian 57 Viking landers 57 sols 57 spacecrafts 57 twin rover Opportunity 57 unmanned probes 57 Mars 57 orbiter 57 orbiting spacecraft 57 robotic explorers 57 landers 56 Mars Rover 56 twin rover Spirit 56 robotic geologists 56 orbiting Mars 56 Mars Reconnaissance Orbiter 56 rover Spirit 56 Nasa spacecraft 56 martian surface 56 Mars rover Opportunity 55 martian 55 robotic probes 55 Voyager probes 55 robotic geologist 55 robotic landers 54 Hubble 54 Mars Odyssey 54 lunar surface 54 Curiosity rover 54 Voyager spacecraft 54 orbiters 54 Cassini Huygens 54 twin Mars rovers 54 NASA Mars rover 53 Stardust spacecraft 53 robotic spacecraft 53 Martian terrain 53 Kepler spacecraft 53 unmanned spacecraft 53 orbiting telescope 53 Saturn Cassini 53 Martian arctic 53 Phoenix Mars lander 53 Mars orbiters 53 Lunar Reconnaissance Orbiter LRO 53 Orbiter MRO 53 orbiters landers 53 LRO 53 Hubble telescope 53 Nasa 53 Opportunity rover 53 crewed spacecraft 53 Martian plains 52 reconnaissance orbiter 52 astronauts 52 Mars Lander 52 flyby spacecraft 52 lunar rovers 52 Phoenix Mars Lander 52 Huygens 52 Saturnian moon Titan 52 APXS 52 outer planets 52 orbiter lander 52 robotic rover 52 Mars orbiter 52 Odyssey orbiter 52 Apollo missions 52 Spirit Mars rover 52 Global Surveyor 51 rover Sojourner 51 twin Voyager spacecraft 51 six wheeled rover 51 simulated lunar 51 interplanetary spacecraft 51 Cassini 51 Mars Color Imager 51 Mars Reconnaissance Orbiter MRO 51 Mars Odyssey spacecraft 51 Comet Churyumov Gerasimenko 51 robotic lander 51 satellites orbiting Earth 51 Martian equator 51 Mars Odyssey orbiter 51 ExoMars rover 51 GRACE satellites 51 Mars rover Spirit 51 NASA Spirit rover 51 Mars lander 51 Orbiter 51 Phoenix lander 50 NASA Jet Propulsion Laboratory 50 lunar orbiters 50 orbiting 50 Jet Propulsion Laboratory 50 Martian winters 50 JPL 50 tumbleweed rover 50 Martian landscape 50 MER rovers 50 NASA 50 stationary lander 50 dust devils 50 NASA twin rovers 50 stabilizing gyroscopes 50 Terrestrial Planet Finder 50 Huygens probe 50 Hubble Space Telescope 50 BepiColombo 50 satellites orbiting 50 NuSTAR 50 microscopic imager 49 lunar landers 49 Mars Pathfinder lander 49 Cassini orbiter 49 Gusev crater 49 PASADENA Calif. NASA 49 comet Churyumov Gerasimenko 49 Mars Surveyor 49 orbiting observatory 49 NASAs 49 dexterous robot 49 lunar rover 49 spacecraft orbiting 49 Mars Global Surveyor 49 Churyumov Gerasimenko 49 Purgatory Dune 49 interplanetary missions 49 robotic arm 49 Nasa Mars Reconnaissance 49 Mars moon Phobos 49 Martian rocks 49 Hayabusa probe 49 Apollo astronauts 49 Enceladus flybys 49 spacesuits 49 Comet #P/Churyumov-Gerasimenko 48 exoplanetary 48 Mars Opportunity rover 48 NanoSail D 48 Martian soil 48 Lunar Rover 48 pressurized rovers 48 robotic rover Spirit 48 Manned missions 48 COROT 48 Jovian moons 48 Martian rover 48 dexterous robots 48 highly elliptical orbits 48 MESSENGER 48 Astrobotic 48 O OREOS 48 Hubble Space Telescope HST 48 retroreflector 48 orbiting observatories 48 Nasa Jet Propulsion 48 Martian arctic plains 48 orbital rendezvous 48 starboard SARJ 48 Meridiani Planum 48 Cryosat 48 HiRISE 48 NASA Cassini 48 Stardust NExT mission 48 Rover MER 48 CHIPSat 48 lunar orbits 48 orbiting Hubble Space Telescope 48 wheeled rover 48 celestial observations 48 NASA Mars Reconnaissance 48 New Horizons spacecraft 48 icy moons 48 MARSIS radar 48 satellites 48 Huygens mission 47 MESSENGER spacecraft 47 NASA Lunar Reconnaissance 47 NASA Kepler telescope 47 Jupiter icy moons 47 Martian atmosphere 47 Orion spacecraft 47 infrared telescopes 47 Shenzhou spacecraft 47 Ulysses spacecraft 47 lunar 47 Martian crater 47 moon 47 NASA Kepler spacecraft 47 Viking orbiters 47 NASA Stardust spacecraft 47 Viking Lander 47 lunar equator 47 asteroid Steins 47 telescopic observations 47 Victoria Crater 47 Reconnaissance Orbiter 47 Stardust capsule 47 OSIRIS REx 47 Interior Laboratory GRAIL 47 Solar Terrestrial Relations 47 Opportunity parachuted 47 LCROSS 47 Cassini spacecraft 47 lunar missions 47 SMOS 47 lunar regolith 47 Earthlike planets 47 SIM PlanetQuest 47 Lunokhod 47 Jupiter Icy Moons 47 CALIPSO 47 asteroid Eros 47 Rosetta spacecraft 47 Solar TErrestrial RElations Observatory 47 payloads 47 HiRISE camera aboard 47 HiRISE camera 47 Peter Theisinger 47 Philae lander 47 EPOXI 47 ESA Huygens probe 47 Yinghuo 1 47 IBEX spacecraft 47 lunar lander 47 SSETI Express 47 Upsilon Andromedae 47 unmanned airships 47 WISE spacecraft 47 NASA Mars Odyssey 47 moonbase 47 Orbital Express 47 Landsat satellites 47 Martian craters 47 John Callas rover 46 extravehicular activities 46 SIRTF 46 KBOs 46 legged lander 46 asteroid Itokawa 46 Ranging spacecraft 46 Galileo spacecraft 46 Jupiter Ganymede 46 Galileo satellites 46 Jet Propulsion Laboratory JPL 46 impactor 46 ST5 46 ion propulsion 46 Venusian surface 46 CubeSats 46 laser altimeter 46 moons Phobos 46 Compton Gamma Ray 46 JIMO 46 Mars Orbiter Camera 46 LCROSS spacecraft 46 Rosetta orbiter 46 arctic plains 46 Orbiter LRO 46 spacecraft Cassini 46 GOCE 46 Hubble Space telescope 46 Endurance crater 46 Mini TES 46 LORRI 46 Orbiters 46 shuttle orbiter 46 moons orbiting Pluto 46 Mars CRISM 46 spaceships 46 flyby Cassini 46 Huygens lander 46 THEMIS satellites 46 comets asteroids 46 martian soil 46 Chandrayaan 46 nanosatellite 46 SPHERES 46 lunar probes 46 NASA Ames 46 planetary missions 46 Mars Polar Lander 46 ESOC 46 gray hematite mineral 46 LROC 46 spaceborne 46 Webb Telescope 46 Ed Weiler NASA 46 MARSIS 46 extrasolar planets 46 Lunar Reconnaissance Orbiter 46 polar craters 46 Lunar Orbiter 46 LADEE 46 highly elliptical orbit 46 Mars Orbiter 46 Martian dust 46 orbit 46 scramjets 45 NASA THEMIS 45 spacefarers 45 Cassini Huygens spacecraft 45 orbiting telescopes 45 SuitSat 45 Comet Hartley 2 45 Mars orbiting spacecraft 45 NASA Voyager spacecraft 45 asteroids 45 wheeled robot 45 comet Boethin 45 orbiting Saturn 45 Titan flyby 45 heatshield 45 Kepler Mission 45 thermal emission spectrometer 45 microshutters 45 reusable spacecraft 45 SMOS mission 45 Mars Gusev Crater 45 GSLV rockets 45 NASA STEREO 45 hardware Theisinger 45 Pilot George Zamka 45 lander Beagle 2 45 Discovery astronauts 45 Context Camera 45 space shuttles 45 Opportunity Mars rovers 45 WISE telescope 45 Epoxi 45 sister rover Spirit 45 STEREO spacecraft 45 asteroid 45 Eurobot 45 spaceflyers 45 orbit GEO 45 lunar craters 45 Opportunity Mars rover 45 Nasa Mars Odyssey 45 alien civilizations 45 planets circling 45 Lunar Roving Vehicle 45 moon landers 45 Mini Thermal Emission 45 lunar landings 45 Chandrayan 45 Interplanetary Internet 45 thrusters 45 Hubble Telescope 45 ADM Aeolus 45 GOES satellites 45 Hayabusa 45 Mars Spirit rover 45 TDRS K 45 solar arrays 45 Mastcam 45 Pete Theisinger 45 spacemen 45 Eagle lunar lander 45 Benthic Rover 45 Moessbauer spectrometer 45 EPOXI mission 45 International Space Station 45 Meridiani plains 45 Comet Hartley 45 orbits 45 MAVs 45 planetary Transits 45 martian landscape 45 Director Charles Elachi 45 extrasolar 45 NASA Kepler 45 phyllosilicates 45 Deep Impact flyby spacecraft 45 ion thrusters 45 asteroid Vesta 45 EVAs 45 Applied Physics Laboratory 44 geostationary orbits 44 OGLE TR #b 44 micrometeoroids 44 PSLV rocket 44 STEREO mission 44 Squyres 44 uncrewed 44 Rosetta lander 44 Lunar Prospector 44 Wilkinson Microwave Anisotropy Probe 44 Stardust NExT 44 RSRMs 44 heat shield 44 lunar module 44 ESA Rosetta 44 NASA Jet Propulsion 44 Chandrayaan II 44 planetary surfaces 44 autonomous robots 44 Laboratory MSL 44 Thermal Emission Spectrometer 44 ExoMars 44 LISA Pathfinder 44 lunar meteorites 44 Mossbauer spectrometer 44 satellites orbiting Mars 44 Gusev Crater 44 Solar Dynamics Observatory 44 LRO spacecraft 44 shuttle Endeavor 44 LCROSS mission 44 Commander George Zamka 44 deorbiting 44 Diviner instrument 44 lunar orbiter 44 rocket thrusters 44 Foton spacecraft 44 Jean Luc Margot 44 Globalstar constellation 44 probe Huygens 44 Russian cosmonaut Alexander Kaleri 44 aboard Atlantis 44 remote sensing satellite Resourcesat 44 orbiting satellites 44 durable twin rovers 44 Kepler Space Telescope 44 NASA EPOXI mission 44 NASA Opportunity rover 44 Mars rovers Opportunity 44 orbiting astronauts 44 spokesman Michael Braukus 44 lunar landing 44 moonwalkers 44 Shenzhou capsules 44 rock outcrop 44 rover robotic arm 44 Kaguya 44 planetary exploration 44 comet particles 44 spaceward 44 Martian orbit 44 comet #P/Churyumov-Gerasimenko 44 Cartosat 2B 44 ESA Mars 44 Earth orbiting satellites 44 Chang'e 44 Gravity Probe B 44 CloudSat 44 Climate Experiment GRACE 44 Zeta Ophiuchi 44 Theisinger 44 SpaceLoft XL rocket 44 moon Charon 44 shuttle astronauts 44 AUVs 44 Atlantis astronauts 44 comet Wild 44 Sojourner Rover 44 telescope 44 geosynchronous satellites 44 Canadarms 44 Gliese #d 44 planets 44 unmanned lunar orbiter 44 binary asteroid 44 GeneSat 44 spaceship 44 parachutes malfunctioned 44 planetary rovers 44 Comet Machholz 44 NASA Mars rovers 44 GEochemistry 44 lander rover 44 orbited Jupiter 44 HiRISE images 44 shuttle underbelly 44 Proba 2 44 JAXA 44 Mars Descent Imager 44 spacecraft Chandrayaan 1 43 impactor spacecraft 43 NASA satellites orbiting 43 Keck telescopes 43 robotic Canadarm 43 Hubble gyroscopes 43 THEMIS 43 manned missions 43 NFIRE satellite 43 bedrock outcropping 43 SMOS Soil Moisture 43 NFIRE 43 spacecraft thrusters 43 Lunar Excursion Module 43 LRO LCROSS 43 Earth orbit 43 CRISM 43 orbital velocity 43 orbiting complex 43 CryoSat mission 43 thruster 43 Deep Impact eXtended Investigation 43 Huygens descent 43 scramjet engines 43 director Charles Elachi 43 comets 43 detect gravitational waves 43 exo planets 43 SSMEs 43 FASTSAT 43 Jupiter flyby 43 Huygens Probe 43 YES2 43 NASA ICESat 43 electric ion propulsion 43 spokesman Paul Foerman 43 microgravity experiments 43 Foton M2 43 Mössbauer 43 celestial bodies 43 Valery Tokarev 43 manned lunar landing 43 comet flyby 43 martian rocks 43 GLONASS satellites 43 GeneSat 1 43 NEOSSat 43 Charles Elachi 43 Lunar Reconnaissance Orbiter Camera 43 ENose 43 Kepler Telescope 43 Gerhard Schwehm 43 NASA rover Spirit 43 Martian lander 43 orbital insertion 43 Martian polar 43 Bonneville Crater 43 irregularly shaped asteroid 43 INSAT 4CR 43 meteorological satellites 43 neutrino detectors 43 Thermal Emission Imaging 43 gamma ray detectors 43 Bowersox Pettit 43 Sensing Satellite LCROSS 43 Orion capsule 43 orbiting ISS 43 SETI @ home 43 Deep Impact mission 43 DEPTHX 43 Cassini flyby 43 geostationary satellites 43 optical telescopes 43 Fuk Li Mars 43 GALEX 43 Space Agency 43 Ken Edgett 43 unstowed 43 ringed planet 43 interstellar particles 43 duration spaceflights 43 Launch Abort System 43 Phobos 43 THEMIS spacecraft 43 twin Viking landers 43 Tempel 1 43 Galaxy Evolution 43 Itokawa 43 Earth asteroids 43 NASA rover Opportunity 43 geosynchronous orbit 43 onboard Chandrayaan 43 GOCE satellite 43 asteroids comets 43 C1XS 43 Astronaut Gregory Chamitoff 43 gradiometer 43 HiRISE image 43 amateur skywatchers 43 Shuttle astronauts 43 radioisotope thermoelectric generators 43 spacesuit 43 Evolution Explorer 43 Saturn moon 43 GNSS receivers 43 permanently shadowed crater 43 icy comets 43 unmanned lunar 43 NASA Cassini spacecraft 43 Spacecraft 43 moon Io 43 Hubble Space 43 SOHO spacecraft 43 Deep Impact comet 43 reentry capsule 43 Jules Verne ATV 43 gamma ray spectrometer 43 GLAST 43 retrorockets 43 Herschel Planck 43 planets orbiting 43 nanosatellites 43 Lander robotic arm 43 JWST 43 Tunguska asteroid 43 meteoroid 43 Itokawa asteroid 43 planet Gliese #g 43 Phobos Grunt 43 Cargo Launch Vehicle 43 gyroscopes 43 outermost planets 42 telescopes 42 rover Spirit landed 42 Messenger spacecraft 42 coronal loops 42 Chang'eI 42 ICESat 42 NASA MESSENGER spacecraft 42 Padalka 42 LightSail 1 42 Professor Pillinger 42 Elachi 42 Saturn Enceladus 42 Spacesuits 42 lunar exploration 42 manned lunar 42 space shuttle Discovery 42 Planetary Society 42 Shenzhou VI craft 42 Flyby spacecraft 42 regolith 42 asteroid Lutetia 42 Isidis Planitia 42 Epsilon Eridani 42 Foton M3 42 Hubble servicing 42 NASA LCROSS 42 Kibo JPM JEM Pressurized 42 Comet Tempel 1 42 BepiColombo mission 42 martian atmosphere 42 ESA GOCE 42 ISS crewmembers 42 rover Curiosity 42 Herschel Space Telescope 42 shadowed craters 42 DIXI 42 JPL rover 42 journal Meteoritics 42 Planetary Transits 42 tile fillers 42 Titan flybys 42 Kepler telescope 42 interplanetary 42 manned spacecraft 42 exoplanets orbiting 42 DubaiSat 1 42 lander Philae 42 Lunar CRater Observation 42 astrobiological 42 cosmos 42 Solar Orbiter 42 micrometeorite 42 STORRM 42 astronaut helper 42 meteoroids 42 Clementine spacecraft 42 sunlike star 42 Shenzhou VI 42 Desert RATS 42 Near Infrared Camera 42 comet nucleus 42 Polar Lander 42 Eyharts 42 William Borucki Kepler 42 axial tilt 42 space shuttle 42 Planck satellite 42 Galilean satellites 42 Mars Squyres 42 Relations Observatory STEREO 42 Ares IX 42 Wolszczan 42 Endurance Crater 42 Mars rover Curiosity 42 EPOCh 42 astrophysical phenomena 42 interstellar grains 42 DISR 42 microbial ecosystems 42 Laboratory Pasadena 42 Kuiper belt objects 42 NASA Lunar Prospector 42 northern arctic plains 42 GLAST spacecraft 42 Herschel Space 42 geosynchronous 42 NASA Stratospheric Observatory 42 Spacewatch 42 CDR SO 42 Ares IY 42 moons orbiting Saturn 42 NIRSpec 42 solar flares 42 HiRise camera 42 ultraviolet spectrograph 42 TanDEM X 42 DART spacecraft 42 Upsilon Andromedae b 42 shuttle Atlantis 42 innermost planet 42 spaceplane 42 Interstellar Boundary Explorer 42 Sensing Satellite 42 swingby 42 Astrosat 42 probe Chang'e 42 Higgs bosons 42 Mars northern latitudes 42 comet 42 Soyuz 42 NASA Cassini orbiter 42 ExoMars mission 42 Space ENvironment GEochemistry 42 THEMIS mission 42 ship thermal shielding 42 infrared spectrometers 42 Dragon capsule 42 lunar module Eagle 42 spectrometers 42 lunar orbit 42 WFC3 42 polar orbiting satellites 42 permanently shadowed craters 42 lunar outpost 42 Explorer WISE 42 orbiter robotic arm 42 Shuttle Atlantis blasts 42 shuttle Discovery 42 Alpha Particle 42 Viking Orbiter 42 LASP 42 ROCSAT 2 42 Jupiter moons 42 Russian Valery Tokarev 42 D CIXS 42 ornithopter 42 geostationary earth 42 situ measurements 42 NASA Deep Impact 41 orbiting Kepler telescope 41 Pluto Charon 41 periodic dimming 41 NASA COBE satellite 41 visit http:/saturn.jpl.nasa.gov 41 Galilean moons 41 Pancam 41 rover panoramic 41 CoRoT satellite 41 Resourcesat 2 41 lunar dust 41 Herschel telescope 41 Pluto Kuiper Belt 41 Extrasolar planets 41 moon Europa 41 orbiter Cassini 41 astronomical observations 41 Jodrell Bank 41 interplanetary voyage 41 planemos 41 lunar explorer 41 Jupiter Icy Moons Orbiter 41 High Resolution Imaging 41 ChemCam 41 Rock Abrasion Tool 41 Mars# 41 Minotaur rocket 41 AMASE 41 interstellar dust samples 41 Saturn moons 41 enters lunar orbit 41 Alexander Samokutyayev 41 Exploration Rover 41 robots 41 spaceflights 41 electromagnetic signals 41 occultations 41 shepherding spacecraft 41 experimental payloads 41 Skripochka 41 meter Keck telescopes 41 cosmic ray collisions 41 NEOWISE 41 Steven Squyres 41 Convection Rotation 41 Space Telescope Imaging Spectrograph 41 Canadian Julie Payette 41 NASA Cosmic Background 41 Atlantis undocks 41 penetrating radar 41 Observatory SDO 41 smallest exoplanet 41 extraterrestrial 41 hubble 41 crewed missions 41 Mission Control 41 navigate autonomously 41 Discovery STS 41 PharmaSat 41 MSL rover 41 Delta Cephei 41 exosphere 41 Tropical Rainfall Measuring 41 NROL 41 AERONET 41 MESSENGER MErcury Surface 41 Cosmic Origins Spectrograph 41 Krimigis 41 microgravity 41 Corot 7b 41 comets orbiting 41 Descent Imager Spectral Radiometer 41 giant gaseous planets 41 ALMA antennas 41 interstellar dust grains 41 Rover Spirit 41 Mercury flyby 41 simulated Martian 41 hypersonic speeds 41 subsurface ocean 41 geo stationary orbit 41 Mikhail Tyurin 41 Solid Rocket Boosters 41 Cancri e 41 copper fortified impactor 41 Particle accelerators 41 Russian cosmonaut Salizhan Sharipov 41 Samuel Oschin Telescope 41 moonlets 41 X ray spectrometer 41 STSS Demo 41 Honeybee Robotics 41 Microscopic Imager 41 Comet Halley 41 VASIMR engine 41 flyby 41 Gerstenmaier 41 XMM Newton 41 transiting planets 41 GSLV rocket 41 X #B 41 VIRTIS 41 ATV Johannes Kepler 41 substorms 41 unpiloted aircraft 41 impactors 41 gaseous planet 41 elongated orbits 41 planetary 41 planetary geologists 41 moons orbiting 41 ion propulsion engines 41 X #As 41 Cosmic Origins Spectrograph COS 41 synchronous orbit 41 comet chaser 41 faraway planets 41 Express orbiter 41 Spaceguard Survey 41 ESA ERS 41 David Southwood ESA 41 Earth orbiting spacecraft 41 comet Tempel 1 41 Ares IX rocket 41 Steve Squyres 41 Climate Sounder 41 Gliese #c 41 mere fly bys 41 lunar poles 41 Infrared Astronomy 41 Apollo lunar 41 NASA Gamma ray 41 infrared observatory 41 lidar instrument 41 exo planet 41 lunar module pilot 41 plucky rover 41 Fomalhaut b 41 robotic 41 Global Surveyor spacecraft 41 NextSat 41 planets orbiting distant 41 carry heavier payloads 41 NASA Lunar Crater 41 DMSP F# 41 Faint Object 41 gamma ray observatory 41 MARSIS instrument 41 Soyuz spaceship 41 micrometeorite impacts 41 observatory 41 Planck spacecraft 41 manager Jennifer Trosper 41 perched atop rover 41 colliders 41 mission Chandrayaan 41 Spacehab module 41 Soyuz rocket 41 clumpy soil 41 MetOp satellite 41 Crew Exploration 41 aboard Skylab 41 orbital mechanics 41 Lunar Module 41 earthlike planets 41 lunar soil 41 CitySense 41 Kounotori vehicle 41 Asteroid Itokawa 41 Characterization EPOCh 41 Stardust 41 Beta Pictoris b 40 orbiting Hubble telescope 40 Permanent Multipurpose Module 40 moon Phobos 40 CryoSat satellite 40 Martian arctic plain 40 Berinstain 40 thruster firings 40 orbit insertion 40 cosmic ray 40 Envisat satellites 40 Hubble Servicing 40 geo synchronous 40 FORCAST 40 Ariane launchers 40 Endeavour astronauts 40 Kepler 9c 40 Martian microbes exist 40 Ken Bowersox 40 shuttle missions 40 LightSail 40 Giove B 40 lunar crust 40 SCISAT 40 lunar orbiter Chang'e 40 Shuttle Endeavor 40 Hesperian epoch 40 NASA Landsat 40 Lunar Module Eagle 40 Shenzhou VI spacecraft 40 sunlike stars 40 commander Leroy Chiao 40 Jing Haipeng 40 Prof Pillinger 40 GSAT 5P 40 CMEs 40 teleoperated 40 Comet 9P/Tempel 1 40 manned Shenzhou 40 orbital maneuvers 40 mooncraft 40 David Wettergreen 40 Apophis 40 frequent woo hoos 40 unmanned rover 40 orbiting Cassini spacecraft 40 NLSI 40 Multi Object Spectrometer 40 Planetary Laboratory 40 Jennifer Trosper mission 40 NASA MESSENGER 40 Shepherding Spacecraft 40 Landsat Data 40 electro optical sensors