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(Click for frequent words.) 66 natural radioactive decay 63 radioisotope thermoelectric generator 61 Galactic cosmic rays 61 Voyager probes 61 X ray spectrometer 59 NASA THEMIS 59 planetary Transits 59 cosmic ray particles 59 neutron spectrometer 59 JIMO 58 orbiting observatories 58 gamma rays emitted 58 NASA Lunar Prospector 58 interplanetary spacecraft 58 Jupiter Icy Moons 58 ion propulsion engines 57 ray Space Telescope 57 Martian rovers 57 ion thrusters 57 ion thruster 57 Convection Rotation 57 plutonium dioxide 57 fission reactors 57 Solar TErrestrial RElations Observatory 57 tokamaks 57 radioisotope 56 IBEX spacecraft 56 thermoelectric generator 56 Heterodyne Instrument 56 superheavy element 56 orbiting telescopes 56 O OREOS 56 neutron detectors 56 storable propellants 56 Fermi LAT 55 particle physics experiments 55 gamma rays X rays 55 cryogenically cooled 55 photometer 55 NASA Stardust spacecraft 55 Climate Experiment SORCE 55 ESA XMM Newton 55 Alpha Particle 55 ESA Integral 55 uncharged particles 55 Super Kamiokande 55 nuclear fission reactors 55 Geminga 55 VHE gamma rays 55 WMAP satellite 55 Neutrons 54 Nasa spacecraft 54 Voyager spacecraft 54 interstellar dust samples 54 neutrino experiments 54 helium atom 54 laser excitation 54 gamma ray radiation 54 ion propulsion 54 X ray telescopes 54 neutron beams 54 thermonuclear fusion 54 APXS 54 moon Mab 54 antineutrinos 54 magnetic spectrometer 54 gamma ray telescope 54 X ray observatory 54 IKAROS 54 gamma ray spectrometer 54 Heliospheric Observatory SOHO 54 spatially resolved 54 Cherenkov 54 nuclides 54 Higgs bosons 54 gamma ray detectors 53 radiometers 53 parahydrogen 53 energetic electrons 53 Galileo spacecraft 53 Lunar Prospector 53 chemical reactants 53 detect gamma rays 53 inertial confinement fusion 53 NASA twin STEREO 53 fission reactor 53 spacecrafts 53 ROSAT 53 galactic cosmic rays 53 orbiting gamma ray 53 Cavity Ring Down 53 ESO VLT 53 radial velocity technique 53 Cometary 53 gamma ray photons 53 UV photons 53 Ray Spectrometer 53 emits alpha 53 NASA Far Ultraviolet 53 electric ion propulsion 53 coronal mass ejections CMEs 53 ionized gases 53 powered icebreaker Rossiya 53 Lunar Laser Ranging 53 Benthic Rover 53 SOLO TREC 53 absorb neutrons 53 NASA Cassini 53 spacecraft 53 jovian 53 neutron detector 53 Optical Spectroscopic 53 particle beam 53 Earth orbiting satellites 53 Ulysses spacecraft 53 VASIMR 53 infrared spectrometers 53 interstellar particles 52 cogeneration plants 52 highly elliptical orbits 52 laser altimeter 52 xenon propellant 52 photosynthetically 52 force microscopes AFMs 52 particle detectors 52 Spectrograph 52 Mars Pathfinder lander 52 Impactor 52 electron accelerators 52 Phobos Grunt 52 NASA MESSENGER spacecraft 52 hyperspectral imager 52 nuclide 52 Fermi telescope 52 exo planet 52 electron neutrinos 52 Neutron Detector 52 robotic landers 52 Magnetospheric 52 radioactive decay 52 thorium reactor 52 NASA ICESat 52 Solar Optical Telescope 52 neutrino detectors 52 Exoplanet 52 robotic explorer 52 unmanned probes 52 Molten Salt 52 Hinode spacecraft 52 interferometers 52 extrasolar planets orbiting 52 radio occultation 52 comets asteroids 52 NuSTAR 52 ionise 52 permanently shadowed craters 52 bipropellant 52 particle detector 52 CERN LHC 52 HESS telescopes 52 Yinghuo 1 52 Churyumov Gerasimenko 52 Nanoprobe 52 WISE spacecraft 51 Fermi Telescope 51 VASIMR engine 51 retroreflector 51 Hubble Space Telescope HST 51 icy comets 51 neutron radiation 51 Multi Object Spectrometer 51 twin Voyager spacecraft 51 Gamma ray 51 NASA Cosmic Background 51 SSMEs 51 Supernova #A 51 PWRs 51 reentry capsule 51 Cosmic Microwave Background 51 extrasolar 51 KamLAND 51 submarine Nautilus 51 Satellites GOES 51 cryogenic cooling 51 quantum cascade lasers 51 X ray Spectrometer 51 satellites orbiting Earth 51 hydrazine propellant 51 experimental payloads 51 Planetary Transits 51 muon neutrinos 51 Relations Observatory STEREO 51 NASA Gamma ray 51 ion batteries 51 gamma ray observatory 51 scramjet engines 51 Helioseismic 51 colliders 51 moons orbiting 51 propulsion 51 Thermal Emission Spectrometer 51 airglow 51 electron accelerator 51 bi propellant 51 X rays gamma rays 51 ultrahigh voltage 51 Cloud Aerosol Lidar 51 Cassini Huygens 51 Orbiter MRO 51 Gamma rays 51 robotic probes 51 radioactive nuclei 51 neutralinos 51 emits gamma rays 51 BepiColombo 51 Krimigis 51 lunar landers 51 millimeter wavelength 51 nanomotors 51 detect neutrinos 51 Solar Arrays 51 Stardust spacecraft 51 Solar Sail 51 ultracold temperatures 51 Mars moon Phobos 51 Epsilon Eridani 51 Laser Interferometer Gravitational Wave 51 ionised 51 controlled thermonuclear fusion 51 photochemical reactions 51 tritium radioactive 51 Viking landers 51 lunar meteorites 51 Perseus Cluster 51 Planck spacecraft 51 manned spaceships 50 exoplanets planets 50 strontium cesium 50 Philae lander 50 corona discharge 50 ultrashort pulses 50 deuterium tritium 50 spacecraft Chandrayaan 1 50 NASA Wilkinson Microwave 50 Particle accelerators 50 X ray pulses 50 NASA Fermi Gamma 50 photons emitted 50 imaging spectrometer 50 ESA Planck 50 Planck telescope 50 neutrino particles 50 piezoelectric crystal 50 ESA ERS 50 Martian lander 50 Mercury magnetosphere 50 Kepler Space Telescope 50 geoneutrinos 50 stellar occultations 50 spectroscopic analysis 50 Earth biosphere 50 thermoelectric cooling 50 Mars rovers Spirit 50 particles emitted 50 Huygens lander 50 tardigrades 50 situ measurements 50 alpha particles 50 terawatt 50 Fermi Gamma ray 50 Variable Specific 50 exoplanetary 50 lunar regolith 50 crewed spacecraft 50 Landsat satellites 50 SPIRE instrument 50 NASA Voyager spacecraft 50 mercury cadmium telluride 50 Observatory STEREO 50 Upsilon Andromedae 50 cryocoolers 50 Hubble Space telescope 50 Uninhabited Aerial Vehicle 50 Infrared Pathfinder Satellite 50 NASA Kepler telescope 50 muon neutrino 50 GJ #b [001] 50 scanning electron microscopes 50 Total Ozone Mapping 50 linearly polarized 50 photoelectron 50 maneuvering thrusters 50 New Horizons spacecraft 50 proton particles 50 Spectrometers 50 Bug Nebula 50 infrared telescopes 50 line HORIZONS ephemeris 50 photoelectrochemical 50 airgel collector 50 Lunokhod 50 outermost planet 50 silicon monoxide 50 emission spectrometer 50 Opportunity rovers 50 Planck satellite 50 tokamak 50 thorium oxide 50 coronal loops 50 TGFs 50 Geosynchronous 50 ST5 50 yttrium barium copper 50 molten salts 50 RAD# [002] 50 proton antiproton collisions 50 antineutrino 50 EPOCh 50 helium ions 50 strange quark 50 hydrogen nuclei 50 powered icebreakers 50 STEREO spacecraft 50 lunar equator 50 D CIXS 50 Mixed Oxide Fuel 50 Magnetic Imager HMI 50 batteries flywheels 50 extragalactic 50 cosmic phenomena 50 solar neutrinos 50 THEMIS spacecraft 50 ESA Cluster 50 Carina constellation 50 NEOSSat 50 CCGTs 50 THEMIS probes 50 liquid oxygen LOX 50 Curiosity rover 50 cosmological distances 49 Rosetta orbiter 49 isotope molybdenum 49 noble gases 49 Foton M3 49 renewably generated 49 CMB radiation 49 helium atoms 49 OGLE TR #b 49 Radioisotope 49 Westinghouse pressurized 49 multiwavelength 49 magnetic resonance NMR spectroscopy 49 IRC +# 49 MARSIS radar 49 proton proton collisions 49 XMM Newton 49 subsurface ocean 49 Hydrokinetic 49 supergiant star 49 Martian dust 49 Foton M2 49 Mars orbiter 49 tau neutrinos 49 NIRCam 49 cadmium zinc telluride 49 nickel cadmium battery 49 cosmic radiation 49 krypton 49 synchrotron radiation 49 auroral emissions 49 Henri Becquerel 49 Superconductors 49 Burst Alert 49 Diviner instrument 49 docking compartment 49 HIAPER 49 photon beam 49 hydroxyl molecules 49 fission reactions 49 Cosmic rays 49 Extrasolar Planet 49 MINOS experiment 49 AERONET 49 Soyuz booster 49 oxidiser 49 terahertz rays 49 inelastic scattering 49 Mars orbiting spacecraft 49 Jupiter Icy Moons Orbiter 49 magnetically levitated trains 49 Rosetta comet chaser 49 mimics photosynthesis 49 SCIAMACHY 49 Kuiper belt object 49 Nasa Mars Odyssey 49 lunar craters 49 cosmic neutrinos 49 BaBar experiment 49 chlorophyll fluorescence 49 proton decay 49 Magnetically 49 spectral measurements 49 photons particles 49 pulsed plasma 49 Russian Elektron oxygen 49 High Flux Isotope 49 americium 49 Io Europa 49 Dynamics Observatory SDO 49 Higgs Boson particle 49 Imaging Spectrometer 49 -# º C [002] 49 UVOT 49 particle acceleration 49 VLTI 49 CAIs 49 rocket thrusters 49 radiometer 49 millimeter wavelengths 49 Atmospheric Infrared Sounder 49 electrons protons 49 Exoplanets 49 microshutters 49 emits photons 49 THEMIS mission 49 SMOS Soil Moisture 49 Anthony Remijan 49 scramjet engine 49 Ocean Salinity 49 LENR 49 cosmic gamma rays 49 lowest levelized 49 RHESSI 49 microgravity environment 49 starquakes 49 Mars Odyssey spacecraft 49 NFIRE 49 Radiation Belt 49 Orbital Debris 49 axions 49 intrinsic brightness 49 RTGs 49 ultracold molecules 49 Deuterium 49 ramjets 49 ENVISAT 49 Cherenkov radiation 49 orbiting observatory 49 californium 49 Neutron scattering 49 exo planets 49 NASA STEREO 49 ions electrically charged 49 Magellanic Stream 49 THz radiation 49 RXTE 49 hypersonic missiles 49 Hubble gyroscopes 49 EPOXI mission 49 colliding protons 49 Upsilon Andromedae b 49 Cassini flybys 49 Hydro Cluster 49 Envisat satellite 49 SIM PlanetQuest 49 Main Injector Neutrino 49 Faint Object 49 ExoMars rover 49 Foton spacecraft 49 orbital rendezvous 49 Extrasolar Planet Observation 48 sodium atoms 48 superlaser 48 SQUIDs 48 particle accelerators 48 W bosons 48 particle colliders 48 moon Enceladus 48 X ray observatories 48 Spectroscopic 48 terahertz waves 48 SF6 48 scramjets 48 distant supernovae 48 ADM Aeolus 48 quantum cascade laser 48 polaritons 48 infall 48 piezoelectric crystals 48 Nasa Mars Reconnaissance 48 laser spectroscopy 48 Matritech proprietary 48 Viking Lander 48 fissionable 48 Samuel Oschin Telescope 48 tabletop fusion 48 sized planets orbiting 48 laser irradiation 48 positron emitting 48 superconducting cables 48 Nuclear fission 48 smallest exoplanet 48 hydrogen maser 48 comets orbiting 48 Mini Thermal Emission 48 radioactive nuclides 48 infrared telescope 48 afterglows 48 Kuiper Belt Objects 48 MeV 48 tunable diode laser 48 Gas Chromatograph Mass Spectrometer 48 infrared photons 48 interferometric technique 48 THEMIS satellites 48 Jupiter Ganymede 48 Ytterbium 48 hypersonic speeds 48 scintillator 48 Fregat upper stage 48 Gamma ray bursts GRBs 48 Planck observatory 48 Evolution Explorer 48 CoRoT satellite 48 NASA LCROSS 48 unmanned Progress 48 Apollo missions 48 ITER fusion 48 Mars landers 48 Keck telescopes 48 spectral fingerprints 48 Titan smoggy 48 EDXRF 48 Hot Jupiters 48 unpressurized cargo 48 Alfven waves 48 Supercapacitors 48 spectroscopic measurements 48 alpha particle 48 superconducting magnetic 48 Moon Mineralogy Mapper M3 48 Infrared Astronomical Satellite IRAS 48 CPR# 48 Magnetosphere 48 Cosmic Microwave Background CMB 48 -# degrees centigrade [001] 48 Kepler spacecraft 48 scanning tunneling microscopes 48 millisecond pulsar 48 spontaneous fission 48 uranium deuteride 48 SpaceX Dragon 48 planets orbiting distant 48 blazars 48 NASA Stratospheric Observatory 48 combustors 48 alpha emitters 48 Deep Impact Extended 48 Ionosphere 48 Mercury exosphere 48 sulfate aerosols 48 Lunar Rover 48 spacecraft orbited 48 shuttle aft compartment 48 Synthetic Aperture Radar 48 distant supernovas 48 gravitationally lensed 48 Transmission Electron Microscope 48 geo synchronous 48 Near Infrared Camera 48 electromotive force 48 sunlike stars 48 CoRoT 9b 48 rubidium vapor 48 moons orbiting Saturn 48 radioactive tracers 48 Alfvén waves 48 cometary material 48 supercooled liquid 48 polar orbiting 48 fusion fission 48 cosmic ray collisions 48 NIF laser 48 Mars Atmosphere 48 Infrared Astronomy SOFIA 48 polarimetry 48 Hubble Telescope 48 backscattered 48 high voltage capacitors 48 emit X rays 48 Cancri e 48 piezoelectric transducer 48 3He 48 Force Microscopy 48 earth orbiting satellites 48 infrared emitters 48 GRACE satellites 48 SMOS 48 Launch Scrubbed 48 absorption chillers 48 Mars Pathfinder 48 Ranging spacecraft 48 atmospheric neutrinos 48 orbited Jupiter 48 ionization mass spectrometry 48 solid propellants 48 Miniature Thermal Emission 48 meteorological satellites 48 cryostat 48 Spectroradiometer 48 visit http:/saturn.jpl.nasa.gov 48 comet particles 48 Fermilab accelerator 48 propellant tanks 48 MSL rover 48 synchrotron X ray 48 cryo electron microscope 48 quantum dot lasers 48 Orbiter LRO 48 spectral fingerprint 48 Shenzhou spacecraft 48 NPO Lavochkin 48 NFIRE satellite 48 Interplanetary Kite craft 48 Hayabusa spacecraft 48 microwatts 48 fission fusion 48 orbiter lander 48 Diviner 48 Photons 48 NextEra Energy collectively 48 planet HD #b 48 superheavy elements 48 Solaren 48 superfluid helium 48 SIRTF 48 Magnetars 48 BLG #Lb 48 magnetars 48 ramjet 48 Titan ETEM 48 robotic spacecraft 48 chlorophyll molecule 48 thermoelectric generators 48 enters lunar orbit 48 NASA Solar Terrestrial 48 extraterrestrial civilizations 48 exosphere 48 pulsed laser 48 Earth Observation satellites 48 Boiling Water Reactors 48 nightglow 48 robotic rovers 48 orbiting Hubble telescope 48 compressed nitrogen 48 moons Phobos 48 cosmogenic 48 planetary missions 48 cryocooler 48 celestial observations 48 cosmic particles 48 Saturnian moon 48 Mössbauer 48 synthetic aperture 48 QCLs 48 Pressurised Heavy Water Reactors 48 #.#kWh 48 nanostructured solar 48 DIXI 48 GJ #b [002] 48 metallic nanostructures 48 magnetic resonance spectrometers 48 Fermilab Tevatron 47 voltaic 47 hydrogen isotopes 47 shadowed craters 47 Cosmic Background Explorer COBE 47 submillimetre 47 robotic lander 47 martian atmosphere 47 GALEX 47 gravitational slingshot 47 protoplanetary 47 AB Aurigae 47 type Ia supernova 47 interstellar clouds 47 SpaceWire 47 Pebble Bed 47 gravitational tides 47 polar orbiting satellites 47 ionized particles 47 rovers Spirit 47 uranium dioxide 47 ChemCam 47 electron positron 47 spectroscope 47 COROT 47 plasma torches 47 quantum oscillations 47 ITER reactor 47 cosmic ray detectors 47 Mossbauer 47 neutron beam 47 micro meteorites 47 Piezoelectric 47 Suzaku X ray 47 comet Churyumov Gerasimenko 47 HFIR 47 Seaswarm 47 Tropospheric Emission Spectrometer 47 Z Backscatter X ray 47 spaceborne 47 masers 47 Saturn outermost 47 spectrographic 47 Chlamydomonas reinhardtii 47 orbiter Cassini 47 Atacama Large Millimeter submillimeter 47 Chemical Oxygen Iodine 47 cryogenic engines 47 zirconium alloy 47 cosmic ray 47 Manned missions 47 photosynthesising 47 dexterous robot 47 Beamline #.#.# 47 toxic hydrazine 47 rooftop photovoltaic panels 47 hydrogen isotopes deuterium 47 geo stationary orbit 47 Huygens descent 47 Turanor 47 conduction convection 47 hyperpolarization 47 ultracold atoms 47 Saturn Enceladus 47 liquid propellant 47 Great Lakes steamships 47 Permanent Multipurpose Module PMM 47 Neutral Mass Spectrometer 47 #P/Churyumov-Gerasimenko 47 DEIMOS 47 exhaust plume 47 Lithium Polymer 47 radial velocity method 47 GOCE satellite 47 Mars Lander 47 exoplanets orbiting 47 laser desorption 47 Transfer System SSPTS 47 attosecond pulses 47 planet magnetosphere 47 ultrafast lasers 47 Spacecraft 47 radar altimeter 47 LightSail 1 47 breathable oxygen 47 Array ALMA 47 AlumiFuel 47 rubidium clocks 47 Mars Orbiter 47 Shippingport Pa. 47 pyroelectric 47 neutron flux 47 NASA Cassini orbiter 47 Thorium Power OTCBB THPW 47 SSM# 47 rechargeable lithium sulfur 47 metal hydride canisters 47 particle smasher 47 SWIR cameras 47 SPICAM 47 Composite Infrared Spectrometer 47 MESSENGER MErcury Surface 47 SMBHs 47 Advanced Microwave Scanning 47 circularly polarized 47 #W light bulb 47 Gamma ray burst 47 Solid Oxide Fuel Cells 47 supersymmetric particles 47 gamma ray emission 47 VIRTIS 47 neutral mass spectrometer 47 ionisation 47 Kola NPP 47 neutrino beam 47 spacecraft Cassini 47 Proba 2 47 Lithium ions 47 PureCycle R 47 xenon 47 UVIS instrument 47 Seebeck effect 47 Comet Halley 47 astrophysical phenomena 47 Planetary Fourier Spectrometer PFS 47 CHIKYU 47 lunar orbiters 47 Observatory SDO 47 supersonic combustion 47 nanosatellites 47 ammonia coolant tank 47 reactor coolant pumps 47 Synchrotron radiation 47 NASA satellites orbiting 47 hydrogen H2 47 electron beam accelerator 47 LightSail 47 ionizes 47 Astro E2 47 NASAs 47 ultraviolet imaging spectrograph 47 UUV 47 xenon ion 47 cosmic rays 47 Moderate Resolution Imaging 47 Venusian atmosphere 47 X ray telescope 47 primitive meteorites 47 Eurostar E# platform 47 Cat Paw Nebula 47 Descent Imager Spectral Radiometer 47 atmospheric aerosols 47 Reflection Radiometer 47 galaxy M# [001] 47 Fast Reactor 47 Borexino 47 constellation Eridanus 47 methyl cyanide 47 Nova NanoSEM 47 technetium #m 47 Aqua satellite 47 laser optics 47 QWIP 47 antimatter particles 47 Antenna LISA 47 HD# [002] 47 KAGUYA 47 electrolysers 47 Interferometer 47 breeder reactors 47 transuranic elements 47 Organic Rankine Cycle 47 stabilizing gyroscopes 47 microquasar 47 Ultracold 47 coronal holes 47 gluon 47 starboard SARJ 47 spectroscopic technique 47 comet nuclei 47 Minotaur rocket 47 marketable byproducts 47 hematite mineral 47 Kepler Mission 47 ALTI ESS 47 planets circling 47 NASA Spacecraft 47 Mitochondrial DNA testing 47 BepiColombo joint 47 Beta Pictoris 47 Anisotropy Probe WMAP 47 Comet Hartley 47 FNPP 47 UV Visible 47 distant quasar 47 Area Telescope LAT 47 photoelectron spectroscopy 47 Telescope Array 47 TerraSAR X 47 icy moons 47 Saft Li ion 47 WISE telescope 47 vaporisation 47 ESO telescopes 47 reconnaissance orbiter 47 SMOS mission 47 RBSP 47 transiting exoplanet 47 mineralogical composition 47 OCO satellite 47 hypernova 47 solar radii 47 pressurized module 47 blackholes 47 Astronomers Find 47 ionized plasma 47 Space ENvironment GEochemistry 47 multipartite entanglement 47 Mastcam 47 neutrons emitted 47 Neutrinos 47 Saturn magnetosphere 47 Comet Hartley 2 47 Far Ultraviolet Spectroscopic Explorer 47 electromagnetic signals 47 Ocean Salinity SMOS 47 Rockot launcher 47 Wilkinson Microwave Probe 46 imaging spectroscopy 46 gamma rays 46 infrared ultraviolet 46 HySTAT 46 Keck Interferometer 46 deorbited 46 Analyzer TEGA 46 Rubidium 46 helium neon 46 PSLV C# rocket 46 plutonium extracted 46 parabolic trough collectors 46 kilo watts 46 multiband imaging photometer 46 particle physics detector 46 grid V2G 46 AGNs 46 ultraviolet photons 46 MESSENGER spacecraft 46 positron 46 spectrographs 46 cosmic cannibal 46 Deep Impact eXtended Investigation 46 Kepler 9c 46 Earth magnetotail 46 isotopic ratios 46 SSPTS 46 Comet #P/Churyumov-Gerasimenko 46 coronagraph 46 ESA Envisat 46 Characterization EPOCh 46 molten salt storage 46 tomography PET 46 nuclear fission 46 atomic 46 Comet 9P/Tempel 1 46 interferometric 46 -# ° Fahrenheit 46 axial tilt 46 electronvolts 46 GE CF6 engines 46 ion microscope 46 lanthanides 46 Ion propulsion 46 virtually inexhaustible 46 moon Io 46 protostar 46 Type Ia supernovas 46 TDRS K 46 NASA MESSENGER 46 Suntech solar panels 46 Scramjet 46 volcanic moon Io 46 Space Capsule 46 Glenn Seaborg 46 Commissariat à l' Énergie 46 electro optic modulator 46 infrared observatory 46 deorbiting 46 discover Fomalhaut 46 Carbon Dioxide ERC 46 millisecond pulsars 46 VVER reactors 46 #-# GWe 46 Supernovae 46 surface plasmon 46 MG J# +# 46 Underwater Vehicle 46 spectro 46 frictional heating 46 Pressurized Heavy Water Reactors 46 graphite whiskers 46 Climate Experiment GRACE 46 ALTB 46 Huygens Probe 46 aboard ESA Envisat 46 NASA Kepler spacecraft 46 NASA rovers 46 Pegasus XL 46 gravitational radiation 46 BP Piscium 46 Spirit Mars rover 46 manned lunar 46 proton proton 46 Stimulated Emission 46 Multi Object Spectrograph 46 sub picosecond 46 polymer electrolyte membrane 46 Neutron Spectrometer 46 LRO spacecraft 46 femtosecond pulses 46 fluorometer 46 protoplanetary discs 46 energetic gamma rays 46 vibrational modes 46 nonradioactive 46 Keck telescope 46 Impact Probe MIP 46 antihelium 4