{"group":"make","pic":"living/pics/comms-city.webp","question":"How do you call home from another planet?","answer":"You write instead. Even at the speed of light a message takes minutes to cross, so families trade video letters, and a field of dishes beyond the town keeps the link open day and night.","play":{"label":"Write home from your living room","href":"play/"},"earth_mars":{"earth":{"head":"Home is a call away"},"mars":{"head":"Home is a star in the sky"},"pics":{"earth":"living/pics/em-comms-earth.webp","mars":"living/pics/em-comms-mars.webp"},"rows":[{"label":"Distance","icon":"globe","earth":"Anyone on the planet is a phone call away.","mars":"Earth is {cm_d_lo} to {cm_d_hi} million km away, depending on where the planets are."},{"label":"A reply","icon":"signal","earth":"Ask, and you hear back straight away.","mars":"The fastest reply takes {cm_rt_lo} to {cm_rt_hi} minutes, so people trade video letters."},{"label":"Finding your way","icon":"target","earth":"{cm_gps_n} GPS satellites put your phone within {cm_gps_m} m.","mars":"No GPS and no compass. Radio masts put you within {cm_fix_lo} to {cm_fix_hi} m."}]},"pin":[0.41,0.4],"reach":{"title":"Why the masts stand tall","line":"Mars curves away fast. Pick an antenna and see how far it can talk to a suit out on the plain.","pick":[{"id":"suit","name":"Another suit","h":"suit"},{"id":"rover","name":"A rover","h":"rover"},{"id":"mast","name":"A town mast","h":"mast"}],"range":"Talks to a suit up to {km} km away","you":"You","horizon":"Horizon"},"steps_title":"The line home","steps":[{"title":"Masts across the plain","text":"Masts {cm_mast} m tall carry the town's radio to every suit and rover, so nobody outside is ever out of touch."},{"title":"Glass underground","text":"Fibre in buried trenches carries everything between the town, the port and the dishes."},{"title":"Dishes aimed at home","text":"Big dishes, {cm_gs_lo} to {cm_gs_hi} km out of town, follow Earth across the sky day and night."},{"title":"A laser for big files","text":"A laser telescope sends films, letters and the day's news to Earth as pulses of light."}],"orbit":{"title":"How far is home today?","line":"Earth and Mars where they really are right now, drawn to scale.","sun":"Sun","earth":"Earth","mars":"Mars","now":"A message sent now arrives in {min} minutes","near":"Closest: {cm_lt_lo} min","far":"Farthest: {cm_lt_hi} min","text":"Light crosses the gap in {cm_lt_lo} to {cm_lt_hi} minutes, depending on where the two planets are in their orbits."},"masts":{"title":"Finding your way with three masts","line":"The masts send timed signals, like GPS satellites standing on the ground.","you":"You","text":"Each mast's signal says how far away you are. Hear three and there is only one place you can be: within {cm_fix_lo} to {cm_fix_hi} m."},"scene":{"pic":"living/pics/scene-comms.webp","look":"03-13","shot":"field_dusk","focus":"50% 50%","zoom":1.6,"spots":[{"at":[0.12,0.44]},{"at":[0.25,0.7]},{"at":[0.475,0.46]},{"at":[0.8,0.6]}]},"proof":{"pic":"living/people/comms-proof.webp","place":"Gale crater, Mars","when":"{cm_efm_date}","head":"The evening star is Earth","credit":"Photo: NASA/JPL-Caltech/MSSS/TAMU, taken by the Curiosity rover (public domain)"},"fact":"On {cm_efm_date}, {cm_efm_min} minutes after sunset, Curiosity photographed Earth from Mars: the brightest star in the sky, with the Moon just below it. Someone standing there could have seen both with their own eyes.","quiz":[{"q":"Why do the radio masts stand so tall?","a":["Mars curves away fast, so a tall mast sees further","To stay above the dust storms","To catch signals from Earth"],"why":"On a small planet the horizon is close. A {cm_mast} m mast reaches a suit about {cm_mast_km} km away; two suits on their own reach about {cm_suit_km} km.","more":"Ingenuity, the Mars helicopter, lost touch whenever it landed out of the rover's sight behind the ground: once for {cm_ing_days} days.","cite":"03-13 F-15, F-16, LC-03, F-17","from":"problem","fig":0,"icon":"signal"},{"q":"How does a suit know where it is with no GPS?","a":["Timed signals from three or more masts","A magnetic compass","The pole star"],"why":"The masts send timed signals like GPS satellites standing on the ground. Hearing three puts you within {cm_fix_lo} to {cm_fix_hi} m.","more":"Mars has no global magnetic field, so a compass is no use, and its sky has no pole star either.","cite":"03-13 LC-06, F-22, F-27","from":"why","fig":1,"icon":"target"},{"q":"How long does the fastest reply from Earth take?","a":["{cm_rt_lo} to {cm_rt_hi} minutes","A few seconds","About a day"],"why":"Light takes {cm_lt_lo} to {cm_lt_hi} minutes each way, depending on where the planets are in their orbits.","more":"On {cm_tweet}, astronaut Timothy Creamer sent the first live tweet from space.","cite":"02-01 F-39, 03-13 F-10","from":"why","fig":0,"icon":"signal"}],"notes":[{"beat":"fix","text":"Every suit, rover and robot carries a beacon that calls for help by itself if its wearer falls, every {cm_bcn} seconds for at least {cm_bcn_h} hours.","cite":"03-13 LC-10"}],"everyday":{"title":"Keeping in touch","items":[{"time":"07:30","head":"Letters overnight","text":"Three video letters from home arrived while you slept, waiting on the wall screen with breakfast.","tag":"{cm_lt_lo} to {cm_lt_hi} min each way","pic":"living/pics/comms-city.webp","pos":"25% 40%"},{"time":"13:15","head":"Comms check","text":"Before a walk outside the airlock calls a comms check, and every suit is on the radio from the moment the door opens.","tag":"Masts {cm_mast} m tall","pic":"living/pics/scene-comms.webp","pos":"12% 50%"},{"time":"20:45","head":"An evening star","text":"After sunset you step up to the viewing room and find home: Earth, the brightest star in the sky.","tag":"{cm_d_lo} to {cm_d_hi} million km","pic":"living/pics/em-comms-mars.webp","pos":"60% 20%"}]},"living":"Home is a bright evening star, and never far away. Video letters arrive every morning, the town's radio finds you anywhere on the plain, and a message to anyone on Earth is on its way the moment you press send.","from":"From the viewing room","later":{"pics":{"city":"living/pics/comms-city.webp","far":"living/pics/comms-far.webp"},"city":{"tag":"Dishes and masts","text":"A few big dishes and a laser station keep the link with Earth, and masts carry the town's radio across the plain."},"far":{"tag":"A field of dishes","text":"Rows of dishes and laser stations, and satellites overhead that tell you where you are anywhere on Mars."}}}