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One of Earth's closest neighbours, Mars, is still at least 56 million km away - so what would it take to put a human on the Red Planet? The BBC asked scientists from Imperial College London to design a mission which could land crew on Mars and get them back safely.

They came up with a spinning spacecraft to create artificial gravity on the nine-month trip, combating bone loss and muscle wastage. A shield would be needed to protect against radiation from the Sun and cosmic rays. A robot sent ahead of the mission would generate fuel from the Martian ice for the return journey.

Here is an edited version of the session which was held on Wednesday 24 July 2013.

Question from @mobzi_malone: What's the fastest a spacecraft can travel with people in? Is 9 months the shortest journey time? #BBCMars

Martin answers: 9 months is the time it takes if you let gravity do most of the work so you don't need as much fuel #BBCMars

Question from @SJSumner: How fast (rpm) would the tethered landing and habitat modules be rotating to create an earthlike gravity? #BBCMars

Martin answers: Because of our 60m tether we only need a 3rpm (20s rotation period) to simulate Earth gravity. #BBCMars

Question from Jav Hidalgo via email: I was just wondering if you thought that a crew of three is enough? #BBCMars

Martin answers: 3 people is based on the ISS expeditions, though there are 2 at a time. With more people you need more resources on the way! #BBCMars

Question from @A_M_Swallow: How are you going to power the ground installations? Nuclear? #BBCMars

Martin answers: We've decided to simply use giant solar panel arrays. Nuclear is not ideal for manned missions. #BBCMars

Question from Brian via email: Where is the energy to electrolyse the water for the return trip fuel going to come from? #BBCMars

Martin answers: We make methane rocket fuel by hydrolysing water and combining with CO2 in Martian atmosphere, this would be solar powered #BBCMars

Question from @B_R_Moss: How much thicker would the Martian atmosphere have to be for astronauts to be able be outside w/out pressure suits? #BBCMars

Martin answers: Mars's atmosphere would need to be 100x thicker to match Earth's, but it certainly wouldn't be nice and breathable, too much CO2 #BBCMars

Question from @Seabird_2: Do you think that the first settlers to Mars will become "aliens" to Earth and will not be able to return home?

Martin answers: Mars isn't very hospitable to humans at the moment, so I don't see people settling there long term just yet. #BBCMars

Question from @jamesrichwalls: Isn't radiation and the effects of gravity on the body a huge obstacle for any long term human Mars endeavour? #BBCMars

Martin answers: You can simulate gravity through spinning. Radiation is still a slight worry, especially solar storms. It'd be a risky trip! #BBCMars

Question from @flypie: What about growing food on the way there and on the surface? #BBCMars

Martin answers: People have tried to mock this up in remote locations on Earth in bio-domes. A great way of producing oxygen too #BBCMars

Question from @bobcrawf: How difficult is entry into the Mars atmosphere attempting a landing compared to Earth re-entry? #BBCMars

Martin answers: Landing on Mars is a lot more difficult than on Earth or even the Moon. It has a thin atmosphere but dust storms too. #BBCMars

Question from: @jamesrichwalls: Have the psychological factors, attributed with long term space travel, been fully addressed? #BBCMars

Martin answers: The Russians did a mock Mars mission on the ground to see about this, but you can't fake the real psychological effects. #BBCMars

Question from @simon_rp84: Do you think that existing launch vehicles are suitable, or will new concepts need to be developed for Mars? #BBCMars

Martin answers: Current rockets, particularly say SpaceX's Falcon series, have the capabilities of sending fairly large craft to Mars. #BBCMars

Question from @tfprophet: Realistically, what are the prospects of ever being able to 'terraform' planets, particularly Mars? #BBCMars

Question from @ddetisi: Could we ever influence or improve the atmosphere to make it habitable? Even over a long period of time? #BBCMars

Martin answers: People have thought about terraforming Mars and making it more habitable. I don't think we really have the technology yet. #BBCMars

Question from @adrianstrand: Should humans hone tools & techniques on the Moon first for as long as a Mars mission - as a stepping stone? #BBCMars

Martin answers: People have thought about using the Moon as a fuelling station for longer trips. But it is quite a different place to Mars. #BBCMars

Question from @NickF75: In case of space craft problems, has any thought gone into sending a spare craft to await in Mars orbit? #BBCMars

Martin answers: Our concept mission actually sends robotic craft beforehand to monitor the planet and start making fuel for the trip home. #BBCMars

Question from @Gturv: How do you cope with unexpected medical emergencies on long space flights? Appendix etc? #BBCMars

Martin answers: Astronauts would have health monitors on at all times and would have some medical training. Experts on Earth could help too. #BBCEarth

Question from @pault14761: What's the minimum time you'd have to stay on Mars, to be able to have a nine-month journey back to Earth? #BBCMars

Martin answers: You'd need to wait 3 months for Earth to be in the right place relative to Mars to return. Otherwise it'd be an 18 month wait. #BBCMars

Question from @tikiteyboo: Could a small earth-like magnetic field be used to protect the crew from cosmic rays? #BBCMars

Martin answers: A magnetic field would be good for protecting against solar particles, some cosmic rays could probably get through like on Earth #BBCMars

Question from @asteven5: When do you think the first human might actually walk on the surface of Mars? #BBCMars

Martin answers: It would take probably 20 years or so just to build our concept mission and get to launch #BBCMars

Question from @RupertM4 : Will we ever go zero gravity skiing on the north pole of mars? #BBCMars

Martin answers: Mars's gravity is about a third of the Earth's so you could potentially go skiing but it'd take longer to build up speed #BBCMars

Twitter Q&A produced by Jeremy Gahagan

## More on this story

• How to put a human on Mars

• How to put a human on Mars: Artificial gravity