British planetary scientist Colin Pillinger, best known for his 2003 attempt to land a spacecraft on Mars, has died aged 70, his family have said.
Prof Pillinger was at his home in Cambridge when he suffered a brain haemorrhage and fell into a deep coma.
His family said he later died at Addenbrooke's Hospital without regaining consciousness.
His death was "devastating and unbelievable", they said in a statement.
Dr David Parker, the chief executive of the UK Space Agency, led the tributes.
He told the BBC that Prof Pillinger had played a critical role in raising the profile of the British space programme and had inspired "young people to dream big dreams".
The Science Minister David Willetts called him a "delightful man and a free spirit". And added: "His vision of space exploration and his dedication to it inspired the nation."
And Prof Mark Sims, the mission manager on the 2003 Beagle-2 probe, recalled: "Colin was a top-rate scientist. You might not have agreed with him but he always went for what he believed in. It was a privilege to have known him and worked with him, both as a friend and colleague."
Prof Pillinger was the driving force behind Beagle-2, which was built to search for life on Mars.
The little craft was carried piggyback to the Red Planet on a European satellite, but vanished without trace after being dropped off to make its landing.
Prof Pillinger continued to push space agencies to complete what he called "unfinished business on Mars", and was sometimes critical of the delays that have seen Europe's follow-up rover mission, ExoMars, slip back to 2018.
Fans took to Twitter on Thursday to pay tribute to the scientist, with author Keith Mansfield calling him a "great advocate" for space and Mars.
Phil Ford, a writer on Dr Who, said: "Very sad to see Prof Colin Pillinger has died. A proper British boffin who will be fondly remembered for the Beagle Mars mission."
At the age of 62, Prof Pillinger was diagnosed with multiple sclerosis, which made it difficult for him to walk.
He said the illness would not diminish his research, and his motorised buggy was often seen racing around scientific conferences.
"Bloody-minded," was how he described his own approach to life. "If I ever said as a child 'I can't do this', my father would always say, 'There's no such thing as can't'," he recalled on the BBC's Desert Island Discs programme.
With colleagues at the Open University, where he headed the Department of Physical Sciences until 2005, he was keenly looking forward to this year's Rosetta mission.
The pan-European venture plans to put a lander on a comet this November, and an OU instrument will help investigate the object's chemistry.
"It's important to note that Colin's contribution to planetary science goes back to working on Moon samples from Apollo, as well as his work on meteorites," said Dr Parker.
"While we still don't know for certain what happened to Beagle-2, I'd say that the project was a turning point in bringing together the space science and industrial communities in the UK - which didn't used to speak with one voice. Beagle-2 wasn't built in Colin's backyard: it was the product of UK brains and hard-work in many companies and universities."
For the British media, Prof Pillinger was often the go-to man for a comment when a new piece of space science was published.
The press appreciated his straight-talking, and the whiskers and the Bristolian accent just added to his appeal.
He had an especially sharp eye for a good headline, once demonstrating the relatively small scale of Beagle-2 by loading a replica into a supermarket trolley and wheeling it through the car park of the Open University. The footage was picked up by the satirical programme Have I Got News for You? ensuring that news of the mission reached a far wider audience.
On the publication of his biography in 2010, My Life On Mars, he recalled an event that made him realise that the lost probe would be his legacy.
"I pulled into the OU car park and there was this huge lorry, a guy delivering a load of bricks - a builder, obviously," he told the BBC.
"I looked at this guy and I thought 'he's going to take a while', so I dashed in front of him in my car to get into the parking space. Well, the door opened on the lorry and this huge man got out - you could eat your dinner off his hands - and he started walking towards the car. And I thought, 'Bloody hell, I'm going to get thumped'.
"Well, he stuffed this huge paw through the window and said, 'You're the man who launched Beagle-2, aren't you? I want to shake your hand, mate'. And that to me says everything. There's nobody in the UK I didn't reach."
Prof John Zarnecki, a colleague at the OU, remembered Colin Pillinger as "a boss, a friend, a rival, a confidant, a fellow football fan and many more".
"Working with Colin was never easy or quiet! But our aims were the same - to do the best science that we possibly could. And with Colin, woe betide anybody or any organisation who got in the way of that objective," he told me.
"Life was never dull - he never fired me (as most of my colleagues claim to have been) but I do remember a particularly fiery meeting at which he accused me of being a traitor (to what I'm not sure I remember).
"I can think of nobody else who could have made Beagle-2 happen - he was so passionate, determined and thick-skinned.
"He refused to work 'by the rules'. Although it was a 'long shot', it definitely could have worked - and I committed myself to the extent of providing an instrument for Beagle-2. The Christmas Day we spent trying to make contact with Beagle-2 was so painful - and so much so for Colin who had invested his very soul - and more - into that effort. He bore it with great dignity.
"Our community will be that much poorer without Colin."
Prof Pillinger was married to Judith with whom he had two children, Shusanah and Nicolas.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos