Fare-paying "space tourist" Gregory Olsen has blasted off on a Russian Soyuz rocket bound for the International Space Station.
Mr Olsen says he is neither tourist nor astronaut
His rocket streaked into the clear blue sky at Kazakhstan's Baikonur launch site at 0955 local time (0455BST).
The US businessman and scientist is taking a 10-day trip to the ISS. He is the third person to holiday there.
The ticket price was not disclosed, but it is believed the electronic sensors expert paid up to £11m.
Two other people have so far taken a holiday on the station: fellow American Dennis Tito in 2001, and South African Mark Shuttleworth in 2002.
"I'll be most relaxed and happy after the rocket takes off," Mr Olsen said at Kazakhstan's Baikonur cosmodrome ahead of the launch.
"Nowadays, everybody can fly every week. The same will be true with spaceflight - there will be more tourists after me," he was quoted as saying by the Associated Press.
He was joined in his Soyuz capsule by Commander William McArthur from the US, and Russian Flight Engineer Valery Tokarev, who make up the 12th International Space Station crew.
The rocket entered orbit nine minutes after take-off, having shed three booster segments on schedule.
They are heading to the orbiting outpost to relieve Expedition 11, Sergei Krikalev and John Phillips, who have been in space since April.
Krikalev and Phillips will return to Earth along with Mr Olsen on 11 October.
Mr Olsen, 60, is the chief executive of New Jersey-based research firm Sensors Unlimited.
The company develops and produces high-sensitivity film and photo cameras and works with the US space agency, Nasa.
Apart from testing new Sensors Unlimited equipment in orbit, Mr Olsen plans to take a number of self-designed experiments. These will involve growing crystals on board the ISS and may have applications in his imaging business.
His trip, therefore, will be something of a working holiday - which is one reason why he prefers the term "space flight participant" to space tourist.
Tokarev labelled Mr Olsen as "scientific investigator of the International Space Station."
"The term 'tourist' doesn't do justice to all the work I've put in, or the work that the people at the Gagarin centre (outside Moscow) put in preparing us," Mr Olsen said.
However, he said: "I will not participate pretending that I'm an astronaut or cosmonaut. There is so much knowledge needed to operate this vehicle."
McArthur, 54, a retired US Army colonel, is a veteran of three space shuttle flights, including one to the ISS and one to the Russian space station Mir.
Tokarev, 52, a colonel in the Russian Air Force, has made one previous spaceflight, to the ISS aboard a space shuttle.
During their stay, Expedition 12 will do two or three spacewalks, to install equipment and carry out maintenance.
And alongside nearly four dozen scientific experiments to be conducted on the station, the two men will do some commercial filming for a Japanese TV advert about instant noodles.