A billionaire’s private SpaceX flight has filled its two remaining seats with a science teacher and a data engineer whose college friend won the spot but gave him the prize.

The new passengers are Sian Proctor, a community college educator in Tempe, Arizona, and Chris Sembroski, a former Air Force missileman from Everett, Washington.

They will join flight sponsor Jared Isaacman and another passenger for three days in orbit this autumn.

Mr Isaacman also revealed some details about his Inspiration4 mission, as the four gathered on Tuesday at Nasa’s Kennedy Space Centre.

He is head of Shift4 Payments, a credit card-processing company in Pennsylvania, and is paying for what would be SpaceX’s first private flight while raising money for St Jude Children’s Research Hospital in Memphis.

Their SpaceX Dragon capsule will launch no earlier than mid-September, aiming for an altitude of 335 miles. That is 75 miles higher than the International Space Station and on a level with the Hubble Space Telescope.

Mr Isaacman, 38, a pilot who will serve as spacecraft commander, will not say how much he’s paying. He is donating 100 million dollars (£73 million) to St Jude, while donors so far have contributed 13 million dollars (£9.5 million), primarily through the lottery that offered a chance to fly in space.

Hayley Arceneaux, 29, was named to the crew a month ago. The St Jude physician’s assistant was treated there as a child for bone cancer.

That left two capsule seats open. Ms Proctor, 51, beat 200 businesses for a seat reserved for a customer of Mr Isaacman’s company. An independent panel of judges chose her space art website dubbed Space2inspire.

“It was like when Harry Potter found out he was a wizard, a little bit of shock and awe,” she said last week. “It’s like, ‘I’m the winner?’”

Mr Sembroski, 41, donated and entered the lottery but was not picked in the random drawing earlier this month — his friend was. His friend declined to fly for personal reasons and offered the spot to Mr Sembroski, who worked as a Space Camp counsellor in college and volunteered for space advocacy groups.

“Just finding out that I’m going to space was an incredible, strange, surreal event,” he said.

Ms Proctor, who studied geology, applied three times to Nasa’s astronaut corps, coming close in 2009, and took part in simulated Mars missions in Hawaii.

She was born in Guam, where her late father worked at Nasa’s tracking station for the Apollo moonshots, including Neil Armstrong and Buzz Aldrin’s.

She plans to teach from space and create art up there too.

“To me, everything that I’ve done… has brought me to this moment.”