In 30 years as a broadcaster, I've interviewed many of the great men and women of jazz. But I've only met one angel. His name was Sun Ra, he was born 100 years ago, and he came from Saturn.

Ra arrived at the Jazz FM studios, where I was a DJ, just after midnight one night in June 1990. Aged 76, he was dressed in gorgeous flowing robes and an impeccably coiffed fur hat. His beard was dyed bright orange, and he wore a star-speckled chiffon scarf and a heavy gold neck chain. My first question was going to be: "Sun Ra, you're a man who has made hundreds of records and travelled the planet …" But I didn't get past the word "man" before he interrupted me. "I'm not a man – I'm an angel." It was clear this would be no ordinary interview.

Sun Ra was born Herman Blount in 1914 in Birmingham, Alabama – at that time perhaps the most segregated city in the world. A fine student, he devoured books and could sight-read music from an early age. Health problems added to his sense of isolation, and his piano became a loyal friend. Defying the second world war draft, he became one of the first black conscientious objectors and served time in prison.

When free, he headed to Chicago, where he played extensively with one of his heroes, the big-band pioneer Fletcher Henderson. In the 50s, he began to piece together his own groups: firstly "space trios" and eventually the Arkestra, the big band that would define his career. It was around this time that the legend of Ra really began to emerge. He disowned the "slave name" he had been given at birth, becoming instead Le Sony'r Ra. His rehearsals became infamous, lasting for 12 hours or more, with Ra stopping the music to lecture on his favourite subjects: Egyptology and space. He began to claim he had been abducted by aliens and was in fact from Saturn.

Over time, the Arkestra evolved from travelling band to musical commune. Relocating to New York, most of the group moved into the first of a series of "Ra houses" where they would live, eat and play music together. Ra would often wake them at 4am to play something he'd just written.

Ra was hardly the first bandleader to drive his band hard, but he took it to another plane. Musicians who didn't focus or turned up late were locked in cupboards or, even worse, given the "royal treatment". At the Arkestra's next gig, they would be placed on a chair at the front of the stage, and Ra would announce that, because of a lack of discipline, they would not be allowed to play. It might sound ludicrous, but this musical naughty step exemplified the power that Ra and his music wielded over band members. Many remained with him for decades, honouring his vision long after his death from pneumonia in 1993. Saxophonist Marshall Allen joined the Arkestra in 1958 and, at the age of 90, leads the band to this day.

In the 1960s, the hippy movement caught wind of Ra's weirdness. By now, the Arkestra had begun wearing Egyptian robes and space-themed headpieces, and their stage shows had become extravagant "cosmo-dramas". John Sinclair, manager of proto-punk band MC5, brought the Arkestra to Detroit, where they began to win a new, white audience. Ra's music was that rare thing – both avant garde and accessible. Nothing displayed this more than his 1972 album Space Is the Place, a beautiful space opera with catchy singalongs and big-band freak-outs side by side. It spawned a psychedelic blaxploitation movie of the same name, featuring a spaceship powered by free jazz.

From the mid-60s onwards, the Arkestra were on a 25-year-long world tour, with the band selling copies of their records from the stage. Ra had launched his own label in the 50s and, over the next four decades, released hundreds of records. In downtime, the musicians would stick on labels and decorate the covers with elaborate drawings. Today, hand-painted Ra originals are among the most collectable records in the world.

By the time I met Ra, he was near the end of his life – yet he'd hit a purple patch, appearing on TV and magazine covers and playing up to 250 gigs a year. But his strangeness often overshadowed the fact that he was an extraordinary composer and musician. His output ranged from exquisite solo piano recordings, through traditional big-band swing to space-age funk and the proto-rap of the incredible Nuclear War ("It's a motherfucker – if they push that button, your ass gonna go!"). Today, there are numerous tribute bands, including Jerry Dammers' brilliant Spatial AKA Orchestra.

Beyond the showbiz, there was a serious and studied message. His self-declaration as an angel was profound. He didn't want to be associated with what he saw as a failed species. He was a conscientious objector to the end; he wanted no part in "being human". As anyone who was lucky enough to meet him will tell you, Sun Ra really was like no other human. He was a musical genius, a music-business pioneer, a moral philosopher. Both out to lunch and ahead of his time, he was a brother from another planet.

• Jez Nelson's documentary Travelling the Spaceways: the Cult of Sun Ra is on Radio 4 on 18 June. The Sun Ra Arkestra are at Cafe Oto, London E8, 16, 17, 25, 26 June; and at Glastonbury on 27 June.