If you're an investor in XM Satellite Radio (NASDAQ:XMSR) or Sirius (NASDAQ:SIRI), you can rest easy when you hear that satellite radio provider WorldSpace is going public. The country's duopoly is secure; there is no third entity clawing away at the domestic airspace.

In fact, if you like XM or Sirius, WorldSpace may be an intriguing possibility for your ultimate growth portfolio. The company raised its IPO price to a range between $16 and $18 a share yesterday -- $2 higher than originally planned.

WorldSpace has been pumping out satellite radio streams in Europe, Africa, the Middle East, and parts of Asia since 1999. It was originally launched as a free service but rolled toward the more promising monthly-subscriber model a year later.

WorldSpace was an early investor in XM. It also provided XM with some of the necessary technology to kick off its business. Yesterday, XM returned the favor by announcing a $25 million investment in WorldSpace that would also expand their partnership.

Digital radio is clearly one of the more exciting emerging industries. XM and Sirius expect to combine for 8.2 million subscribers by the end of the year. The potential of WorldSpace and its coverage map, which spans the globe, is enormous. So, however, are the risks.

Satellite radio isn't the only terrestrial-radio killer out there. Companies like Apple Computer (NASDAQ:AAPL), Yahoo! (NASDAQ:YHOO), Napster (NASDAQ:NAPS), and RealNetworks (NASDAQ:RNWK) are all selling song downloads that give music fans a tailored listening experience. Podcasting's popularity and the growth spurt at Audible (NASDAQ:ADBL) are opening up options for aficionados of the spoken word.

That's why WorldSpace may be even more compelling than the domestic providers, since it taps into developing markets that lack similar distractions. Naturally, a lack of disposable income in many of WorldSpace's broadcasting regions will be working against its marketing efforts. No one said it would be easy. Then again, perhaps that's why it seems so worthwhile.

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Longtime Fool contributor Rick Munarriz thinks it's cool that Sirius broadcasts The Motley Fool Radio Show four times over the weekend. He does not own shares in any of the companies mentioned in this story, though he is a satellite radio subscriber.He is also part of theRule Breakersnewsletter research team, seeking out tomorrow's ultimate growth stocks a day early.The Fool has adisclosure policy.