It's been six long years since world-famous physicist Stephen Hawking got a taste of weightlessness during a zero-G airplane flight from NASA's Kennedy Space Center — but he still wants to feel the real deal aboard Virgin Galactic's SpaceShipTwo rocket plane.

The 71-year-old Hawking has been living with neurogenerative disease for decades, but his illness hasn't kept him from taking on adventures that might tax younger, fitter humans. On Tuesday, during a London talk sponsored by the charity Breathe On UK, Hawking noted that he has required assistance with his breathing since his tracheotomy in 1985.

"Being on a ventilator has not curbed my lifestyle," he told the audience, using his instantly recognizable computer-generated voice. "I have been to Brussels, the Isle of Man, Geneva, Canada, California ... and I hope to go into space with Richard Branson's Virgin Galactic. It is possible to have quality of life on a ventilator."

That's music to the ears of Breathe On UK, which was created to help kids who need long-term breathing assistance. It's also a compliment to Virgin Galactic, which put SpaceShipTwo through its supersonic paces for the first time this week. If all the tests go right, SpaceShipTwo could be taking passengers on suborbital space trips as early as next year.

Richard Branson, Virgin Galactic's billionaire founder, promised to consider Hawking for one of those trips even before the good doctor took his ride into weightlessness in 2007. The invitation still stands, according to George Whitesides, the company's president and CEO.

"Richard and the team would love to welcome him on board," Whitesides told NBC News on Tuesday.

Hawking's health is the big issue for any future spaceflight, just as it was for the zero-G flight years ago. The physicist would have to be fully checked out, and even if he was cleared for takeoff, medical staff would almost certainly have to ride along. Deceleration could be the toughest part of the trip. SpaceShipTwo's flight profile calls for up to 6 G's of force on the way down. That's more force than most space shuttle astronauts have felt, and it ranks right up there with the world's rockiest roller-coaster rides.

If Hawking were to fly into space sometime in the next few years, he'd take the No. 2 spot on the list of the world's oldest astronauts. The only person older would be senator-astronaut John Glenn, who flew on the space shuttle Discovery at the age of 77 and is now 91 years old.

Six years ago, Hawking declared, "Space, here I come!" Should he keep that dream alive, or should he focus on earthly adventures instead? Feel free to register your opinion in our informal survey, and weigh in with your comments below.

More about Stephen Hawking:

• Hawking lays out his case for a godless big bang
• Stephen Hawking visits LA stem cell lab
• Cosmic Log archive on Stephen Hawking

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Mark Greenberg / Virgin Galactic

A bearded Richard Branson (center) gets a congratulatory hug from SpaceShipTwo designer Burt Rutan. Sierra Nevada Corp.'s Mark Sirangelo, who was involved in the development of SpaceShipTwo's hybrid rocket engine, can be seen just to the right of Rutan.

British billionaire Richard Branson's Virgin companies operate airplanes and trains, sell music and phones, offer games and radio shows. He's an adventurer who has flown balloons over oceans, has swum with sharks and whales, and has even started up his own ocean exploration venture. He's had his own reality-TV series and played cameo roles in "Around the World in 80 Days," "Casino Royale" and "Superman Returns." But what really gets the 62-year-old's juices flowing is outer space: Even in a Virgin Mobile TV commercial, Branson's dream of going weightless serves as the kicker.

So it's debatable whether anyone was happier than Branson to see Monday's first blastoff by SpaceShipTwo, the rocket plane that he hopes will take hundreds of regular people (with $200,000 to spend) on quick suborbital trips into outer space. Over the past eight and a half years, Branson has spent tens of millions of dollars to get his Virgin Galactic venture this far, and if the tests continue to go smoothly, he and his kids may soon be getting on the space plane themselves.

Exactly when will that be? Branson's predictions have been uniformly over-optimistic: 2007? 2008? 2012? 2013? Now he says commercial service will start next year. The fact that the future time frame is shrinking suggests that Branson is getting closer to being right. In a quick Q&A, the rebel billionaire talked about the "very long road" behind him and the road that lies ahead:

Cosmic Log: You've talked about how you and your family are looking forward to this. After today's launch, are you looking forward to it even more?

Richard Branson: Of course. It was a thrilling day today. Everything went absolutely according to plan. It looked magnificent. The pilots just loved the experience. I think they were tempted to go straight into space, but knew they'd get fired if they did. We're very much looking forward to getting there either at the end of this year or very early next year.

Mark Greenberg / Virgin Galactic

Virgin Galactic's billionaire backer, Richard Branson, gets a "high-ten" hand-slap from SpaceShipTwo pilot Mark Stucky. George Whitesides, Virgin Galactic's CEO and president, is to Branson's right.

Q: What has this effort meant to you? I don't know if people could have predicted that it would take eight and a half years to get to this point after SpaceShipOne. Has this been a longer road than you thought it would be? Does that make it taste sweeter when things go right?

A: Yes, it's been a very long road. But as far as putting people into space, Virgin Galactic is the only company that has gotten this far. Quite a few other companies have also been working hard to get this far. Today was such an important milestone, in that we knew the rockets were finally working. We knew the spaceship worked on its own. But we obviously needed to test the two together to make sure that the designers got it right. We're absolutely delighted that it broke the sound barrier on its very first flight, and that everything went so smoothly. So we really are on the way now. We've overcome the biggest hurdle, and there are no major hurdles left except for the normal test flights that are needed before we go into space.

Q: How many test flights do you think will be needed? You've already mentioned that you are hoping the first spaceflights could happen by the end of this year, and commercial service would follow. Now that the first powered test has taken place, what does the schedule ahead look like?

A: There will be many test flights between now and the end of the year, before we actually go into space. We'll do as many tests as we feel are necessary before we actually turn it over to myself, my children and other people. We'll be working with the FAA and others to get as many flights under our belts as we feel are needed, but I do think we'll be ready by the end of the year. 

Q: When you saw SpaceShipTwo fire up its engine, were there any surprises, or was it totally the way you expected it to go. Did you ever think to yourself, "Whoa, I didn't think it was going to work that way"?

A: Fortunately, there were no surprises. Until it happens, you have to be nervous, even though you have the best team in the world working with it. What was incredible was how clear it was, just looking up without binoculars. You could visibly see the spaceship getting faster and faster. There's an old saying, "It's not rocket science." But this is rocket science, and that's why it's taken eight and a half years to get this far.

Q: You have more than 500 people who have already put money down for a flight, and many more who are interested in the idea of flying into outer space. What would you say to them about the significance of today's test, and what they can expect in the years ahead?

A: Today was the most significant day in the program. I think that for those people who have been good enough to stick with us for the last eight years, who signed up early on, their time to become astronauts is very soon now. I'd just say, 'Thank you very much for sticking with it.' We'll soon be able to make their dreams come true.

More reactions to the SpaceShipTwo test:

• Charles Lurio, writer of The Lurio Report on private space development: “It’s been a long eight and a half years, but this is the kind of thing that happens in development programs.”
• Commercial Spaceflight Federation: "We are one step closer to achieving safe, routine and cost-effective access to space that will create abundant opportunities for space-based research and that will inspire the next generation of engineers and scientists."
• House Majority Whip Kevin McCarthy, R-Calif.: SpaceShipTwo's supersonic flight is a "major milestone in commercial space travel, bringing us one step closer to offering private commercial space travel and solidifying the Mojave Air and Space Port as our nation’s premier aerospace research, development and test flight center for this emerging space industry."
• Spaceport America: "Today's successful powered flight means we are getting closer to the day when the first Virgin Galactic passenger flight will be taking place from Spaceport America in New Mexico."

More about SpaceShipTwo:

• SpaceShipTwo goes supersonic
• Tom Cruise might be up for space
• Cosmic Log archive on SpaceShipTwo

Slideshow: The making of SpaceShipTwo

Click through scenes from the construction of Virgin Galactic's suborbital passenger spaceship.

Launch slideshow

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log and the rest of NBCNews.com's science and space coverage, sign up for the Tech & Science newsletter, delivered to your email in-box. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Virgin Galactic's SpaceShipTwo rocket plane lit up its engine for the first time in flight on Monday, taking a giant supersonic leap toward outer space.

The crucial 16-second blast took place at about 7:50 a.m. PT (10:50 a.m. ET), high above California's Mojave Air and Space Port. Virgin Group's billionaire founder, Richard Branson, was on hand to watch the proceedings.

"Today was the most significant day in the program," Branson told NBC News afterward. "I think that for those people who have been good enough to stick with us for the last eight years, who signed up early on, their time to become astronauts is very soon now. ... We'll soon be able to make their dreams come true."

Branson wasn't the only one watching: Rocket aficionados flocked to viewing areas near the airport to see the blastoff. Until Monday, Mojave-based Scaled Composites, which is building and testing the plane for Virgin Galactic's eventual use, had tested SpaceShipTwo only by dropping it from its WhiteKnightTwo carrier airplane and having its pilots guide the plane back through unpowered glides back to the runway. The engine, powered by a rubber-based solid fuel and nitrous oxide, had been fired only on the ground.

Monday's test was radically different: WhiteKnightTwo released SpaceShipTwo from its traditional drop zone, at an altitude of around 47,000 feet. But after the rocket plane glides clear from the mothership, its pilot lit up the engine and pointed SpaceShipTwo upward into the sky, reaching a maximum height of 56,200 feet. The plane coasted back to a landing back at the Mojave airport, about 13 minutes after blastoff.

Test pilots Mark Stucky and Mike Alsbury were at SpaceShipTwo's controls for Monday's flight, Virgin Galactic said. Afterward, the company said in a tweet that the pilots confirmed "SpaceShipTwo exceeded the speed of sound on today's flight!" The reported maximum velocity was Mach 1.2.

Virgin

SpaceShipTwo fires up its rocket engine for the first time in flight on Monday.

MarsScientific.com / Clay Center Observatory

A 16-second rocket blast sends SpaceShipTwo toward the heavens.

Virgin Galactic via W. Christine Choi

A boom camera on Virgin Galactic's SpaceShipTwo plane shows the rocket engine firing.

George Whitesides, Virgin Galactic's president and CEO, said in a news release that the flight test "went as planned, with the expected burn duration, good engine performance and solid vehicle handling qualities throughout."

Eventually, SpaceShipTwo could break the space barrier as well as the sound barrier — just as its predecessor, SpaceShipOne, did in 2004. When the single-piloted SpaceShipOne made repeated flights beyond an altitude of 100 kilometers (62 miles), which is the internationally accepted boundary of outer space, it won the $10 million Ansari X Prize for private spaceflight. Ever since then, Virgin Galactic has been funding the multimillion-dollar development effort to create a fleet of passenger space planes.

In the grand scheme of things, suborbital spaceflight isn't exactly new: The U.S. Air Force's X-15 rocket plane blazed that trail to manned spaceflight a half-century ago. The new twist is that it's being done by private companies rather than government programs. 

Scaled and Virgin Galactic have mapped out a series of flight tests that would gradually push the envelope, potentially leading to suborbital spaceflights over California's Mojave Desert by the end of this year. Virgin Galactic's goal is to begin passenger service, for tourists as well as researchers, at New Mexico's Spaceport America as early as next year. More than 500 people — including celebrities such as Ashton Kutcher — have already put down money for a $200,000 ride.

The six-passenger, two-pilot plane is designed to give riders a commanding view of the curving Earth beneath the black sky of space, a few minutes of free-floating weightlessness, and a roller coaster ride back down to Virgin Galactic's spaceport. Other companies — including XCOR Aerospace and Blue Origin — are planning to get into the suborbital space passenger business as well, but if SpaceShipTwo's flight tests go well, Virgin Galactic is likely to become the market leader.

Branson has said he and his family would be among the first to fill SpaceShipTwo's passenger seats.

"Like our hundreds of customers from around the world, my children and I cannot wait to get on board this fantastic vehicle for our own trip to space and am delighted that today's milestone brings that day much closer," he wrote in a blog post.

Slideshow: The making of SpaceShipTwo

Click through scenes from the construction of Virgin Galactic's suborbital passenger spaceship.

Launch slideshow

More about SpaceShipTwo:

• Virgin billionaire can't wait for space ride
• Tom Cruise might be up for space
• Cosmic Log archive on SpaceShipTwo

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log and the rest of NBCNews.com's science and space coverage, sign up for the Tech & Science newsletter, delivered to your email in-box. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

This story was originally published on Mon Apr 29, 2013 10:29 AM EDT

MarsScientific.com / Clay Center Observatory

Cold oxidizer streams from the back of SpaceShipTwo's engine during an unpowered test flight on April 12. Richard Branson, the billionaire founder of Virgin Galactic, says the rocket plane could go supersonic when its engine is lit up for the first time in flight, as early as Monday.

Virgin Galactic's billionaire founder, Richard Branson, says his company is planning to fire up SpaceShipTwo's rocket engine for the first time in flight on Monday — a "historic" blast that is expected to send the space plane supersonic.

"We're hoping to break the sound barrier," Branson told the Las Vegas Sun. "That's planned Monday. It will be a historic day. This is going to be Virgin Galactic's year. We'll break the sound barrier Monday, and from there, we build up through the rest of the year, finally going into space near the end of the year. I'll be on the first official flight, which we look to have in the first quarter of next year. We're doing a number of test flights into space first."

Branson made his comments on Monday during a visit to kick off Virgin America's airline service to Las Vegas. In just one paragraph, the British entrepreneur and adventurer capped off weeks of rumors and laid out a new timeline for starting up passenger flights to outer space.

SpaceShipTwo builds on the heritage of SpaceShipOne, which powered its pilots beyond the 100-kilometer (62-mile) altitude mark in 2004 to win the $10 million Ansari X Prize for private-sector spaceflight. SpaceShipOne is now hanging in the Smithsonian, but SpaceShipTwo has been under development for years at Scaled Composites in Mojave, Calif. Scaled has been conducting a series of unpowered tests at the Mojave Air and Space Port. During an April 12 "cold flow" flight test, Scaled's pilots rehearsed every step for a powered flight, short of lighting up the hybrid rocket engine.

So far, SpaceShipTwo has been attached to the belly of its WhiteKnightTwo mothership, carried up to altitudes of around 50,000 feet and then dropped into the air to make a glider-like landing. During powered tests, SpaceShipTwo's engine will be lit up after the drop. The rocket plane will make a spectacular blast into the heavens and then glide back to the runway.

"We’ve experienced about a year’s worth of vertical flight tests and captive-carry flight tests by a number of tenants, and now we’re entering the phase of manned research flights," Stuart Witt, CEO of the Mojave Air and Space Port, told NBC News. "We’re excited about that: The industry has been waiting for this for a long time – since 2004."

Virgin Galactic's CEO and president, George Whitesides, stressed that the test schedule was dependent on several factors, including the weather. He said the first rocket-powered test could easily slip to a later time.

Like Branson, Whitesides said the first powered flight, known as PF01, would be merely the first step in the next phase of testing. "PF01 will be the start of a series of increasingly longer-duration burns (PF02, PF03, etc.) that should lead us to space altitude before the end of the year and commercial ops start soon after that," he said in an email to NBC News.

Virgin Galactic plans to conduct commercial spaceflights from Spaceport America in New Mexico, with passengers charged $200,000 for a ride. More than 500 customers have already put money down for flights. The passengers would experience breathtaking views of the curving Earth beneath the black sky of space, go weightless for a few minutes and then strap themselves back in for a powerful plunge back to the runway.

The first supersonic flight of a new spaceship isn't always auspicious: When SpaceShipOne came in for a landing in Mojave after breaking the sound barrier for the first time on Dec. 17, 2003, its left landing gear collapsed and the plane ran off the runway. Fortunately, no major damage was done, and pilot Brian Binnie was unhurt. Binnie went on to fly SpaceShipOne into space on Oct. 4, 2004, to win the $10 million prize.

Blue Origin, a rocket venture backed by Amazon.com billionaire Jeff Bezos, put its prototype spacecraft through its first unmanned supersonic flight at a Texas rocket range in August 2011 — but the company said the craft had to be destroyed when it experienced a "flight instability" at an altitude of 45,000 feet. Blue Origin recovered from that setback and is continuing to work on suborbital as well as orbital spacecraft.

Slideshow: The making of SpaceShipTwo

Click through scenes from the construction of Virgin Galactic's suborbital passenger spaceship.

Launch slideshow

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email inbox every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

MarsScientific via Virgin Galactic

A trail of oxidizer streams behind the SpaceShipTwo rocket plane during Friday's gliding test flight.

Virgin Galactic's SpaceShipTwo rocket plane successfully glided through a test on Friday that sent oxidizer flowing through its engine — a sight that led observers to speculate that the suborbital spacecraft's first powered flight could be imminent.

SpaceShipTwo has been tested in the air for more than three years. Its hybrid rocket engine has undergone extensive development and testing, including multiple test firings on the ground. But the rocket has not yet been lit up in flight — and that's a crucial step in Virgin Galactic's plan to put tourists in outer space.

Based on rumblings coming from Mojave, Calif., where Virgin Galactic and Scaled Composites have been testing the six-passenger plane, the first powered test flight could come on April 22. Virgin's British billionaire founder, Richard Branson, hinted that something big was coming in a weekend blog posting: "I look forward to seeing you all in Mojave soon," he wrote.

During Friday's test, Virgin Galactic's massive WhiteKnightTwo carrier plane took off from the Mojave Air and Space Port, carrying SpaceShipTwo under its belly. The mothership rose to an altitude of about 50,000 feet, then released the rocket plane for a 10.8-minute-long gliding descent back to the runway. Mark Stucky and Mike Alsbury were the test pilots for what was characterized as a "mission rehearsal" for the first rocket-powered flight.

Virgin Galactic said SpaceShipOne successfully went through every step in preparation for that milestone flight, "apart from actually igniting the rocket."

"Importantly, and for the first time in the air, oxidizer was flowed through the propulsion system and out through the nozzle at the rear of the vehicle — thus successfully accomplishing the 'Cold-Flow' procedure," the company said in a news release. "As well as providing further qualifying evidence that the rocket system is flight-ready, the test also provided a stunning spectacle due to the oxidizer contrail, and for the first time gave a taste of what SpaceShipTwo will look like as it powers to space."

Slideshow: The making of SpaceShipTwo

Click through scenes from the construction of Virgin Galactic's suborbital passenger spaceship.

Launch slideshow

Virgin Galactic hasn't announced when the first powered flight would come. "We have to do a full review of the data before we finalize our next flight milestone, but we’re getting close now," the company's CEO and president, George T. Whitesides, was quoted as saying on the Space Coalition blog.

April 22 has been the focus of speculation because that would be the 69th birthday of the late millionaire adventurer Steve Fossett, who became the first person to make a nonstop solo flight around the world in 2005. That trip was financed by Branson, a friend and fellow flight enthusiast, and was accomplished with a Virgin GlobalFlyer airplane that looked like a lighter version of WhiteKnightTwo.

"Flying the space plane under power on his birthday would be a poignant tribute to Fossett, who died in a plane crash in California’s Sierra Nevada mountains two years after making his solo around-the-world flight," Parabolic Arc's Doug Messier wrote. Citing personal sources, independent consultant Charles Lurio also said April 22 was a target date.

The first powered flight would represent the biggest step yet in SpaceShipTwo's development effort, which builds upon the history-making suborbital space missions that were flown by the SpaceShipOne rocket plane in 2004. Virgin Galactic's plan calls for an increasingly ambitious series of flights from Mojave that will eventually take SpaceShipTwo's test pilots beyond 100 kilometers (62 miles) in altitude, which is the internationally accepted boundary of outer space.

As soon as next year, paying passengers may get their turn to climb aboard SpaceShipTwo at Spaceport America in New Mexico. The aerial launch from WhiteKnightTwo would lift them up to take a look at the curving Earth and the black sky of space. There'd be a few minutes of free-floating weightlessness at the top of the ride. Then SpaceShipTwo's innovative folding-wing design would slow down the supersonic plunge back toward Earth. The outer-space trip would end with a glide back to Spaceport America's 2-mile-long (3.2-kilometer-long) runway.

Virgin Galactic says more than 500 people have signed up for the $200,000 suborbital space tour.

More about SpaceShipTwo:

• Tom Cruise might be up for space
• What it's like to ride on SpaceShipTwo
• Cosmic Log archive on SpaceShipTwo

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log and the rest of NBCNews.com's science and space coverage, sign up for the Tech & Science newsletter, delivered to your email in-box. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Luke Colby / Virgin Galactic

Virgin Galactic's SpaceShipTwo rocket plane glides over its Mojave test range in California with its rocket motor components installed for the first time.

Virgin Galactic's SpaceShipTwo rocket plane has gone through more than 20 unpowered glide test flights, but today's test was special: It marked the first free flight during which the actual rocket motor components were installed.

"It was also the first flight with thermal protection applied to the spaceship's leading edges," the company said in today's status report. "It followed an equally successful test flight last Friday which saw SpaceShipTwo fly in this configuration, but remain mated to its WhiteKnight carrier aircraft."

Virgin Galactic said all the objectives of both flights were successfully met. At least two more such glide flights are expected to take place before the spacecraft's builders move on to the next phase of testing: powered flights, which call for the plane to fire up the hybrid rocket engine for its ascent.

SpaceShipTwo is being developed at California's Mojave Air and Space Port, with the aim of beginning passenger spaceflights as early as next year. The multimillion-dollar project follows up on the flights of SpaceShipTwo's predecessor, SpaceShipOne, which won the $10 million Ansari X Prize for private-sector spaceflight in 2004.

The new plane's hybrid rocket motor, built by Sierra Nevada Corp., is powered by a rubber-based fuel and nitrous oxide — much like SpaceShipOne's engine was. Virgin Galactic said the propellant tanks were installed on SpaceShipTwo for the latest tests, but did not indicate that the tanks were fueled up ... yet.

SpaceShipTwo's flight profile calls for the plane to ride on the WhiteKnight mothership to an altitude of 50,000 feet, then drop away and light up the rocket. That blast would power the craft to a height beyond 62 miles — giving up to six passengers a few minutes of weightlessness and a great view of the curving Earth beneath the black sky of space. The plane would then re-enter the atmosphere and glide back down to a runway landing.

Although the test program is being conducted in California, Virgin Galactic's business plan calls for commercial spaceflights to originate from Spaceport America in New Mexico. In recent months, some observers have questioned whether the $209 million New Mexico space project will match initial expectations on the economic development front. Spaceport officials insist, however, that the futuristic-looking facility will be ready for Virgin Galactic as well as other tenants.

Virgin Galactic was founded by British billionaire Richard Branson. The company says more than 500 people, including celebrities such as Ashton Kutcher, have signed up for the $200,000 suborbital space tour.

More about the commercial space race:

• Golden Spike wants to fly you to the moon ... for $1.4 billion
• Blue Origin's spaceship prototype aces pad-escape test
• Virgin Galactic's future at Spaceport America is uncertain
• Gallery: Ten players in the commercial space race
• Cosmic Log archive on the new space race

Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Noel Vasquez / Getty Images

Ashton Kutcher, seen here at a Los Angeles Lakers basketball game in February, is the 500th customer to sign up for a Virgin Galactic suborbital spaceflight.

Virgin Galactic says uber-celebrity Ashton Kutcher is the company's 500th customer to sign up for a suborbital trip into outer space.

"I gave Ashton a quick call to congratulate and welcome him," Virgin Galactic's billionaire founder, Richard Branson, said today in a blog post announcing that Kutcher was coming on board. "He is as thrilled as we are at the prospect of being among the first to cross the final frontier (and back!) with us and to experience the magic of space for himself."

Kutcher, who got his start in television on "That '70s Show" and is now one of the stars of the highly rated sitcom "Two and a Half Men," is said to have a net worth of around $140 million. So the $200,000 fare for a ride on Virgin Galactic's SpaceShipTwo rocket plane, also known as the VSS Enterprise, shouldn't break the bank. He's as well-known for his online presence (with 9.8 million Twitter followers) and his gossip-column appeal (due to last year's breakup with Demi Moore) as he is for his filmography.

All this makes him arguably the highest-profile prospective spaceflier confirmed to be on Virgin Galactic's list — although Tom Hanks, Katy Perry, Brad Pitt and Angelina Jolie are reportedly on the list as well. Beyonce and Jay-Z are among other celebs considering a flight. The stars are reportedly all paying their own way, except for physicist Stephen Hawking, who is receiving a free ride courtesy of Branson.

For Stephen Attenborough, Virgin Galactic's commercial director, getting to the 500-passenger point is as big a milestone as getting Ashton Kutcher to sign on. "It's great to get to No. 500," he told me.

It's way too early to put Kutcher's flight on the calendar. SpaceShipTwo is still in the midst of free-glide flight tests, with rocket-powered test flights expected to begin this year. Last year, Branson said he was holding out hope that he and members of his family will be able to take a ride into space as this year's Christmas present. That suggests 2013 could mark the start of commercial service, although Virgin Galactic and its partners at California-based Scaled Composites say the schedule is totally dependent on the outcome of tests at the Mojave Air and Space Port.

The current plan calls for commercial flight operations to be based at Spaceport America in New Mexico. SpaceShipTwo, which is capable of carrying six passengers and two pilots, would be linked up to its wide-winged WhiteKnightTwo carrier airplane and brought up to an altitude of 50,000 feet. At that height, SpaceShipTwo would drop from its mothership and turn on its hybrid rocket engine. The blast would take the craft beyond 62 miles (100 kilometers), the internationally accepted boundary of outer space — a commanding height from which Kutcher and his fellow passengers could see the arc of planet Earth and the dark sky of space above.

There'd be a few minutes of weightlessness, and then the passengers would return to their seats for the descent. After weathering up to 6 G's of acceleration, the fliers would glide down to the landing — and get their astronaut wings back at Spaceport America.

Attenborough said Virgin Galactic is already giving a lot of thought to determining who would fly when. One of the factors in the formula would give priority to customers "roughly in the order that they signed up," he said, but the schedulers also would consider customer preferences and the possibility of achieving firsts in spaceflight (for example, ahem, first prime-time TV star in space). "We're expecting to be able to keep everyone happy," Attenborough told me. 

How long do you think it'll be before Kutcher is clicking his camera on the final frontier? And do you suppose there'll be a deal to document everything for reality TV? Feel free to weigh in with your comments below.

More about celebrity spaceflight:

• Fly Justin Bieber into space, scientist says
• Bunnies in jetpacks? It's Playboy Club in space!
• Director James Cameron targets spaceflight
• Celebrities in space?

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.

Frazer Harrison / Getty Images file

Jay-Z and Beyonce strut their stuff during a performance at the 2006 BET Awards. The couple, who just had a baby girl last week, are reportedly talking about doing a music video together in space.

Record executives are reportedly talking about sending Jay-Z and Beyonce into space to make a music video — and although Virgin Galactic's executives aren't yet in on the discussions, they say the pop-music power couple are welcome to take a ride. They can even bring their celebrity daughter along.

The idea came to light on Thursday in The Sun, a British tabloid that specializes in the exploits of the rich and famous.

"The label people have been talking about making a music vldeo in space," The Sun's Harry Haydon quoted a source as saying. "Beyonce and Jay-Z seemed the obvious option. Everything is being done to make it happen."

Executives are reportedly inquiring behind the scenes about making the video on Virgin Galactic's SpaceShipTwo rocket plane. Scenes for the video would be shot during the few minutes of weightlessness that come at the peak of the suborbital space ride, more than 62 miles (100 kilometers) above Earth's surface.

SpaceShipTwo isn't due to start commercial service until the end of this year at the earliest, which is probably a good thing, because I'm guessing that Jay-Z and Beyonce have their hands full right now with Blue Ivy Carter, their newborn daughter. Blue Ivy has already hit the Billboard music charts, and eventually she might hit the heights of outer space as well.

"We're having no talks with Beyonce and/or Jay-Z (or indeed Blue Ivy!) but would be more than happy to take mum and dad or the whole family to space at some point!" Stephen Attenborough, Virgin Galactic's commercial director, told me in an email today.

Celebrities lining up
Beyonce is already known to have a soft spot for space: She recorded a special audio message for the crew of Atlantis during last summer's final space mission. "You inspire us all of us to dare to live our dreams, to know that we're smart enough and strong enough to achieve them," she told the astronauts.

If the project comes together, Jay-Z and Beyonce could make the first professional music video performed in outer space. But they wouldn't be the only celebrities in space. The glitterati rumored to be on the list of prospective Virgin Galactic passengers include Katy Perry and Russell Brand (who recently broke up), Brad Pitt and Angelina Jolie, Paris Hilton, Tom Hanks and Victoria Principal.

Then there's Stephen Hawking, the superstar quadriplegic physicist who just turned 70. Hawking took a zero-gravity airplane flight five years ago and is still up for a Virgin Galactic spaceflight. Attenborough said that Virgin Galactic's founder, British billionaire Richard Branson, "was delighted to attend his 70th birthday celebrations last weekend and reiterated Virgin Galactic's commitment to do everything possible make it happen."

No freebies for the stars
SpaceShipTwo is designed to carry just six passengers and two pilots — which means celebrities can bring a small entourage along with them. Hawking would reportedly ride for free, but other celebs shouldn't expect freebies: The cost of a space travel package is $200,000 per seat, and Branson said last year that he wouldn't be offering any special deals.

"I'm in the airline business, and a lot of people ask for upgrades, and we're not going to get the same thing happening with our space program," he told The Canadian Press.

So when might the tourist treks to space begin? Attenborough didn't mention an exact date in his email, but he said "2012 is going to be the most exciting year yet."

"We are poised on the edge of the final stretch of flight testing, with the commencement of SpaceShipTwo powered tests expected in the not-too-distant near future and ramping up, if all goes well, to space flight within the 12-month period," he said. "Clearly we will also require the FAA licences before we embark on passenger flights, but are continuing to see a major gear-up in the Spaceport America-based operation in preparation for that."

Virgin Galactic has more than 450 would-be spacefliers signed up, and Attenborough said "we are continuing to see near-record growth in the numbers of future astronauts."

"I expect in the early part of this year we will have more paid-up customers than have been to space to date — which will be an exciting milestone," he said.

More about Beyonce and Jay-Z:

• 'Bootylicious' fly named after Beyonce
• Slideshow: Beyonce, a star at last
• Beyonce baby complaints at hospital dismissed
• Jay-Z raps about baby, mentions miscarriage

More about Virgin Galactic:

• Planning the next steps in a new space race
• New Mexico's desert spaceport makes a splash
• Slideshow: The making of SpaceShipTwo
• Gallery: Ten players in the commercial space race
• Cosmic Log archive on the new space race

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

If you think America's space effort is in a state of flux now, you ain't seen nothing yet: Just wait until billionaires Richard Branson and Robert Bigelow are vying to offer orbital hotels, or until there are as many brands of spaceships built in the United States as commercial jets.

Or not.

That's the curious thing about Space Race 2.0: It's definitely a marathon, not a sprint, and the field of contestants have had dropouts (like the bankrupt Rocketplane Kistler) as well as drop-ins (like the Boeing Co.).

If any of the racers make it to the finish line, NASA will once again be able to send U.S. astronauts to the International Space Station on U.S.-built spacecraft, ending the post-shuttle spaceship gap. There may also be opportunities for businesses and foreign governments to purchase their own presence in space, in the form of private-sector space stations. Regular folks may be able to buy vacation packages that include a quick up-and-down on a suborbital spacecraft, or even a stay on one of those space stations.

There'll be new opportunities for space research and manufacturing as well. Alan Stern, a planetary scientist at the Southwest Research Institution as well as an adviser to the Blue Origin space venture, has called low-cost space research the "killer app" for the space travel industry — right up there with space tourism and space station resupply.

But what steps lie ahead for private space ventures, and what's the time frame for taking those steps?

A crucial year
For the companies seeking NASA's business, the next six months to a year will be crucial: Four companies — Blue Origin, Boeing, Sierra Nevada Corp. and SpaceX — are receiving hundreds of millions of dollars from NASA to develop spaceships capable of ferrying astronauts to the space station and back. SpaceX and yet another company, Orbital Sciences Corp., have already been receiving NASA funding to support the development of unmanned cargo spaceships.

In February, SpaceX is due to launch a test cargo shipment to the space station and bring the capsule back to Earth. Orbital Sciences, meanwhile, is gearing up for its first test flight of its Taurus 2 launch vehicle in the same time frame. By 2013, both companies should be cleared for orbital cargo deliveries as part of a $3.5 billion combined deal with NASA.

The development effort for crew vehicles is more complex, due to the higher safety requirements. Last month, Congress settled on an allocation of $406 million for the next phase of the commercial crew development program, or CCDev. That's less than half of the $850 million requested by the Obama administration, and NASA hasn't yet laid out a revised plan for the next development round.

Based on the space agency's previously announced plans, the money for the next phase would be given out starting next July, for the development of an integrated system that includes a space-taxi capsule as well as the rocket it rides on. SpaceX can already offer the full package, which combines its Falcon 9 rocket with its Dragon capsule. The other contenders will have to buddy up with rocket builders — either United Launch Alliance, which offers the Atlas 5; or ATK and EADS Astrium, which have proposed creating a hybrid rocket called Liberty. Right now, the Atlas 5 is the favored vehicle in the rocket race, but the next phase of CCDev provides an opportunity for dark horses like ATK to get back in the race.

As long as no one crosses the finish line, NASA is stuck in the position of paying the Russians $50 million or more for each seat filled by a U.S. astronaut heading to the space station. So the space agency has a powerful interest in making sure that at least one space-taxi operator succeeds. NASA expects that it'll be using U.S.-built space taxis in the 2017 time frame, but warns that reduced funding levels will slow down the timeline.

Suborbital space race
Meanwhile, additional companies are aiming for suborbital space business, either for research or tourism purposes. Among the major players in this particular race are Armadillo Aerospace, Virgin Galactic and XCOR Aerospace,

Virgin Galactic says it's on track to begin powered test flights of its SpaceShipTwo craft early next year, with an eye toward offering suborbital trips at $200,000 a seat in 2013. Branson, the company's founder, is aiming even higher: "We're starting by suborbital trips, we'll then go to orbital trips, we're then going to look at space hotels. We're going to look at intercontinental travel at a speed much quicker than you can currently travel," he told me during an interview in October.

XCOR Aerospace plans to start testing its Lynx rocket plane in the air within a year, and wants to take on tourists starting in the 2013-2014 time frame.

Armadillo has partnered up with Space Adventures, the company that has sent seven paying passengers to the space station, to develop a suborbital launch system capable of carrying passengers or scientific experiments. The New Mexico Spaceport Authority says Armadillo ran a successful test of a reusable sounding rocket known as STIG A on Dec. 4. The rocket rose to an altitude of 137,500 feet (41.91 kilometers), and carried a scientific package from Purdue.

Blue Origin, which was founded by Amazon.com billionaire Jeff Bezos, is also working on a suborbital spaceship project that's separate from the NASA-funded orbital effort. (The company is bouncing back from the crash of a suborbital test vehicle in August.)

Next giant leap
Of course, there's no guarantee that any of these companies will get off the ground on the timetable they expect. This space race is notorious for slowing down the pace: Spaceship builders have been predicting that the golden age of private spaceflight is just two years away for the past 15 years.

The interesting thing is that the different companies are coming together in combinations that make the space race look more like a square dance: Space Adventures is teaming with Armadillo on suborbital tourism, with Boeing on orbital tourism, and with the Russians on trips to the space station and even the moon. Sierra Nevada is relying on Virgin Galactic's help for atmospheric tests of its prototype orbital vehicle, while Virgin Galactic is relying on Sierra Nevada to provide the hybrid rocket engine for SpaceShipTwo. Boeing is a partner with Lockheed Martin in United Launch Alliance, which plans to provide rockets for Boeing as well as two of its CCDev competitors.

Bigelow Aerospace, which has already put two of its inflatable space modules into orbit on Russian rockets, could conceivably purchase launch services from SpaceX or United Launch Alliance to establish future private-sector space stations — and it's teaming up with Boeing and Space Adventures to make the arrangements for orbital trips by tourists and researchers.

Where could all this lead? Would you believe to Mars? At least that's what SpaceX founder Elon Musk expects. He's teaming up with NASA's Ames Research Center on a proposal for an unmanned Mars mission in the 2018 time frame, and he has said SpaceX's rockets could send humans to Mars in the next 10 to 15 years if that's what NASA wants to do.

"The reason to do space and to try to push the boundary of space is that it's one of the coolest things that humanity, or we as a country, can do," he told me. "We want there to be cool things. Life cannot just be about solving problems. If that's all it's about, why get up in the morning? There's got to be things that are inspiring and make life worth living — and I think pushing the boundaries of space and the outer frontier is one of those things."

More on the future of spaceflight:

• SpaceX chief aims for Mars
• Boeing runs hard in the new space race
• Future spaceflight goes virtual at Sierra Nevada
• Blue Origin spruces up its rocket report
• Slideshow: The making of SpaceShipTwo
• Gallery: Ten players in the commercial space race
• Cosmic Log archive on the new space race

This report draws upon videos that are part of a Future of Technology package produced by msnbc.com's Matt Rivera. Stay tuned for a new twist in the saga of future spaceflight on Tuesday.

Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.

Matt Rivera / msnbc.com

Virgin Galactic founder Richard Branson joins in on the acrobatics performed by the Project Bandaloop dance troupe during the dedication of Spaceport America's terminal/hangar facility.

British billionaire Richard Branson christened the world's first facility designed for private-sector space travel in grand style today, rappelling down the glass-sheathed side of Spaceport America's spaceship complex with a champagne bottle in hand.

Branson's acrobatics served as the climax of a spaceport dedication ceremony that also featured some aerobatics from the SpaceShipTwo rocket plane and its massive WhiteKnightTwo mothership. The double-plane flight system is designed to carry passengers to the edge of outer space from Spaceport America in the New Mexico desert. Branson's company, Virgin Galactic, could conceivably start passenger service on SpaceShipTwo in late 2012 or 2013, depending on how the flight tests go.

Branson and his children, Sam and Holly, popped up as surprise guest performers during a wall-walking dance staged by the California-based Project Bandaloop. The champagne bottle was lowered down to Branson on a rope. He uncorked the bottle and gave Spaceport America's alien-looking, 110,000-square-foot terminal/hangar facility a celebratory splash of bubbly. Then he declared that the building would be called the "Virgin Galactic Gateway to Space," and finished off the rites with a deep swig of champagne.

Matt Rivera / msnbc.com

Virgin Galactic founder Richard Branson sprays champagne from a bottle as he hangs from a rope along the side of Spaceport America's nearly completed terminal/hangar facility in New Mexico.

When Branson was lowered to the ground, he told New Mexico Gov. Susana Martinez that he hoped the performance would be "the first of many safe landings at Spaceport America."

Virgin Galactic's chief executive officer, George Whitesides, said it was Branson's idea to get in on the act, and Branson told me that no one rejected the idea. "I think they've given up trying to convince me not to do things," he said.

Today's ceremony drew hundreds of onlookers — including Martinez and other New Mexico politicians as well as Apollo 11 moonwalker Buzz Aldrin — to a site 30 miles east of Truth or Consequences, N.M., in an area so isolated that cellphone coverage was hard to come by. Spaceport director Christine Anderson joked that the $209 million facility might be out in the middle of nowhere, but it's a "beautiful middle of nowhere," set amid the San Andres Mountains.

Mark Greenberg / Virgin Galactic

Dancers perform acrobatics on the glass facade of Spaceport America's terminal/hangar facility, which was christened as the Virgin Galactic Gateway to Space.

Among those in the audience were more than 150 spaceflight fans who have already put down deposits for SpaceShipTwo's $200,000 fare. Virgin Galactic says that, in all, it has taken more than 450 reservations for suborbital spaceflights. One of the would-be fliers, Namira Salim of Dubai, said that today's dedication marked "an exciting moment."

"It's not just about going into space — the dream of the common man," she told me. "It's so much more historic and significant. The development of the private space industry and the contribution that Virgin Galactic is making to private spaceflight will actually enable payloads, researchers, scientists, all kinds of people to go up to space."

Alan Stern, a space scientist at the Southwest Research Institute who has reserved his seat to conduct scientific experiments on SpaceShipTwo, said the rise of suborbital spaceships would be a "game-changer" for his field. Just last week, in fact, Virgin Galactic said it had worked out contract arrangements with NASA for up to three research flights on SpaceShipTwo, with a value of up to $4.5 million. 

Virgin Galactic's flight plan calls for the WhiteKnightTwo carrier plane (also known as VMS Eve) to take SpaceShipTwo (also known as VSS Enterprise) up to a height of 50,000 feet, and then let it go to fire its rocket engine. The smaller plane would blast up to more than 62 miles (100 kilometers) in altitude, past the internationally accepted boundary of outer space. After a few minutes of experiencing weightlessness and gazing at the curving Earth, the riders would glide back to a landing at Spaceport America. The up-and-down trip would take about three hours in all.

Eventually, Branson plans to extend Virgin Galactic's reach to point-to-point suborbital flight as well as orbital outings. "This is a Virgin birth," he joked.

Frederic J. Brown / AFP - Getty Images

Virgin Galactic's WhiteKnightTwo carrier plane flies over Spaceport America in New Mexico with the SpaceShipTwo rocket plane nestled between its twin fuselages.

Matt Rivera / msnbc.com

The SpaceShipOne rocket plane is nestled between the twin fuselages of its WhiteKnightTwo carrier airplane.

During today's half-hour demonstration flight, SpaceShipTwo remained firmly attached to its carrier airplane, and the altitude was capped at about 10,000 feet. But the plane has already gone through 16 free-flying, unpowered glide tests at its Southern California test site — including one last month that was characterized as a "nail-biter." Sometime in the next year, SpaceShipTwo's developers at Scaled Composites are expected to begin powered tests with a hybrid rocket engine made by Sierra Nevada Corp.

Meanwhile, construction work will continue at Spaceport America. The facility is said to be more than 90 percent complete, and the exterior of the "Gateway to Space" bseems to be essentially finished. But lots of work remains to be done on the interior — particularly in the facility's terminal area, where the floors and the walls are little more than bare concrete and unadorned drywall. The hangar, which is designed to hold as many as two WhiteKnightTwo motherships and five SpaceShipTwos, seems cavernous today.

Matt Rivera / msnbc.com

Hundreds of visitors take their seats inside the spaceship hangar at Spaceport America in New Mexico.

Another year should make a big difference, both for the Gateway to Space's interior and for Branson's space aspirations. He's hoping to take a suborbital space trip on SpaceShipTwo with his family for Christmas in 2012. Does that sound too ambitious, coming from a guy who has just rappelled down a 60-foot-high building? You tell me, by registering your comments below.

Matt Rivera / msnbc.com

Virgin Galactic founder Richard Branson walks alongside the WhiteKnightTwo carrier airplane parked at Spaceport America in New Mexico.

More about commercial spaceflight:

• Private spaceship factory open for business
• PhotoBlog: Another look at Richard Branson's antics
• Slideshow: The making of SpaceShipTwo
• Gallery: Ten players in the commercial space race

Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding me to your Google+ circle.