On Tuesday June 5, Earthlings will be able to observe a rare celestial event: the silhouetted disk of the planet Venus will slowly pass in front of, or transit, the disk of the Sun. The event will last just under 7 hours, and will be directly visible by people living in the U.S. and parts of S. America, Africa, Europe, and Western Asia.For those not able to view the event (because it will be night-time), the magic of the Internet will enable them to watch in real-time through the streaming Webcams and telescopes of others halfway around the planet. NASA, for example, will be streaming the event live at http://venustransit.nasa.gov/transitofvenus/.
There's a lot of hype over this celestial event because it occurs so rarely. The orbit of Venus is tilted slightly (about 3 degrees) relative to the orbit of the Earth, so even though Venus is closer to the Sun than we are and thus passes between the Earth and the Sun roughly every 584 days, the different orbit tilts of the planets make Venus usually pass above or below the disk of the Sun as viewed from Earth. Thus, rare transits of Venus across the Sun's disk only occur when the Earth, Venus, and Sun line up just right. In fact, these kinds of alignments occur only about once a century (more detailed diagrams, descriptions, and links can be found at http://eclipse.gsfc.nasa.gov/OH/transit12.html).
In detail, Venus transits occur in pairs about 8 years apart, but with either 105 or 121 years until the start of the next pair. The June 5 transit is the second event in the only pair this century -- the first one in this pair occurred in June 2004. The previous pairs were in 1874 and 1882, and 1761 and 1769 (the latter famously observed from Tahiti in a scientific expedition) by Captain James Cook and crew. The next pair of Venus transits won't be until 2117 and 2125. So, unless you are lucky and healthy enough to live for another 105 years, this will be your last chance to see a Venus transit from the surface of the Earth.
But -- aha! there's the catch -- "from the surface of the Earth." I think the odds are good that this need not be the last transit of Venus that you will ever see. Let me explain why.
The future of human exploration of space is at a crossroads. Except for one glorious adventure, NASA and other space agencies have mostly focused on sending people into low Earth orbit -- just a few hundred miles up -- to conduct medical or scientific experiments, to launch and repair satellites, or just to maintain a presence at the high frontier. That one glorious adventure, of course, was the Apollo program of six successful human missions to orbit, land, walk, and drive on the Moon, more than 40 years ago. But then we never went back, or beyond.
With the recent retirement of the Space Shuttle fleet, NASA is now getting out of the humans-to-low-Earth-orbit business, and instead trying to focus on building a new deep space rocket that can carry astronauts back to the Moon, or to nearby asteroids, or all the way to Mars and its moons. Low Earth orbit may instead become the realm of private enterprise. Aerospace companies large and small have won a variety of NASA contracts to deliver cargo and, eventually, people to/from the International Space Station (ISS). Witness the recent historic launch and docking of the world's first private corporate spacecraft -- SpaceX's Dragon capsule -- with the ISS! It's a heady and exciting time, akin to the beginnings of commercial aviation in the first third of the 20th century -- a business that was also initially seeded by heavy government contracts and incentives.
The thing is, there's no reason that private, commercial space operations need to be restricted to low Earth orbit. There are plans and ideas for hiring corporate spacecraft to potentially deliver fuel and other supplies to farther destinations, like to geosynchronous orbit, lunar orbit, to more distant "parking orbits" called Lagrange Points, where the gravitational balance between the Earth, Moon, and Sun are in balance, or even back to the lunar surface. If there are going to be space delivery and maintenance services zipping around in and near the Earth-Moon system, then it's only natural to believe that some of that precious cargo will eventually include paying tourists. And that's where Venus transits come back in.
I'm a big believer in the future of what I call "space adventure tourism." In the not-too-distant-future, I believe that a lucrative industry will emerge that is comparable to the cruise line or adventure tourism industry today, but in space. Space Cruises, Inc. (and competitors) will take people on the ultimate adventure, to see the Earth from high above in zero-g, to skim past the Moon's surface or zip past a small near-Earth asteroid. Initially the prices (and risks) will be very high and only affordable by the very rich, like the billionaires who, already, purchase tourist rides today to low Earth orbit from the Russian Space Agency. But eventually -- maybe by the middle to the end of this century, I believe -- the prices (and risks) will come down and such adventures will be affordable to middle class families, like some ocean cruises or tours to tropical or exotic destinations are today.
Traveling away from our home world offers people new perspectives, and I mean that literally: the angles between the Sun, Earth, Moon, and planets change once you get off the Earth's surface. Things you couldn't see from down here could become visible from up there, if you're in the right place at the right time. For example, a skilled space navigator could maneuver his spaceship full of tourists to a place high above (or below) Earth's poles and catch the disks of Venus or Mercury transiting in front of the Sun much more frequently than can be seen from Earth's surface. Maneuvering to other positions far above Earth's equator and getting the timing right could allow space tourists to routinely witness total solar eclipses -- another rare event for those of us on the surface, but potentially a routine space tourist experience in the future. Many other kinds of "manufactured celestial events" will be possible to witness once access to near-Earth (and lunar) space becomes more routine. Such access may never become as routine as airline travel is today, but the trend will surely be for it to become safer and more affordable over time.
I plan to watch the tiny black orb of Earth's sister planet Venus slowly march across the setting Sun's disk on Tuesday June 5, no doubt along with millions of other space enthusiasts. It will indeed be the last time any of us will see this rare and lovely natural celestial event from here. But I am planning to see it happen again, maybe just a few decades from now. Next time it will be from a different perspective, on a ship full of fellow space adventure travelers, many of whom will have scrimped and saved to spend their vacation on a new kind of adventure: chasing the thrills of a manufactured celestial event.
Jim Bell is an astronomer and planetary scientist, a Professor in the School of Earth and Space Exploration at Arizona State University in Tempe, and the President of The Planetary Society, the world's largest public space advocacy organization. He is the lead scientist for the Pancam color stereo cameras on the NASA Mars rovers Spirit and Opportunity, is a member of the science camera team on NASA's Curiosity rover, and has authored several space photography books, including "Postcards from Mars," "Mars 3-D," and "Moon 3-D."