Although still three years from starting actual scientific missions, the Stratospheric Observatory for Infrared Astronomy (SOFIA) airborne observatory is tenaciously getting closer to its first job day. After two decades of research and $500 million modding a Boeing 747—including the 2.5-meter telescope itself that you can see tested in this video—SOFIA got a High-speed Imaging Photometer for Occultation two weeks ago, an instrument that will help it to measure objects' surfaces and atmospheres. Now, NASA is completing final tests at their Dryden Aircraft Operations Facility before its first open-door flight later this year.

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying TelescopeS

Inside NASA's 747 Flying Telescope

Why twenty years and so much money spent in this observatory? Well, while it's not as spectacular as a space telescope, the daily challenges that SOFIA will face are greater than those of Hubble. From the high-tech door system in the modified 747 to the technology needed to compensate for the extreme in-flight vibration, SOFIA needs a lot more daily love than Hubble does. In fact, it's not that expensive: The $500 million price tag is a bargain when you consider the $2.5 billion paid for the Hubble Space Telescope's construction. Not to talk about the Hubble's total bill, including the servicing missions, which is estimated at between $4.5 billion to $6 billion without the more than half billion dollars that Europe put into the project. For sure, SOFIA is not as flexible and won't take the same kind of breathtaking photos that Hubble does. The telescope is designed to only work on the infrared and far-infrared light spectrums. But then again, in those light ranges, flying will allow it to get results as good or better than Hubble (it's mirror is almost 4 inches larger than Hubble's). After all, SOFIA won't have to deal with 99% of the atmospheric vapor that disturbs these kind of instruments at ground level. On the other side, repairing it won't require dangerous and costly space missions. If it breaks down, they will fix it in the hangar. Tests for SOFIA will being this month, while actual science missions will start in 2011, getting into full capability in 2014. [NASA]