 limited, but when meteorologists but that boundary condition of that vector into their overall wind models, they get 75 to 80 percent knowledge of the total wind vector.”
The long wait has not tempered expectations for Aeolus.
“We were initially promising that we would be flying in 2007, or at least in early 2008,” said Elfving, who became Aeolus project manager in 2010. “Yes, we have had a 10-and-a-half or 11-year delay. My director said we are not ashamed at all of that. We are very proud to have sustained through all the troubles because there is actually no other comparable mission implemented in the world.”
Engineers determined Aeolus needed an ultraviolet laser to give Aeolus the capability to monitor global winds. A laser operating in visible wavelengths, such as the laser aboard the U.S.-French CALIPSO satellite, would only be tuned to track cloud motion.
Ultraviolet lasers flown in space on previous tech demo and Earth science missions have not succeeded. Most recently, an ultraviolet laser on NASA’s Cloud-Aerosol Transport System instrument on the International Space Station failed soon after it was activated in 2015.
Some of the major challenges that faced the Aeolus team included selecting materials that can cope with the high-temperature output of the laser.
“We pulse our optics 50 times every second, and this goes on year after year, and this intense pulse creates 1,700 degrees on the optical surface of the optics, and will fatigue the optics in the long run,” Elfving said. “So we have had tremendous development of the coatings of the optics to withstand this high laser pulse. We are actually our own sun.”
Managers ordered one of the most significant redesigns of the ALADIN instrument aboard Aeolus to ensure the sensor’s optical surfaces remain free of contamination.
“If there are some remaining hydrocarbons on the surface of the optics, and the UV light hits it, it will carbonize, so blacken, the surface and reduce dramatically the reflection or transmissibility of the optics,” Elfving said. “That has been a pain for many missions trying to do this.”
Despite strict clean room controls on the ground, all space missions launch with some amount of contamination, which can outgas in orbit once heated by the sun. The high-energy laser aboard Aeolus could also lead to outgassing as it warms up materials inside the ALADIN instrument.
Engineers determined they could flush the ALADIN instrument with tiny injections of pure oxygen throughout the Aeolus mission. Two high-pressure tanks carrying a combined load of around 31 pounds (14 kilograms) of liquid oxygen will gradually supply the instrument once in space.
“We have a regulation system which takes the high pressure oxygen down to an extremely low pressure in orbit — 40 to 60 pascal — which is an incredibly light pressure, very few molecules,” Elfving said.
The amount of oxygen gas inside the instrument in space will be equivalent to the oxygen produced by a small plant indoors, according to ESA.
“But it is enough to cause combustion of this organic material which always remains on the optics,” Elfving said. “It helps to catalyze and burn away that, instead of carbonizing it and blackening the optics.
“Those two, I would say, have been the main challenges — laser-induced contamination and laser-induced damage,” he added. “For the rest, we are talking about stabilities in alignment of microns or nanometers, and that’s obviously a tough challenge, especially when we are pumping a lot of energy into the laser.
“That put us between a rock and a hard place,” Elfving said. “So it has taken us about 10 years to get from a prototype laser to a flight-worthy laser with a lifetime that meets our requirements. In the beginning, due to this laser-induced contamination, we lost about 50 percent of the energy within hours, and we now we have run a laser with absolutely no degradation for six months and beyond. So we’re very confident that have achieved the state of the art.”
Concerns about the sensitivity of the Aeolus satellite’s laser instrument drove managers to transport 