AIMing at Earth
Published on Apr 26, 2007 at 1:10 pm.
1 Comment.
Filed under Earth, atmosphere.
Well, that is an ominous sounding title. No, I don’t mean that alien space battlecruisers are aiming destructor beams at Earth. Rather, I am refering to yesterday’s launch of the Aeronomy of Ice in the Mesosphere (AIM) spacecraft aboard a Pegasus XL rocket.
Almost all of Earth’s clouds form in the troposphere, the lowest layer of the atmosphere. However, there are a type of very thin cloud of ice crystals that form in the upper part of the mesosphere, a very poorly studied part of the atmosphere. These clouds are usually at an altitude of about 50 miles (about 80 kilometers). That altitude is the rule-of-thumb definition of how high you need to go to be in outer space. Really, Earth’s atmosphere extends quite a ways beyond that. Technically, the International Space Station, and all of our Low Earth Orbit satellites are really passing through one of the outermost parts of Earth’s atmosphere: the thermosphere. So, these satellites can study the thermosphere. We live in the troposphere. High altitude aircraft can study the lower stratosphere, and balloons can study the upper stratosphere. But, the mesosphere is too high for balloons and too low for satellites. We can look up at it from below, or down on it from above. The only way to study in in situ is with sounding rockets, and then for just brief moments at a time. So, the mesosphere is the least studied and understood part of our planet’s atmosphere. And these clouds that form in the mesosphere are even less studied and understood!
Noctilucent clouds are so named because they are only seen at night, well after sunset or before sunrise, then the Sun is shining on the uppermost part of the atmosphere, but not on the lower layers. This is during astronomical twilight. Typically, noctilucent clouds are seen at very high northern latitudes or very far southern latitudes (near the polar regions). For this reason, they are also given the more official name of Polar Mesopsheric Clouds. But, in recent years, the noctilucent clouds have been seen a farther from the polar latitudes than they have traditionally been observed. Nobody really knows why this is the case. Of course, nobody really knows what causes noctilucent clouds in the first place. Reports indicate that there are more noctilucent clouds after meteor showers, so perhaps the dust from micrometeors plays a role in their formation, though they seem to be ice clouds rather than dust clouds. Suggestions for reasons that the clouds are being seen at somewhat lower latitudes than they used to be range from solar-terrestrial interactions to human induced changes in the Earth’s atmosphere to even variations in interplanetary dust concentrations in the Solar System. But, since nobody understands these clouds, any ideas are pure speculation.
This is where the AIM spacecraft comes in. AIM is a small spacecraft, measuring only 55 inches by 43 inches and weighing only a few hundred pounds. That is why the Pegasus rocket was the perfect launch vehicle for this satellite. AIM has three instruments, one of which is a set of four cameras that look in slightly different directions. This permits a stereo view of the clouds to be generated as the satellite takes pictures from somewhat different perspectives as it passes over the clouds. AIM will look at the ice of the clouds, as well as dust and chemicals in the mesosphere. This will be one of the most complete studies of the mesosphere done from orbit. Reports are that the satellite deployed nicely after yesterday’s launch. After about a month of instrument and spacecraft checkout tests, then AIM will begin its two year mission of studying nocticlucent clouds and the Earth’s mesosphere.
-Astroprof
(Satellite images courtesy of NASA
Noctilucent cloud image courtesy of NASA and Pekka Parviainen)
Ben Anderson WT on March 4, 2009 at 9:51 am: 1
Your article is great. Thanks a lot.