AIM

Launch Date: 25 April 2007
Location: Vandenberg AFB, California, USA
Launch Vehicle: Pegasus
Orbit: Sun-synchronus
Inclination: 97.8 degrees
Period: 96 min, 32 sec

Exploring clouds at the edge of space - AIM website

That pretty much says it all. AIM (Aeronomy of Ice in the Mesosphere) was launched to study the noctilucent (also called polar mesospheric) clouds at the highest levels of our atmosphere (the first to do so). Extending from 50 km up to 80-90 km up, the mesosphere is between the maximum altitude for aircraft and the minimum for spacecraft. Lack of study of this region and the lower thermosphere above it has led to it sometimes being referred to as the ignorosphere.

The AIM mission has been extended until September of 2012.