Simpler, less extraterrestrial explanations should first be considering before jumping to UFO conclusions. There are so many things flying around up there, on purpose.
What was that floating (sometimes stationary) bright, glowing object hovering over the Valley Monday evening?
Did you see it? Some thought it was a UFO, a weather balloon or even an alien spaceship.
The National Weather Service in Phoenix confirmed that it was a student research balloon, part of the Columbia Scientific Balloon Facility.
HASP 643N, the balloon’s official name, launched from the Physical Science Lab at New Mexico State University. located north of Roswell, N.M. That’s approximately 471 miles away from Phoenix or some 412 nautical miles.
The 400-ft wide object (as noted for this type of balloon) can surely be seen from the ground even though it travels at high altitudes. You can read about the project here. More flights are planned so UFO researchers might want to keep track of these launches as obvious explanation for UFO reports.
Benjamin Radford writes more in this piece: Phoenix ‘UFO’ Revealed as NASA Experiment : Discovery News. Several other “mysterious” sightings of things in the sky are not mysterious at all but have man-made explanations.