Was sitting in car looking out window when at my zenith (directly overhead) I saw what I thought at first was a meteorite. It shocked me that it lasted so long and didn't change in luminosity. It was solid light without fluctuation and it appeared to be descending, it was moving very rapidly. I believe it was moving only east to west and it appeared to be falling because of the curvature of the earth and the rate of it's velocity. It continued unwavering eastward from the west valley of phoenix to the east valley. It never changed light intensity until it appeared to be so far off on the horizon (never reaching it before dimming out) that I believe it was actually crossing over to New Mexico. I have witnessed many asteroids and fireballs and I live close to Luke air-force base so I'm accustomed to what regular craft fly like. What astonished me was the distance (below cloud cover), the velocity, and the fact that there was not a sound, no sonic boom whatsoever. My mother who was present at the time also witnessed the event. The object traversed the sky from west valley of phoenix to east valley (approximately mesa) in less than 15 seconds. This places the speed of the object at approximately 19,200 miles per hour - correct me if i'm wrong:approx 80 miles in 15 seconds3600 seconds in an hour80(miles)/15(seconds)=x(miles)/3600(seconds)288000/15=xx=19200 (mph)I have witnessed objects (fireballs) traverse the sky at similar rates and at much higher altitudes than this one and i've heard their sonic booms.... this had no sound whatsoever...