At 10:31 pm five of us saw something like a fireball or burning-up space junk fly very slowly over the Denver area from the south part of the sky, directly overhead, continuing until it disappeared behind houses to the NNE. Two original witnesses were in the back yard getting exercise when they first noticed the light. They ran inside from the back yard and got 3 more witnesses to go out front of the house where we watched it continue across the sky. The white object pulsed brightly several times when straight overhead, at roughly 2 seconds intervals. It had a trail at all times it was visible, about 3 or 4 degrees in length (6 to 8 times the width of the moon). Toward the end of its flight it appeared to break into two pieces. Having the witnesses individually reenact the sighting pinned the timing down to 50 to 55 seconds. I was in the basement and because it took me 15 or more seconds longer to get notified I saw it the least of anyone, but was still able to see a dim rusty trail drift down towards the north-north-east horizon and slowly disappear behind the roof line of a house across the street, probably 5 degrees above the horizon. Its angular velocity appeared faster than an airplane or the ISS (we watch that often), but much slower than any fireball we've ever seen. At its brightest its pulses might have been as bright as the full moon according to the original two witnesses.