High Altitude Strobe Light.

This evening, my husband and I were outside on our deck observing the stars.  As we were watching a satellite traverse from north to south, we were surprised to see a very bright, white strobe light flash in the same vicinity, at a very high altitude.  About 15 seconds later, it flashed again, having moved only a very short distance in the sky.  We observed it flash only 5 times, moving west to east, but at what appeared to be a very slow speed, or at an extremely high altitude.  It moved only a few degrees across the center of the sky before the strobe disappeared.
My husband has been a commercial and military pilot for 35 years.  He was not familiar with any craft that does not either show a constant light, such as a satellite reflecting the sun, or a known commercial or military aircraft with the required flashing white and red lights.  He has never seen a flying aircraft of any kind that carries only a strobe, with 10-15 seconds darkness between flashes, and had no explanation for what this strobe could have been.