Red -orange firey disk that flew past me slowly, and then speedup and was out of sight in about 10 seconds.I was traveling on Highway 85 in southwestern North Dakota near White Butte the highest point in North Dakota. I saw what I believe to be two commercial aircraft flying from east to west approximately 30 miles south of me, actually all that I could see was their strobe lights steadily flashing across the southern sky. As I was driving along and observing these flashing lights, a saucer shaped craft went past me on my right side. The craft was flying vertically, at about 100 feet above ground, about 200 yards to the west of the road, traveling at a speed of about 100mph. The craft then speed up and started gaining altitude,and looked to be heading straight towards one of the commercial aircraft that I was observing.It only took the saucer shaped aircraft about 6 or7 seconds to close the gap between it and the airliner,it looked like the saucer just missed the airliner to its rear and then in about 2 more seconds the saucer was clean out of sight.((NUFORC Note: Date is approximate. PD))