where d = distance between the car and the object.

This formula is arrived at as follows: Calling I_c the intensity of the car headlights at a distance of 10 feet ahead of the car, I_u the intensity of the unknown source at distance d, and P the optical power output of the object we can write:

                 

and

The fact that the headlights were washed out by the unknown source at a ten-foot distance provides a lower limit for I_u. If we assume that we can detect a "just noticeable difference" (JND) between I_u and (I_u + I_c) we write:

which leads to:

or P=150d².

In his report, Dr. Condon estimated the distance at 2,400 feet, which gave an energy of 9 X 10⁸ W (900 megawatts) for the UFO. A more correct estimate is given by the subsequent investigation since the clearing is actually located 1,800 feet from the observation point. The energy output becomes 5 X 10⁸ W (500 megawatts). These figures are approximations only: As David Newton has since pointed out in correspondence with the author, the fact that the car headlights were not radiating uniformly in all directions but were directed onto the road by reflectors, should be taken into account in any refined calculations.

Case no. 4: November 5, 1976. Grenoble (France) -- Classification: MA-1

Another remarkable observation made near Grenoble, France, on November 5, 1976, by a senior French scientist is relevant here. As in the previous case, there were multiple witnesses and the duration was long enough to allow details of the object and its trajectory to be seen and recalled. There were two other remarkable characteristics: first, it was possible to establish the distance of the object with precision; second, the exceptional qualifications of one of the witnesses provided some physical parameters that have rarely been available in UFO cases.

We are indebted to GEPAN, the French government's official UFO investigation task force (now known as SEPRA), for communicating to me the details of the case, which the author had the opportunity to discuss with them at length