Low, slow moving craft over NASA Goddard Space Flight Center

I don't know the exact date I saw this -- I can say that it occurred between 1984 and 1988.
My mother, father and I were at the Cipriano Square shopping center which is right across the street from the main entrance to the NASA Goddard Space Flight Center in Greenbelt, MD. My mother and I had just finished using an ATM when my father, who was waiting next in the parking lot, waived us over quickly to point out the unidentified object.
The object was hovering at approximately 1000-2000 feet, about 1/2 mile SE of the shopping plaza. The shape of the object was not discernable, however it had, as I recall, three white lights, situated what I assumed to be at its perimeter. 
The most noticeable thing about this craft was that it made no noise and that it moved so slowly -- very slowly. The object was easily 300 feet long if all the lights observed belonged to the same craft. It moved very slowly to the NE.
This craft did not have position/navigation lights typical of FAA approved aircraft.
Witness indicates that date of event is approximate.  PD