Bright light seen moving slowly across sky at helicopters height with no noise and a 2 second rear-end emittance.Yellowish-white light abruptly lifted into sky close to or in eastern part of Big Horn Mountains and slowly moved eastward until it vanished into the night. It made no noise and was one bright yellowish-white color very close to the ground, approximately a helicopters height in the sky. At one point within the first few miles it seemed to emit a type of gaseous subststance out of the rear-end, which was only seen because of the haze around it and only lasted about 2 seconds, but it did not seem to speed up any afterwards.((NUFORC Note: Witness elects to remain totally anonymous; provides no contact information. PD))