Bright orange orb flown over by landing aircraft at Seatac.I was a passenger on an Alaska Airlines flight, landing at Seatac. We were on final approach, gear and flaps down, lined up with the runway, landing from the south. I had a window seat, 8F, in a 737-400.A bright orange orb came into view, as if it was hovering or floating, not more than a couple of hundred feet below our aircraft, possibly less than that. We quickly passed overhead, and I lost sight of it as it went out of sight behind our plane. It was orange, as bright as a crab pot buoy, a very vivid, intense orange. It appeared to be about 3-4 feet in diameter, but that was difficult to judge due to our relative speed in flying by.It's possible it was a balloon, but it would have been at about 1,500' altitude, and directly in the path of landing aircraft. I was surprised to see more aircraft landing behind our plane, as the orb was so visible, and very close to the approach flightpath. I would have thought our pilots would have seen and reported it as a hazard to navigation, even if it was a balloon.The sun was just setting behind the mountains on the Olympic Peninsula at the time of the sighting, and I was looking out to the east.