Even though we don't definitely know what causes the 'donuts-on-a-rope' contrails, there are a number of theories.

Theory 1: It's just ordinary contrails behaving strangely.

If you watch the skies for long enough, you'll certainly see some strange formations left by aircraft.

	This is especially true since the introduction of wing-tip fences by major aircraft producers such as Boeing and Airbus, as an aid to increased fuel efficiency by reducing drag caused by wing-tip vortices. These have the effect of creating an airflow that affects the formation of contrails, making them look quite odd. However, these can usually be identified with the aircraft causing them, and tend to evolve over time. While they look peculiar, they don't have the distinct look of the more unusual trails that get people's attention.
The major problem is that by the time the trail gets noticed, the aircraft that caused it is long gone. Occasionaly both are seen and photographed, as in this example (right), taken from Salt Lake City, Utah, by Roger Clark. The aircraft was seen and identified as a four-engined commercial airliner, and yet it produced this peculiar contrail. (Click for a larger image).

Theory 2: It's part of a conspiracy to deposit mind-altering chemicals into the atmosphere.

Oh dear. There's a bunch of people who believe that contrails are actually 'chemtrails'. Fleets of aircraft are criss-crossing the skies, dumping unknown substances into the air in an attempt to modify the consciousness of the public at large. Apparently, the most dangerous are trails that form an 'X' shape. Pity their theory is completely shot full of holes - it might have made an entertaining 'X-Files' story.

Theory 3: It's caused by a high-flying vehicle propelled by a form of propulsion we haven't seen before.

Searching through the 'Black Projects' literature, you'll come across references to a vehicle called 'Pumpkin Seed'. Although there's no evidence for it's existence, some of the reports are consistent. It's been described as looking like a flattened american football, covered in heat-resistant tiles like the Space shuttle, with ducts on the underside that may form the intakes to conventional engines. More unusually, it's described as having a series of openings just aft of it's widest point. This may be due to a 'Pulse Detonation Engine' (PDE). This would operate by emitting a mist of fuel which is then ignited 3 or 4 times a second, pushing the craft forward at high velocity. The result of this would be a contrail that looks like a series of smoke rings.

There are also reports of an aircraft called the 'Pulser', which flies exceedingly fast, but only appears as a rapidly blinking light.

As the 'Pumpkin Seed' wouldn't be able to take off on it's own, it would have to be carried on the back of another aircraft which would release it once it was flying high and fast enough. But what of the carrier aircraft? Could this be 'Brilliant Buzzard'? It certainly fits the bill.

What of the mythical 'Aurora'? Could that produce the contrail? Aurora, as it has been described, has more in common with conventional aircraft, although it's supposed that it would be propelled by scramjets - which wouldn't produced the 'donuts' effect. See 'Black Projects' for more on these aircraft.

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