This could be it. The debris found in the southern Indian Ocean does seem to be consistent in size with parts of a Boeing 777, and the location puts it within the limits of the range that could have been reached by Flight MH370.

If so, this will be an unexpected and extremely welcome turn in the search. Many experts were becoming resigned to the possibility that it could take months, until debris was washed up on some distant shore, before the first firm clues would emerge to this mystery.

Right now the first key to assessing the plausibility of this discovery is: what would float and what would not?

The largest part of an airplane that could remain on the surface is the wing. The wingspan of the 777 is 200 feet. John Young, of the Australian Maritime Safety Authority, has reported that the largest piece to show up on the satellite images is about 78 feet long—length is a critical measure because the 777’s wings are long but also very slender. This size would conform to one half of the wing.

The heaviest part of the airplane are the engines. They are attached to the wings on pylons and under any impact will break away. In the ocean they would sink like bombs.

The most robust part of the airplane structure—it carries all the principle aerodynamic loads—is the wing box, where the wings meet the fuselage. This is also heavy and would probably sink. Like the wings, the tail surfaces—horizontal and vertical—easily break away from the fuselage and float.

The vertical stabilizer of Air France 447 was the first major piece of debris to be found after that Airbus went missing in 2009, instantly recognizable by the airline’s decals. It would be the same with a 777. This is definitely something searchers would look for to confirm the finding.

The fuselage, essentially a long narrow tube, would shatter into several parts, and the outer shell would be buoyant. Then it would be a matter of how the airplane impacted the water, the way it broke up, and the formation of the debris field above and below water. Life savers would be bobbing around among smaller pieces.

One of many intriguing things about this development is the deployment of satellites and how it has been directed. One minute we’re talking about a great void with no watching eyes, the next it would seem that there are assets available that can be rapidly deployed.

Until now it has been the Inmarsat satellite owned by a London company that has been the only means of gauging where the 777 might have gone, from the pinging signals indicating how long the airplane was “alive.” Many countries are collaborating in this search, and some have far more resources than others. Intelligence agencies are very reluctant to reveal their assets and in this case might well have been “silent” partners in the search—satellites can be moved from their regular orbit in an emergency, as they were when the Libyan uprising began.

Malaysian Transport Minister Hishammuddin Hussein described this new satellite imagery as a “credible lead.” Unfortunately we have learned over the past 12 days that the Malaysians themselves are frequently far from credible in their statements, but in this case the Australians, though more cautious, are clearly taking this as a very promising development.