Days before U.S. Defense Secretary Robert Gates is scheduled to meet with his Chinese counterparts to improve strained relations, leaked photos of what appears to be China's first known stealth aircraft have popped up on the internet. The "radar-evading" fighter jet, the J-20, was photographed in late-stage runways tests, suggesting that it's days away from its first test flight. The U.S. military wasn't expecting a Chinese rival to the F-22 Raptor — the world's only fully stealth fighter jet — for several years. (Watch a comparison breakdown of the two jets.) Here, a brief guide to the J-20:

What do the leaked photos tell us?
Experts are divided on whether the photographed plane is an elaborate mock-up or a real, stealth J-20 prototype. If it's real, the J-20 appears to be bigger and heavier than both the F-22 and the smaller U.S. stealth F-35, so presumably it's capable of carrying enough fuel to fly far beyond Chinese airspace. The J-20's size also suggests it would carry air-to-surface missiles and other heavy weaponry.

Why are the photos showing up now?
Many analysts think China wanted them to hit the internet. The J-20's high-speed taxi tests were conducted at an unsecured airfield in Chengdu frequented by camera-wielding air enthusiasts, and China's internet censors have allowed the photos to stay online. "This is their new policy of deterrence," speculates Andrei Chang, in the Canadian journal Kanwa Defense Weekly. "They want to show the U. S., [and] show Mr. Gates, their muscle" right before his visit to Beijing.

How did China catch up so fast in stealth avionics?
The J-20 is at least partly the fruit of China's aggressive, 15-year-old push to modernize all facets of its People's Liberation Army. U.S. officials say they aren't sure how much China has spent on the effort. Another lingering question is "the degree espionage has aided the development of the J-20," says Bill Sweetman in Aviation Week. Starting in 2006, about the same time the J-20 program likely began, the U.S. military and its defense contractors noticed sophisticated infiltration of their computer networks.

Does anyone else have a stealth jet?
Russia is believed to have one in the works. Its stealth fighter, the Sukhoi T-50, made its first test flight a year ago.

How much of a threat is the J-20 to the U.S.?
Not much, says Paul Wallis in Digital Journal. This prototype is "basically a showroom model," and the working model will probably be obsolete before it hits the skies. If China were serious about "air superiority," it would ditch this "fledgling show pony" and just "buy a few thousand Sukhois" from Russia. I wouldn't be so sure, says China military expert Richard Fisher. Judging by the photos, the J-20 has "the potential to be a competitor with the F-22 and to be decisively superior to the F-35."

So what's the big picture?
Whether or not the J-20 is a real rival to U.S. stealth fighters, China's big military investment is now showing results: U.S. officials were unpleasantly surprised at how far China has advanced on its "carrier-killer" ballistic missiles, a threat to U.S. aircraft carriers, and are bracing for the deployment of China's own first aircraft carrier. "From a U. S. perspective it comes down to an issue of whether the United States will be as dominant in the western Pacific as we always have been," says Bonnie Glaser at the Center for Strategic and International Studies. "And clearly the Chinese would like to make it far more complicated for us."

Sources: Bloomberg, Aviation Week, AOL News, Wall Street Journal, New York Times, Digital Journal