A Mars probe's tank is expected to melt during re-entry, so there's no need for a missile intercept, like in 2008, experts say.
Russia's failed Mars probe is due to fall to Earth on Sunday or Monday.
The spacecraft, stranded after a botched launch on Nov. 8, has more than 10 tons of toxic rocket fuel aboard.
The fuel is expected to incinerate during the plunge through the atmosphere, eliminating the need for a missile intercept.
When a botched launch stranded a U.S. spy satellite close to Earth four years ago, the military took action.
Concerned that the satellite's rocket fuel could pose a contamination risk if the uncontrollable spacecraft re-entered the atmosphere over populated areas, the Pentagon ordered the Navy to shoot down the crippled bird so any debris would land harmlessly in the ocean.
The operation was successful and the threat from 100 pounds of rocket fuel averted.
On Sunday, Russia's failed Phobos-Grunt Mars probe, which carries more than 10 tons of propellant, is expected to plunge back to Earth, though experts say this time the fuel, despite its great mass, isn't as much of a concern.
Russia says the spacecraft, which was designed to return soil samples from the Martian moon Phobos, has fuel tanks made of aluminum, which will not survive the fiery plunge through the atmosphere. Phobos-Grunt ("grunt" is Russian for "soil") was left uselessly orbiting Earth after an upper-stage engine failed to fire following launch Nov. 8.
The military won't comment about its hardware, but experts believe the spy satellite's tank was made of stronger, more heat-resistant metals.
"Some say it was titanium, though I think that's a heck of an investment in a tank," Michael Simpson, director of the Secure World Foundation in Colorado, told Discovery News.
"Anyway, there was the belief that the tank was less vulnerable (to heat)," he said. "It wouldn't be surprising to me at all if it was more robust than simply aluminum -- the military does tend to over-engineer."
The United States' decision to shoot down its satellite raised questions about whether the real motive was to test an anti-satellite weapons system. A year earlier, China fired a ballistic missile to destroy one of its defunct weather satellites, generating a gigantic field of orbiting debris and earning widespread condemnation for its act.
"Right now, the Russian Federation and the U.S. are at loggerheads over the United States' deployment of a ballistic missile shield in Europe. And back when the U.S. intercepted its crippled spy satellite, it used a sea-based component of that ballistic missile system to actually perform the intercept. It did so very nicely on the first try," Michael Listner, a New Hampshire-based attorney specializing in international space law and policy, told Discovery News.
"I think it would put the Russian Federation in a very uncomfortable position in terms of the negotiations of our missile defense system in Europe, by asking us to use it to help intercept one of their satellites," he said.
"The United States can't just unilaterally say 'We're going to shoot it down.' That would violate international law, big time. Then the Russian Federation would have a very legitimate gripe if we did that," Listner said.
As of Friday, the 14-ton Phobos-Grunt was projected to re-enter the atmosphere over the Atlantic Ocean, east of South America, on Sunday or Monday. Some fragments are expected to survive re-entry, though with 73 percent of the planet covered by water and vast regions of land uninhabited, the chance any debris will fall on populated areas is very slim.
"The odds that this is going to be a threat to anyone is extremely remote, but when things fall out of the sky it makes people nervous," Simpson said.