Israel is claiming that Iran is thisclose to developing a missile that can hit American soil. But missile and intelligence experts say Tehran has a long, technically complex road to travel before it can threaten Manhattan.

From getting all the rocket thrusters to work properly to developing heat shields that can withstand the stresses of rapid atmospheric reentry, Iran is probably many years away from getting an intercontinental ballistic missile (ICBM). The American spy apparatus, which once hyped the Iranian missile threat, has quietly stopping saying when Iran can hit the east coast. And the irony is that it's taking Iran so long precisely because its missile efforts really are sophisticated.

"The bottom line," says Paul Pillar, a veteran CIA Mideast analyst, "is that the intelligence community does not believe [the Iranians] are anywhere close to having an ICBM."

That, however, isn't the message out of Jerusalem. Israeli Finance Minister Yuval Steinitz told CNBC on Wednesday Iran was "two to three years" away from slamming a missile into New York, Boston or Washington. Its strategic-affairs minister, Moshe "Bogie" Yaalon, issued that same warning earlier this month, but declined to say when Iran's mega-missile would be ready.

Chances are, the Israelis are hyping the Iranian missile threat so their American friends will consider the Iranian threat more acute. They're not happy with Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, for saying on Sunday that an Israeli attack on Iran was "not prudent." But few missile or intelligence experts believe the new claim of an imminent Iranian ICBM is going to change Dempsey's mind, or anyone else's, because it's far-fetched.

It's true that Tehran has a robust missile program. Its stockpiles of Shahab-3 ballistic missiles, which top out at 800 miles, strike fear into the hearts of Arab Gulf states. Israel has real reason to fear the development of its Sejjil medium-range ballistic missile, a more sophisticated weapon, that could maybe reach Israel in a few years. And unlike rogue-state missile flameouts like North Korea, Iran is able to launch satellites into space, which is a key ICBM step (since any intercontinental missile is going to have to fly through space in order to attack a foe so far away).

But none of that adds up to Iran getting a missile that can travel the 6,000 miles necessary for striking America any time soon.

For one thing, Iran needs to master what's called "clustering" of the engines needed to power its missile. Picture a box with an engine – probably from a North Korean Nodong-2, the paterfamilias of Iran's missiles – on each corner. Iran in fact unveiled precisely such a design in 2010.

There's a long way between design and a working set of thrusters, however. Basically, in order to keep the missile on track as it streaks through the heavens, each engine has to provide precisely the same amount of thrust. If not, the pulses of acoustic energy from one engine might destroy another. "That's not an easy thing, to make sure they fire simultaneously and don't shake themselves to death in process," says Greg Thielmann, a former missile analyst at the State Department's intelligence wing.

Then there are additional technical obstacles Iran isn't believed to have overcome. Guidance systems need to be able to withstand the pressures of atmospheric reentry to keep the missile on course. "Then the warhead itself has to function at such extreme physical conditions," says Hans Kristensen, director of the nuclear studies program at the Federation of American Scientists. "There are several really complicated steps they have to go through to do this."

As well as mundane ones. Iran will have to balance durability and weight, most likely leading it to want aluminum alloys for any early long-range missile rather than heavy steel, says David Wright of the Union of Concerned Scientists, and Tehran may not have the aluminum stocks for it. It may not even have the machine tools necessary for boring out an ICBM's diameter. And ICBMs are big, meaning if they stay in one place for too long, they'll be vulnerable to detection – and an Israeli or American bombing campaign. That's why Wright thinks they'll have to be rugged enough to survive being moved along the roads, which can also create convoys ripe for the schwacking.

Even Iran's seriousness about its missiles is a potential time-suck. Unlike North Korea, which rushes to claim military glory even when its missile tests fail, Iran tends to test its missile stockpile thoroughly. Long-range missiles tests will likely mean Iranian ships heading out into the Indian Ocean to collect data and telemetry. Which are also vulnerable to detection, giving the U.S. and Israel some early warning.

"You're gonna know whether this happens. You're gonna see at least one flight test of this bigger stage," Wright says. "We haven't seen them develop reentry vehicles on something this long-range."

Bottom line? Iran is probably "five to ten years away" from an ICBM, Wright thinks. That seems plausible to other experts interviewed for this story, though most demurred from making an actual prediction.

That's similar to what U.S. spy agencies used to estimate. Emphasis on the used to.

In 1993, the CIA told Congress Iran was "10 to 15 years" away from an ICBM. An assessment of the missile threat in 1995 drawn from across the 16 intelligence agencies, called a National Intelligence Estimate, pegged the date for doomsday at 2010.

That actually caused a freakout from Republicans in Congress, who accused the CIA of understating the urgency of the Iranian missile threat to help the Clinton administration stall on missile defense. They in turn asked Donald Rumsfeld to chair a commission on ballistic missiles. Rumsfeld's conclusion, in 1998, was that it would take Iran at most five years to build a long-range missile; and he warned the Iranians may have already decided to do so.

All of those estimates were ultimately proven to be unrealistic. The Iranians didn't have an ICBM in 2003, they didn't have one in 2010, and they don't have one now.

Lately, the spy agencies have sung a different tune. When James Clapper, the director of national intelligence, briefed Congress on the Iranian missile threat this month, he notably declined to predict when Iran would get an ICBM. Same with his predecessor, Dennis Blair.

"That's probably a tacit acknowledgement that they really don't have much to say," says Pillar, who was the top Mideast analyst for the entire constellation of intelligence agencies during much of the last decade. "It's far enough away to say 'they're so many years away,' or you don't say anything at all."

Israel has lots of reasonable fears about Iran, a country whose president denies the Holocaust while implying that he'd love a second one, and which appears to be building a nuclear weapon. It doesn't have to make all kinds of extra concerns up.

Additional research assistance provided by Robert Beckhusen.