Methinks what the passengers saw was a meteor on the last throes of breaking up in the atmosphere.
August and September do have a lot going on when it comes to meteors.
If it really was a MANPAD, it might have been a old Soviet Strela. They have a nasty habit of going anywhere *but* in the direction of the target - assuimg it doesn't detonate on launch or at some random point in flight (usually close to the launcher, ruining the operator's day). There are literally millions of Strelas running around the world arms markets. They're more dangerous to the operator than to an aircraft, but they're cheap and the operator might get lucky. Me, I'd stick to throwing rocks at a landing or launching plane, it's more reliable and more effective. :-P
Of course, what they probably saw in all reality was some kid setting off a big firework or something equally stupid and innocuous.
Could you please explain to me the reasons that this manpad must come down on land? You might want to look at a map of JFK airport first.