Looking at Twitter’s visitor growth charts every month is like watching a rocket go to the moon. ComScore released its U.S. numbers for April, 2009 today and it shows Twitter reaching 17 million unique visitors in the U.S. during the month, an 83 percent increase from March when Twitter had 9.3 million domestic visitors. While Twitter nearly doubled its audience in April, its monthly growth rate did temper down from the 131 percent growth in March.

Given Twitter’s rocket-like trajectory, it is appropriate that just about two hours ago we saw the first tweet from space. But it still has plenty of places to grow here at home. As it spreads into the mainstream, it is getting a boost from celebrities and TV. For instance, now Nightline is developing a new “Twittercast” show called NightTline which will incorporate viewer feedback via Twitter. If Twitter is experiencing a 60 percent abandonment rate every month, as Nielsen recently suggested, those people sure are being replaced at an awfully fast clip. Meanwhile, everyone and their mother is trying to get into real-time search, an area Twitter seems to have a lock on for now.

Visitors are spending on average 7.9 minutes a day on Twitter, which is twice as much as in December. Domestic pageviews are estimated at 428 million, up from 219 million last month. All of these numbers are just for visitors to Twitter’s Web site, and do not include mobile or desktop clients. They also include people who visit Twitter but don’t necessarily have an account (treat it as a proxy).

Global numbers haven’t come out yet from comScore, but in March it estimated Twitter’s global visitors at 19 million, which was about double the domestic number. So is Twitter worldwide now past 30 million? It could very well be.