The Hammer Of God is a balanced mixture of these two ACCs. For the first half of the book, one catches glimpses of the world-building that made Rendezvous with Rama such a compelling read, but for the most part it's a dud. The first half is mostly filler. He probably could have cut 60% of it, but even with the first half intact, this is a very short novel. It's a one-day read, maybe two.
On the other hand, the second half of the book is more than worthwhile. I'm sure that's the half which interested Spielberg when he optioned the book into a movie. Why his production company made Deep Impact instead, I'll never know.
"I have great faith in optimism as a guiding principle."
A little background: humans are living not just on Earth, but on the moon and Mars. One of the world's fastest growing religions has been started by a woman who, inspired by a tour of duty in Desert Storm, decided to combine Christianity and Islam into something new. Partly because Chrislam isn't as prude as most religions when it comes to sex and other modern desires, it becomes popular pretty quickly. When Earth receives what appears to be a deliberate radio signal from another star system, Chrislamists preach it's a message from God. In the same way paranormal investigators unwittingly construct tools to give them the false positives they're looking for, Chrislamists use special methods to insure they get exactly the message they want to extract from the signal. More on these bozos later.
ACC recycles ideas. Since this is the guy who first wrote about the geostationary satellites we rely on today for communications, can you blame him? In Hammer of God more than 90% of all asteroids and comets in the solar system have been cataloged by SPACEGUARD. Does that program sound familiar? It's because he made it up in Rendezvous with Rama, but in the years between that novel and Hammer, it became a reality. In Hammer, he writes about the real SPACEGUARD, interestingly enough, rather than the one he imagined. Nevertheless, it's an amateur astronomer living on Mars who originally detects a doomsday asteroid on a collision course with Earth.
(The real world implications are frightening. Consider: in the next thousand years, a catastrophic collision is expected to occur on Earth, the moon, or Mars. If you plan on staying on Earth for the rest of your life, as most of us will no doubt have to do, this is more or less like being forced to play spin the bottle with a loaded gun and only three players.)
Thankfully, there's a spacecraft within rendezvous distance of the doomsday asteroid (the same thing happened in Rendezvous with Rama), which scientists dub Kali, after the goddess of destruction. Astronauts plan to touchdown on the asteroid and attach a thruster system known as ATLAS, which will nudge Kali out of its current trajectory. ATLAS, however, requires a mindbogglingly large amount of fuel, which takes a month to acquire. By the time they get it, Kali is within the orbit of Mars—frighteningly close to the homeworld. No worries, though, because things seem to be smooth sailing once they get their fuel. The astronauts land on Kali, attach ATLAS, turn the system on and, surprise-surprise: it's been sabotaged by Chrislamists. It turns out they believe only God should decide whether or not the asteroid collides with Earth.
This is when the book gets good. And I mean really good. The astronauts devise one plan after another, only to encounter problems left and right. The scientists and politicians back on Earth decide to take out an insurance policy: a hastily constructed nuke which they plan to fire at the asteroid when all is lost. If the astronauts succeed, the scientists will simply send a deactivation signal. If the astronauts fail, they'll allow the nuke to continue as planned. As you can imagine, things don't work out as simply as that.
I'm tempted to tell readers of The Hammer of God to start at its halfway point. They shouldn't. The second half really is worth the first half.