September 2, 1999
Web posted at: 1:18 p.m. EDT (1718 GMT)

(CNN) -- Earth is in danger! Humanity must act quickly or face extinction!

From "War of the Worlds" to "The War against the Chtorr" to "Independence Day," the premise of an ultimate threat to the birthplace of the human race has produced a seemingly endless array of science fiction stories, novels and films. Some are instant classics, like "Footfall" by Niven and Pournelle. Others, like Hubbard's "Mission Earth" series, are instantly forgettable. The idea of Earth imperiled is a literary flame. Many, many writers are drawn toward it.

Each of them faces the same challenges -- to devise a credible threat and to craft an ingenious response to it. Along the way, each writer has the opportunity to employ the entire arsenal of science fiction themes, from the nature of life on other worlds to the nature of the human spirit.

Two writers take up these challenges in new novels. The authors couldn't be more different. Frederik Pohl is a grand master of science fiction whose connection to the field dates back to the 1930s. He has distinguished himself as an agent and as an editor. But he is best known for his dozens of books. Ken MacLeod, on the other hand, is a relative newcomer. He has written three novels, only one of which has been published in the United States. His first two garnered honors in the United Kingdom.

Their books are as different as the authors. "The Far Shore of Time" is the conclusion to Pohl's "Eschaton Sequence." Despite those rather ponderous names, the novel is almost whimsical in tone. "The Cassini Division," on the other hand, has a very clever title, yet its tone is deadly earnest.

The threat in "The Far Shore of Time" is posed by aliens. A star-hopping race called "Beloved Leaders" by the species they subjugate -- and called "The Others" by everyone else -- is intent on enslaving Earth. Agent Dan Dannerman has been assigned to assess the threat and help devise defenses. Before the opening of this, the third novel in the series, Dan was captured by the Beloved Leaders, whose goal is to bring on the eschaton, the final collapse of the universe. Now, he has been liberated by another race, the Horch. It's not much of an improvement. He's still being tortured. His only consolation is that his jailer from The Others is now his cellmate.

The Horch are bitter enemies of The Others. Battles between them have been raging for generations across great sweeps of the galaxy. War on that scale can be prosecuted because of a great leap of technology, the transporter booth. It can transmit people, weapons, supplies, whatever, over light-year distances instantaneously. It has another useful function. Anything that passes through a booth can be recorded and copied. Anything. Including Dan Dannerman.

It takes him time to realize it, but Dan is actually a copy of himself. He's not sure how many others there might be, but some have actually made it back to Earth. During his captivity by the Horch, he begins to understand the enormity of the threat to his home world. He eventually wins the trust of a Horch and enlists his aid in saving Earth.

It may sound as if this were a gritty story told with grim determination. In fact, it's precisely the opposite. Dan is an inveterate wise guy and can't help making insolent cracks as he narrates the tale.

"It's hard to say what a Horch group sing sounded like," Pohl writes. "It was a little like the howling of a constipated pack of wolves, a little like hogs grunting ferociously as they battled for tidbits in a pen. The big difference was that the Horch were doing all that in unison, and there were lyrics."

Pohl's story unfolds in a fairly straightforward manner, despite the complex nature of the conflict. MacLeod, on the other hand, jumps around quite a bit. That's not surprising, since his protagonist and narrator, Ellen May Ngewthu, has been flitting around the solar system for centuries. She's a soldier in the Cassini Division, a space-based defense force engaged in a "long twilight struggle" with an implacable enemy. They are "post-humans," a frightfully powerful blend of people and technology.

The post-humans broke away from Earth early in Ellen's life. They colonized some of the moons of Jupiter and eventually began changing the nature of that planet so they could live there. They consider themselves the next stage of human evolution and plan to spread throughout the galaxy. They also plan to exterminate mere humans and have been bombarding the inner solar system with electronic viruses that can cripple technology and the human mind.

As a result, civilization on Earth collapsed in a series of technological and ecological disasters. What has grown up in its place is a global network of communalism with a strong strain of radical environmentalism. On Earth, capitalism is a dirty word.

After hundreds of years of conflict, the post-humans make overtures for peace. The Cassini Division is highly skeptical, but Earth's war-weary leaders are willing to take a chance. They demand access to a wormhole the post-humans constructed to a system many light-years away. A breakaway band of humans had made their way through it many years earlier. Earth wants to re-establish contact. Ellen and her crew are dispatched to find the human colony called New Mars.

What they find is capitalism run amok. They are astonished by the "anything for a buck" attitude of the New Martians and suffer major culture shock. Ellen finds that even the most commonplace setting can be disorienting.

"Airports are quiet places," MacLeod writes, "where people make their leisurely way along covered walkways to the waiting craft.... The only barriers you'll encounter are to keep you from wandering into danger.... That's what airports are like.

"Not in capitalism, they're not. When I emerged at the end of a long corridor from the busy landing field on the main concourse ... I was greeted by hundreds of enthusiastic people behind a barrier.... Every square yard that wasn't absolutely required for passengers ... was occupied by a stall or shop or kiosk, each of which had its own fluorescent rectangle above it advertising flights or drugs or socks or cosmetics or lingerie or back-ups or cabs or hotels."

Ellen hopes to forge an alliance with the New Martians against the post-humans. The New Martians, however, have their own agenda. As her efforts toward diplomacy fail, there are new complications back on Earth, and her mission is plunged into grave danger.

Pohl and MacLeod have constructed two completely original threats to life on Earth. Yet their novels come to very similar conclusions. Both are page-turners that sweep the reader along for the ride. Pohl's lighthearted approach to his story detracts not a bit from the pleasure of reading it. MacLeod's book is not without humor, but it is the kind heard in foxholes and submarines, steeped in the knowledge of war and death.

"The Far Side of Time" and "The Cassini Division" are very different books with a lot in common. Both are inventive on a grand scale. Both are brimming over with adventure. Both are well worth reading.

L.D. Meagher is a senior writer at CNN Headline News. He has worked in broadcasting for 30 years.