Clarke tells us of the great number of asteroids out there, so many that they were a nuisance for 19th century astronomers. But the efficient computers of Spaceguard take them all in stride, tracking half a million of these celestial "vermin." Any rock that poses even the most remote threat to Earth would not get by Spaceguard's all-seeing eyes.
Spaceguard first detects the object catalogued as 31/439 just beyond the orbit of Jupiter. First radar contact at such a distance meant that the object had to be very large - about 40 km in diameter. The second thing Spaceguard discovers is that the object is not on a regular asteroidal orbit, but is a true wanderer among the stars, making its first and only visit to the solar system.
That's when human beings start noticing this object, which they now "dignify" with a proper name: Rama. There is an initial buzz in the media, but with very little info to go on about Rama's nature, most of humankind loses interest...except for the astronomers.
Clarke goes into some science content: Rama is seen as a mystery because of its lack of a light curve. Every asteroid is supposed to display this property, yet Rama doesn't seem to. So for months nothing more is known about Rama, because none of the big orbiting telescopes could be spared from their regular job of staring into infinity - the cosmos at large.
A Dr. William Stenton is the astronomer who gets the lucky break in discovering that Rama does actually have a light curve, though just barely noticeable. Rama is apparently spinning at more than 1000 km/hr, so its "day" would last only 4 minutes, whereas it would be several hours on a normal asteroid.
We are then entertained by a wild notion by the imaginative Stenton, that Rama is perhaps a collapsed star - "a madly spinning sphere of neutronium, every cubic centimeter weighing billions of tons." We are given a sense of the calamity that such an invader would bring to the solar system. But Stenton comes to his senses and realizes that no stellar mass could just sneak up on the solar system, because its strong gravitional effect on the outer planets would have been detected long before.
Thus, the scientists know that Rama is not a neutron star, and it's not an asteroid...but what the heck is it?
I'll probably be saying this often, but I like how Clarke inserts clear explanations of scientific concepts in the course of the story. I'm not saying Clarke is unique in this, just that I enjoy this kind of sf, where the author really understands science, and is willing to share that scientific enthusiasm with me, the lay reader. So it's fun to follow the step-by-step procedures the scientists take to try and understand the mystery of Rama.
H.G. Wells's story, The Star, is mentioned in this chapter. I assume it must have been a favorite of Clarke's. Indeed, I assume the works of H.G. Wells must have been a great influence on many sf writers of Clarke's generation, and beyond. However, I personally have never read any of Wells's books, though I'm familiar enough with The War of the Worlds via the movie adaptations.