Science: Theoretically: Falling satellites, laser rockets, mysterious Moon, knowing HIV, Benveniste threatens suit

Say goodbye: Nasa's Lewis satellite plunged into Earth's atmosphere on Sunday and apparently burned up over the south Atlantic off the coast of Antarctica. The 400-kg, $64.8m satellite was intended for a five-year mission, but went into an uncontrolled spin four days after its launch on 22 August. It re-entered the atmosphere at 1158 GMT. Space Command, which had been tracking the wayward satellite, said that it was impossible to determine how the satellite interacted with the upper atmosphere, but that it wasn't designed to survive re-entry and probably burned up. The final orbit varied between 146km and 158km above the Earth.

Wave hello, to the laser-powered rocket. While Lewis was whirling uncontrollably above the Earth, Nasa scientists launched a miniature rocket using a ground- based laser-beam for propulsion. Scientists at White Sands Missile Range in New Mexico aimed a 10-kilowatt pulsed carbon dioxide laser at the "launch vehicle" and raised it two metres off the ground. As if that weren't underwhelming enough, the object is about 14cm in diameter - and weighs only about 55 grams.

But in these things it's the principle that matters, and this uses a reflector at the rear of the rocket which concentrates the laser's power; this then heats the air, which blasts out of a nozzle. The problem is that the laser's energy does dissipate with distance - "but ground-based energy is cheap", a Nasa spokeswoman pointed out. In the next experiment, the miniature rocket will have a more sophisticated guidance system for steering it. Can't wait.

The Moon probably formed about a year after the impact of a giant interplanetary object with the early Earth, according to new computer simulations by a team from Japan and the US. But according to an analysis in the latest edition of Nature, the body that hit the Earth to knock off what became the Moon would have had to be twice the size of Mars. "Earth's Moon is one of the most peculiar bodies in the Solar System," comments Jack Lissauer, of the Space Science division at Nasa's Ames Research Center in California.

Two-thirds of those Americans who are infected with HIV, the virus that causes Aids, already know it, according to American researchers. They reckoned that that's a "surprisingly" high figure, given that they estimate about 775,000 Americans carry HIV, and at least 500,000 have been tested and know their status. The research, from the Center for Disease Control, is the first careful attempt to arrive at a "total infected", using infection data collected by the States. Until now, many experts had guessed that about half of all HIV-infected Americans were aware of it.

"This is encouraging, because it suggests that the majority of persons with HIV have been tested," said Dr Patricia Sweeney, who directed the study. She added: "We need to continue to work to ensure these people have access to recommended treatment."

Water is not just wet as far as Jacques Benveniste is concerned - it's a matter over which to take libel action. The French scientist whose research on the "memory of water" was first published in 1988 - and pooh-poohed by pretty much all the scientific establishment - is planning to sue at least two Nobel prizewinners, according to New Scientist. The focus of his ire is a number of articles which appeared in the French newspaper Le Monde in January: Benveniste is upset that anyone should think him dishonest. "If you say I am a poor scientist, I have no reasons to sue," New Scientist quotes him saying. "If you declare I am a fraud, I am going to sue."