theoretically...

Beware: an asteroid warning is on the way. More than 200 "stray" asteroids greater than a kilometre across are roaming the solar system, and some could cross the Earth's orbit, according to scientists from Colorado and Arizona. In a letter to Nature magazine, they said the asteroids came from the so-called "Trojan swarms" - two groups of asteroids orbiting the sun at the same distance as Jupiter. If a 90-metre asteroid hit Earth it would have the effect of a nuclear bomb; the repercussions increase geometrically with size. Ready yet to fund some telescopes to watch for impending asteroid strikes?

The Jewish priesthood, the Cohanim, are among the true children of Israel, having genetic distinctions that seem to date back to "a unique evolutionary event" - a DNA insert - 29,000 to 340,000 years ago. Biblical accounts say the Cohanim (separate from rabbis) were established3,300 years ago when the first Israelite high priest was appointed. Membership is passed to male children of priests on their Y chromosome. Genetic testing found only 1.5 per cent of Cohanim had the DNA insert, compared with 18.4 per cent of lay Jews, even among those living as far apart as Israel, the US and UK.

Why is it that obese girls tend to enter puberty earlier than average, and lean female ballet dancers much later? A team from the University of California at San Francisco reckons the answer is the hormone leptin, secreted by fat tissue. They injected leptin into pre-pubertal female mice, and found that they began reproducing before a control group. Conclusion: "that leptin acts as a signal triggering puberty", say the authors in last week's Science, "supporting the hypothesis that fat accumulation enhances maturation of the reproductive tract". So if you want to grow up, put on some weight.

If life existed here 3,800 million years ago, as research in Greenland rocks suggests, Heinrich Holland at Harvard University says that either the destructive effects of meteorite impacts had decreased dramatically before then, or life was invented more than once, or our distant ancestors were truly hardy. Writing in Science, he prefers the first - reckoning that geological evidence points to a planet that had already settled down to its present, "somewhat humdrum existence", offering the relatively stable conditions required to support life.

Happy birthday this year to complex numbers (devised in 1797), the sewing machine and modern flute (1847), the discovery of the mountains of the Moon (1647), of tobacco (1497, by a monk who sailed with Christopher Columbus), and of the electron (1897) by Joseph John Thomson at the Cavendish Laboratory in Cambridge. Expect celebrations of the latter centenary around March.