The Hubble Space Telescope has blurred vision, again. One of three cameras in the Near Infrared Camera and Multi-Object Spectrometer (Nicmos), installed in February, can't focus properly. Staff at Nasa reckon that the insulation around the flask cracked and let heat in, expanding the flask's casing and displacing the camera. Meanwhile, nitrogen, used as a coolant, is believed to be leaking away - probably reducing the lifetime of Nicmos to between two and three years. And it had been going so well ...

Green taxes work, according to Sweden's Environmental Protection Agency. Since 1984, when a number of environmental taxes were phased in, acid rain and diesel emissions have been cut. The study, reported in last week's New Scientist, looked at taxes on sulphur dioxide, and found them more effective than environmental regulations in lowering emissions, which fell by 30 per cent between 1989 and 1995. Sweden also has taxes covering nitrogen fertilizers, pesticides, car disposal, water pollution and gravel extraction. The fertilizer tax is thought to have cut their use by 10 per cent.

Everything else has been blamed for obesity, so why not a virus? Scientists at the University of Wisconsin have found circumstantial evidence that viral infection could explain some cases. Researcher Nikhil Dhurandhar said the Ad-36 virus, from a common family of adenoviruses, may play a role in cases of obesity. But infection only seems to be present when the blood cholesterol level is not raised.

Why can't athletes keep running and swimming faster by training more and storing more energy? Not because of the limits of bone or muscle, but because they would need bigger energy-controlling organs - the gut, liver and kidneys - which would, in turn, burn up too much energy at rest. That, in turn, would be bad news in evolutionary terms. Jared Diamond and Kimberly Hammond of the University of California observed in last week's Nature that Tour de France cyclists burn off about 7,000 calories daily. Why not eat more and so leave the pack behind? Because you can't digest and store the energy and then get rid of the waste any faster. Studies with animals confirmed it: there are limits to how much fuel any animal can convert into energy, and the ratio between the resting metabolic rate and the peak. In humans that ratio is, at best, around four.

Some news on how viruses evade the immune system. Cells infected by viruses display fragments of the invading viral protein on their surfaces - the "shop window" of the cell, called MHC class 1. Roving T-cells which detect infection then destroy the cell. But researchers investigating a type of herpesvirus found it could produce a "fake" MHC molecule. It is expressed on the surface of an infected cell which makes it seem that the host cell is working normally. The T-cells thus leave the cell alone.

Two research papers on the finding were published in Nature last week.