"Navigating Celestial Currents: Math leads spacecraft on joy rides through the solar system" (SN: 4/16/05, p. 250) gives the casual reader the distorted view that one could travel the solar system at will by using these methods. These are generally small perturbations on the much larger primary propulsion requirement that is fixed by standard two-body orbital mechanics.
San Diego, Calif.
In the course of its eccentric orbit, the moon moves through the mean positions of the Earth-moon L1 position. Maintaining a space station at L1 would be a dubious proposition.
David R. Burwasser
In "Mood Brighteners: Light therapy gets nod as depression buster" (SN: 4/23/05, p. 261), the use of light therapy was shown to fight depression. I would suggest consideration of the possibility that the light therapy also increased the levels of vitamin D in these patients.
A review published last year by the Cochrane Library also found evidence that bright-light treatment is effective against depression. The review indicated that light treatment might be useful even for patients with serious nonseasonal depression.
Daniel F. Kripke
University of California, San Diego
What a gas
It seems that one of the intriguing potential beneficial applications of hydrogen sulfide-induced torpor ("Frozen in Time: Gas puts mice metabolically on ice," SN: 4/23/05, p. 261) would emerge if it turns out that cancer cells are less sensitive to the gas than healthy cells are. If we could turn down the metabolic activity of normal tissue, reducing its sensitivity to chemotherapeutic drugs, while the cancer cells remained vulnerable, chemotherapy would simultaneously become less toxic and more effective.
If this ability is indeed an ancient adaptation, it might explain how the precursors of small mammals, reptiles, and birds could have survived the Cretaceous and Permian extinctions. Hydrogen sulfide would have been a fairly ubiquitous by-product of the massive die-off of vegetation.
James M. Kelly
East Sandwich, Mass.
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