While NASA's spacecraft Stardust is due for a special Valentine day rendezvous with comet Tempel 1, a report from NASA states that procreation in space may not be possible, as radiation might render people sterile on long voyages.

The Daily Telegraph reported that the Ames Research Center in California has found that radiation in space will sterilize female embryos conceived there. It also damages male sperm counts.

The way to stop the radiation, made up of high-speed protons, from playing havoc with human fertility, is to develop effective shielding on spacecraft. However, heavy shielding makes getting any spacecraft launched much more expensive.

The findings show that plans to colonize the further reaches of space will have to take this into account. Since any mission heading to distant planets will take decades or centuries, sterilization of embryos will hinder population growth.

The report suggests that DNA, which takes care of the development of cells in the body, is vulnerable to radiation in space. The report cites experiments done on animals whereby exposing egg cells of female fetuses to radiation resulted in the destruction of those cells. Researcher Dr. Tore Straume said: One would have to be very protective of those cells during gestation, during pregnancy, to make sure that the female didn't become sterile so they could continue the colony.

But radiation is not the only culprit as the lack of gravity can play tricks on sperm, according to Joseph Tash, a NASA-supported physiologist at the University of Kansas Medical Center. Tash says that the sperm acts in puzzling ways in near-weightless environment but whether it impairs or is conducive to fertility is not known.

Tash, who studied the sperm movement of sea urchins on board NASA shuttle flights, explains that sperm movement commences with a process called phosphorylation -- a chemical reaction widely used by cells to control their own activities. In phosphorylation, an enzyme changes the functioning of a protein within a cell. This sets off a kind of domino chain reaction that starts some type of activity -- like causing the tails of sperm to move, and to propel them forward.

On Earth, the tail movement is halted or modified when a second enzyme, known as a protein phosphatase, kicks in. However, in a state of near weightlessness, the second enzyme responsible for halting the tail movement fails to complete its job on time. The finding suggests that sperm cells move faster in space, but this does not mean it abets fertilization. The effects on the second enzyme imply that others can also be affected.

The current finding by Ames Research Centre, however, adds another obstacle to human fertility in space -- radiation.