It came from space, and now it's returned to space--that's the strange itinerary of a unique device that lies in John Glenn's belly continuously taking his temperature as he orbits the Earth aboard space shuttle Discovery.

As part of his medical experiments, Senator/astronaut Glenn swallowed a 3/4-inch-long, silicone-coated capsule that contains a tiny telemetry system, a microbattery and a quartz temperature sensor. These devices had their roots in the work of spacecraft scientists at the Applied Physics Laboratory as they invented Transit, the world's first navigation-by-satellite system, more than three decades ago.

Later, when APL biomedical scientists, working on a shoestring $75,000 NASA grant, were asked to develop a device that could be swallowed by astronauts as a way to measure their core body temperature, they borrowed technology from their space colleagues to invent the ingestible temperature capsule.

Once it's swallowed, the biomedical marvel continuously transmits temperature from deep within the body to an external monitor/recorder with an accuracy to within 1/10 of a degree Celsius during the 24 to 78 hours it takes to travel through an astronaut's digestive system.

"The ingestible temperature sensor is a key tool for NASA researchers," says Tag Cutchis, an APL biomedical engineer who helped develop the original technology. "By studying the fluctuation of temperature of other physiological functions, along with sleep patterns, they hope to determine the best times for astronauts to perform their assignments in space."

Medical researchers also will use information provided by the "temperature pill" to increase their understanding of the aging process and its similarity to the physical deconditioning experienced by astronauts in the weightlessness of space.

This space-age medical tool was first employed by Hopkins veterinarian Phillip Brown, who used it to monitor large animals' temperature during and after surgery.

The temperature-sensing device was licensed to HTI Technologies in 1988. Since then, ingestible capsules have been used to monitor the core body temperatures of firemen as they battle blazes and divers as they work deep in cold waters. The devices have also been used to monitor critical temperatures in paper manufacturing, in food processing and in jumbo television sets found at sport stadiums.

And now, like John Glenn, the technology that came to prominence early in the Space Age has returned for another trip.