For more than 40 years, the United States has been putting sensors on satellites to detect nuclear weapons testing in the atmosphere. In 1996, many countries around the world signed the Comprehensive Nuclear Test Ban Treaty, which bans nuclear explosions.  But some nations continue to secretly test nuclear weapons.  VOA's Deborah Block tells us about one of the latest nuclear bomb monitoring satellites.

For the past eight years, a U.S. satellite called Forte has been traveling over the Earth, searching for covert nuclear test explosions.  The satellite uses optical and radio frequency sensors that scan the skies looking for the flashes of light and radio waves produced by nuclear weapons. 

Forte was created, in part, by the Los Alamos National Laboratory in New Mexico.  That laboratory has also developed other types of sensors on satellites that monitor nuclear detonation. 

Mark Hodson, who works with the Forte project in Los Alamos, says the satellite is constantly searching for nuclear tests. "They will occur anywhere from slightly below the surface of the Earth out to great distances in space.  And we're looking for them in all those places with our satellite based remote sensors."

Forte can pinpoint the brilliant flash of light from a nuclear explosion.  Even though that flash is similar to lightning, the satellite's complex instruments decipher the information from the sensors to determine the difference. 

Mr. Hodson says another problem is television signals, which occur in the same frequency range as nuclear explosions. "All these tend to mask some of the signals a nuclear explosion might give out that we might like to monitor."

He says the sensors pick up signals from a variety of nuclear devices, some more advanced than others.

"We're looking at the most primitive type that would be indicative of just some very primitive effort, to the most sophisticated type,? says Mr. Hodson. ?People who have these devices, or have gotten them from somebody that could produce them."

Data from Forte is recorded and analyzed several times a day at ground stations in New Mexico and Alaska.  Scientists hope the next generation of sensors will also be able to find biological and chemical weapons.

Although Mr. Hodson says sensors rarely record signals from nuclear explosions in space, they provide a deterrent, by making it difficult for countries that are testing nuclear weapons above the Earth to deny it.