WHEN SIGNALS IN SPACE GET A RESPONSE

What happens in outer space is a never-ending source of mystery and wonder.

Black holes, pulsars, anyone? W despite the awe-inspiring physics behind some of those, despite what they have taught us about the universe, despite the visual spectacles some offer-arguably the most beguiling question of all is an ancient one indeed: "Are we alone?"

Variations: Is there life somewhere out there? Another civilization? Technology comparable to ours? Decades ago, we humans started sending spacecraft soaring through space that, among other things or maybe even peripherally, sought to answer such questions. I don't mean the Sputniks and Apollos and space shuttles, valuable as all those were in their own ways. The missions I am referring to are intended to soar indefinitely, never to return to Earth. They go by the names Voyager, Pioneer, and New Horizons.

Here's a quick rundown of some of these. Pioneer 10 was launched in March 1972. In late 1973, it began sending to Earth images of Jupiter and eventually got to within about 130,000 km of that giant planet. Over the next decade, it flew through the outer reaches of the solar system. It sent back ever-weaker signals till 2003, when it was about 12 billion km from Earth and the signals died out. Pioneer 11 followed about a year after Pioneer 10, also flying by Jupiter and to within 21,000 km of Saturn's surface. Its last signal reached Earth in 1995, from a distance of about 6 billion km.

Voyager 1 and 2 were launched in 1977, to study the large outer planets of the solar system. Both have sent us beautiful images of those planets and their moons.