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Moon and Venus

October 11, 2012

On many cell phone calls, there’s a noticeable delay in the conversation — a lag of a fraction of a second that often has people talking over each other.

That’s a small taste of what it’s like to communicate with people or spacecraft at other worlds. Radio waves travel at the speed of light, which is 186,000 miles per second. When you consider the great distances between worlds, the seconds add up in a hurry.

The round-trip travel time between Earth and Moon is two-and-a-half seconds. That created a lot of double-talk between Apollo astronauts and mission control.

Even so, they could hold two-way conversations, and flight controllers could keep an eye on spacecraft systems. But for spacecraft at other planets, the lag time is much greater.

Tonight, for example, the planet Venus is more than a hundred million miles away. At that distance, it takes radio waves more than nine minutes to travel from Venus to Earth. So if the orbiting Venus Express spacecraft were to encounter a problem right now, it would take almost 20 minutes for engineers to learn about it and send a response — a gap that could endanger the mission.

So when a spacecraft at another world encounters a problem, it shuts down most of its systems and waits for instructions from home — a good while later.

Venus stands just to the left of the Moon at first light tomorrow. The planet looks like a brilliant star — whose light traveled more than nine minutes to reach your eyes.


Script by Damond Benningfield, Copyright 2012

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