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Light-Time and Robots: Communication Across the Solar System

Click to enlarge > Sitting on Earth, a Soviet operator controls Lunokhod on the Moon.

by James D. Burke

The following is reprinted from the July/August 1990 issue of The Planetary Report. James (Jim) D. Burke was the manager of the first series of Ranger lunar flight between 1960 and 1963. He also led the post-Apollo lunar study and field test teams at Jet Propulsion Laboratory (JPL). Jim specializes in lunar exploration and settlement, micro-spacecraft design, field testing of mobile robots, and human-powered and ultralight aircraft.

Lunar rovers can be and have been driven by remote control from Earth. Why then do we conclude that such driving is completely out of the question for Mars? The answer is that Mars is far away and even at the speed of light, our radio commands and the responses to them are delayed.

Here are the relevant numbers: Light and radio waves travel at nearly 300,000 kilometers per second in the vacuum of space. The Moon is about 400,000 kilometers from Earth. Thus, what we call the one-way light time to the Moon is about 1 1/3 seconds. As the driver commands the rover he can never observe its response any sooner than 2 2/3 seconds later. Many Earth-based experiments and the actual Lunokhod experience show that, though demanding of good on-board vision and hazard sensing, operator skill, and patience, such driving is indeed practical--so long as the rover travels so slowly that the driver's delayed reactions can keep up with it.

Now how about a rover on Mars? As an example of the numbers in that situation we can examine the light times for a typical martian mission, the Mars Observer to be launched in 1992. As Mars makes its way around the Sun during its 687-day year and Earth circles faster, going around in only 365 days, the distance between the planets varies widely. During the flight of Mars Observer, Earth and Mars will come within 102,000,000 kilometers of each other when both are on the same side of the Sun, and the distance will stretch to 367,500,000 kilometers when we and Mars are on opposite sides of the Sun. Dividing these figures by lightspeed, 300,000 km/s, we find the one-way light times to vary from 5 minutes and 40 seconds to 20 minutes and 25 seconds.

Obviously nobody wants to leave a dumb rover to its own devices for nearly 41 minutes! It could drive right off an unseen cliff. No, the martian rover absolutely has to be smart. If it were not, its progress would be glacial, and even with considerable-on-board intelligence it does not dare go faster than a human walking pace. The compromise among high mobility and hazard tolerance, travel speed, quality of vision and other on-board sensing, and human control demands is one of the most interesting challenges facing the engineers, who are now preparing for the mobile robotic exploration of Mars.

Also from this issue:

From the Moon Rover to the Mars Rover, by Alexander Kermurjian

The First Rover on Mars: The Soviets Did It in 1971, by Charlene Anderson