Radar Mapping of the Solar System
Radar Mapping of the Solar System
Radar stands for radio detection and ranging. It is a technology that generates radio waves, reflects them from an object, and detects the reflected waves to determine where the object is located in space. An outgrowth of the tremendous advances in radar technology made during World War II, radar astronomy debuted in 1946 with the detection of radar signals reflected from the Moon. Since that modest start, radar has been used to map the Moon, Venus, Mercury, several asteroids, and to detect numerous other bodies in space. Recently, the Magellan space probe took radar to, literally, new heights, mapping the surface of Venus with unprecedented accuracy during a multi-year orbital mission. Orbital radar has also been used to map the Earth's surface, including the seafloor. Radar techniques have become increasingly sophisticated over the past half century, giving astronomers yet another tool with which to explore.
The history of radar actually dates back to the 1880s, when Heinrich Hertz (1857-1894) showed that radio waves exist and could be both generated and detected. Hertz also showed they could be reflected, just as light is by a mirror. Somewhat later, in 1904, German engineer Christian Hülsmeyer developed a device using radio echoes as a navigational aid. This device did not work well, however, and it was left to Guglielmo Marconi (1874-1937; the inventor of wireless radio broadcasts) to suggest a way to make radar useful in 1922. American physicists Gregory Breit and Merle Tuve developed useable radar in 1925, but its use remained limited until shortly before World War II.
During the Second World War, technological advances by Germany, England, and the United States resulted in significant improvements to radar in terms of technology, reliability, and power. Shortly after the war ended, Walter McAfee (1914-1995), an American scientist, was able to determine the strength of a radar signal that would reflect from the Moon and still be strong enough to detect with earth-bound equipment. McAfee's theory was successfully tested in 1946, ushering in the era of radar astronomy. In following years, radar was used to map portions of the Moon, to detect otherwise invisible meteor trails, and, later, to map other objects in our solar system.
Radar mapping is similar to radar detection, but differs in some significant ways. In normal detection, pulsed radio signals are sent towards an object. If the object is made of metal or something that will reflect radio waves, some are reflected back towards the antenna. These signals are picked up and the direction from which they returned shows the direction to the object. The amount of time it takes for a radar signal to return can be used to calculate the distance to the object. Radio waves, traveling at the speed of light, travel almost exactly 300 meters (about 1,000 feet) in a microsecond. By counting the number of microseconds that elapse between sending a signal and detecting its return, a computer can determine its distance.
In radar mapping, a few more steps must occur. It may help to visualize a radar signal as an expanding balloon, expanding into space at the speed of light. As this balloon comes into contact with a planet, it hits the highest points first, and they push into the balloon first, marring the balloon's surface. In quick succession, the rest of the expanding sphere presses into other high points on the planet's surface, followed by steadily lower points until an entire half of the planet is embedded in the balloon's surface. This is where the balloon analogy breaks down for, in radar astronomy, the points that were first touched by radar waves are the first to reflect those waves back at the antenna from which they came. Those first waves are soon followed by reflections from the rest of the planet, separated in time by their relative distance from the antenna. For example, a mountain that is 3,000 meters in height is, at the speed of light, 10 microseconds above the surrounding plain. It takes radar waves 10 microseconds longer to reach the plain than the mountaintop, and 10 more seconds to climb back to the elevation of the mountain. So radar waves reflected from the plain will return to Earth 20 seconds later than those from the mountaintop, because they had to travel the extra distance twice, once going to the planet and once returning. This helps to make radar mapping possible because differences in elevation are exaggerated by this phenomenon. This technique has been used to make radar maps of the Moon, Venus, Mercury, and many asteroids.
Two other points must be mentioned with regards to mapping planets with radar. First, the strength of the radar signal drops off quite rapidly with distance. In fact, signal strength declines as the square of the distance. In other words, sending a signal twice as far requires four times the power if it is to be detected at the receiving end. Simply reaching another planet with a radar signal requires one fourth the power as reaching the other planet and receiving a detectable return signal. This level of power is required because the signal has to travel from Earth to the other planet and back. In addition, it should be pointed out that different surfaces reflect radar differently, based on their geometry and composition. A hard, flat plain, for example, will return radar extremely well because there is nothing to break up, or scatter, the signal. Plains will show up as "radar-bright" objects on radar maps. On the other hand, a jumble of rocks, loose soil, mountains, and similar features will scatter the beam, sometimes greatly, leaving a "radar-dark" area. Similarly, some materials are more likely to absorb radar than others and will show up as darker areas on a map. These factors are all considered when interpreting a radar map.
The impact of radar astronomy was initially huge, then dropped off as space probes were sent to return with photographic maps of these same bodies. However, in recent years, radar astronomy and radar mapping has experienced something of a renaissance with the triumphant Magellan mission and several exceptionally successful terrestrial mapping satellite missions. At the same time, the term "radar astronomy" may no longer be strictly accurate, especially when we turn orbital radar onto our home planet. Radar mapping is a more accurate and more descriptive term that will be used for the rest of this essay.
The first impact of this field came in 1946 when the technology was first demonstrated. Simply bouncing a signal off of the Moon was a triumph because no radar signal had ever traveled so far before. Mapping the Moon with radar was more an exercise in calibrating the technology since radar maps could be compared with what we could actually see through a telescope. The first real test came when radar telescopes were turned towards Venus and, later, Mercury.
The surface of Venus is perpetually covered by thick clouds. For the first half of the twentieth century, considerable debate surrounded the question of Venus's period of rotation, in other words, how long the Venusian "day" was. The matter was finally settled in the 1950s when a series of mapping projects identified three large, distinct areas on Venus: Alpha, Beta, and Maxwell (after the physicist who was instrumental in describing electromagnetic waves). By tracking these areas, scientists were able to show that Venus's "day" was actually longer than its "year." In fact, a single rotation of Venus takes 256 days and, unique in the solar system, Venus was found to rotate in a retrograde (i.e., backwards) direction compared to the Earth. Another interesting finding of these early radar and radio studies was that the surface temperature was over 450 degrees C, hotter than virtually any part of Mercury, in spite of Mercury's closeness to the Sun. These findings were sufficient to permanently dash any illusions of Venus as the home to life or as Earth's sister planet in the solar system.
Mercury was mapped in the 1950s and 1960s in a manner similar to Venus, but the Mariner 10 flyby missions in 1973 and 1974 rendered these radar maps obsolete. However, radar did tell astronomers that, like Venus, Mercury has a very long "day," in this case, 59 days. Recently, radar has been used to map several asteroids with some degree of success.
The primary advantage of radar mapping is that the equipment stays on Earth. This makes maintenance possible, reduces costs enormously, and allows construction of a much larger and more powerful antenna. On the other hand, terrestrial equipment cannot achieve nearly the accuracy, precision, and resolution of spacecraft in orbit around the planet being mapped, making such craft indispensable for mapping Venus and other cloud-shrouded bodies.
The most recent advances in this field utilize such orbital craft. Magellan's mission to Venus in the early 1990s resulted in mapping 98% of the planet's surface with a resolution of about 100 meters (328 feet), meaning the radar could detect objects 100 meters in size or larger. More area has been mapped on Venus than on the Earth, Moon, Mars, and Mercury combined, thanks to Magellan. This same technology has been turned Earthward, too. The Radar Ocean Satellite (ROSAT) launched by the United States and the French TOPEX craft have mapped the ocean surface with unprecedented accuracy. This, in turn, has allowed oceanographers to construct a very accurate map of the seafloor because, as it turns out, water mounds up slightly over top of underwater mountains and is slightly lower over subsea valleys and trenches.
P. ANDREW KARAM
Morrison, David. Exploring Planetary Worlds. Scientific American Library, 1993.