Astronomy Now January 1999
Bouncing radar signals off Solar System bodies is one way of touching the Solar System
When Galileo put his newly-invented telescope to good astronomical use in the early 17th century, our relationship with the local planetary neighbourhood became more intense yet remained on a "view only" basis. Until astrophotography arrived in the mid-19th century, most astronomers simply looked at celestial objects and learned from the visual images presented in the eyepiece. Generations of lunar and planetary observers had to be content to draw what they saw through the eyepiece, and it is fair to say that much of their scientific credibility rested upon their sketching skills and descriptive prose.
Different coloured filters placed in the eyepiece have allowed astronomers to see and to photograph far more lunar and planetary detail than can be discerned in normal integrated light. To give one planetary example, when photographed in yellow, red and infrared light, the features of Mars' surface stand out well. But in blue, violet or ultraviolet light, only the most prominent dark Martian surface features are visible. Instead, at these shorter wavelengths the secrets of Mars' tenuous atmosphere are - clouds and large scale meteorological "clearings" - are revealed in all their glory.
There is little obvious colour on the Moon, but from time to time the lunar surface displays temporary anomalous activity known as transient lunar phenomena (TLP) which may take the form of small red or blue coloured glows. Although the cause of TLP remains something of a mystery, their detection is made easier if the Moon is scanned using a special filter called a "Moonblink", which is a manually-operated rotating filter wheel onto which are attached red, blue and neutral filters. For example, a strong blue anomaly will jump out at the observer because it will appear to blink on and off when alternate filters are rotated into the field of view. Because TLP tend to occur in certain known areas dotted about the lunar surface, the few amateur astronomers conducting TLP searches tailor their monitoring programmes around a select few features.
Since Newton's experiments at splitting light's components with a prism, we have known that the visual spectrum forms only a tiny fraction of the radiation given off by heavenly bodies. Looking at the heavens in other wavelengths has greatly extended our knowledge. In infrared wavelengths, the heat from the Moon and planets can be detected. The Moon is a dark body and reflects just seven percent of the light it receives from the Sun; the rest of the energy is absorbed by the surface and re-radiated in infrared wavelengths. In studies began in 1869 with his 1.8 metre "Leviathan" reflector at Birr Castle, the 4th Earl of Rosse (Laurence Parsons) was the first to measure the Moon's heat. He used a device known as a thermocouple, a special sensitive electric circuit which responds to heat by producing a small but measurable flow of current. Subsequent investigations with thermocouples enabled the heat received from most bodies in the solar system to be measured to a reasonable degree of accuracy.
With the arrival of the New Astronomy and an ever-more-impressive battery of sophisticated equipment capable of detecting anything from short wavelength gamma rays to long wavelength radio waves, the human eye has long been superseded (in professional astronomy at least). The era of the New Astronomy has seen a revolution in the means of detection and the sensitivity of instruments. It is a measure of the importance to which we attach images in our understanding of the universe that we demand to see false colour pictures of every detectable source of radiation, rather than wavy line tracings on graph paper. Computer technology allows scientists to do this with great efficiency and, dare I say, with a commendable degree of artistic merit.
Over the past 50 years technology has allowed us to become truly interactive with the solar system. Probes can be sent to distant parts of the solar system, and the stuff of planets, satellites, asteroids and comets can be directly sampled and returned to Earth. We are no longer mere watchers of the skies but participants in the action hungry for a bigger role.
Radio astronomy began in 1931 when Karl Jansky of the Bell Telephone Laboratories in New Jersey detected radio waves from our galaxy, the milky way. Bigger, more sensitive radio telescopes soon followed, and the construction of the fully-steerable 75 metre diameter Lovell Telescope at Jodrell Bank (1953-57) will stand as radio astronomy's greatest achievement this century.
Touching the Moon with radar
In January 1946 a group of investigators in the US Army Signal Corps based at Belmar, New Jersey, sent a series of high frequency radio pulses (using a 3 kW transmitter) towards the Moon and successfully detected their returning echoes. The radar beam, being an electromagnetic wave, travelled at the speed of light and had a round-trip time measured to be 2.56 seconds. The achievement was the first "feel" of any planetary body, and the feat was repeated a month later by the Hungarian Z Bay working independently of the US team, using much the same sort of radio equipment.
Using radar, the Moon's distance was determined to an accuracy of under half a kilometre, and the slow rocking of the Moon on its own axis called "libration" could be measured. Early radar observations gave clues as to the Moon's surface texture, and the lunar surface was shown to be covered with a porous layer of soil or pumice-like material up to a depth of ten metres. In 1965 T W Thompson detected radar echoes from individual lunar features. The strongest echoes were found around the Moon's ray craters, including Copernicus, Eratosthenes, Kepler, Langrenus and Tycho.
Lunar communications satellite
It is often claimed that the 30 metre diameter aluminised balloon called Echo 1 became the first passive communications satellite, serving as a giant reflector for radio waves transmitted from the Earth in 1960. While it is true that Echo 1 was the first artificial communications satellite, nine years previously the Moon became the first natural communications satellite. In November 1951 a US government project (collaborating with the Collins Radio Company) actually beamed a message from a station at Iowa to one in Virginia (1,200 km apart) via the distant lunar surface. The phrase chosen for the historic first message was the same one that Samuel Morse had first transmitted in May 1844 through his new telegraphy equipment linking Washington and Baltimore - "What God Hath Wrought".
Currently, the best way of measuring the distance of the Moon is to aim short pulses of laser light at a reflecting point at a known location on the lunar surface and then to accurately time the faint returning light echoes. In May 1962, scientists at the Massachusetts Institute of Technology used a ruby laser to send pulses of red laser light to the Moon. The laser light was calibrated with a 150 mm mirror to an accuracy good enough that the width of the beam hitting the Moon was less than three kilometres in diameter. The reflected light was incredibly faint but it was recorded with a light intensifier coupled to a 1.25 metre telescope. An estimated 100 sextillion photons (a hundred thousand million million million particles of light) were sent to the Moon - only a dozen managed to return to be detected back on Earth 2.56 seconds later.
The laser method of determining the Moon's distance (among other properties) has now become very refined. The passive laser retroreflectors left by astronauts on the lunar surface are small reflecting devices pointed towards the general direction of the Earth. Laser beams bounced off these useful (cost-free) instruments have been used to measure the distance of the Moon within a few metres.
Viewing veiled Venus
The planet Venus is permanently swathed in thick cloud layers. Sometimes the telescopic observer can see a little detail in these clouds, but they are so dense that the Venusian surface can never be seen, not even for a brief moment. The landscape of Venus at midday is illuminated by the same kind of light levels experienced in Moscow on an overcast winter's day, even though the planet is around 41 million kilometres closer to the Sun than the Earth.
Radar has provided an excellent means of seeing surface detail. Radar was first used in 1964 to produce coarse images of the Venusian surface. In 1969 by R M Goldstein and H Rumsey used the 26 metre and 64 metre radio dishes of the Goldstone Tracking Station in California to prepare more detailed maps of Venus. Based on studies on 17 dates around the time of Venus' closest approach to the Earth (inferior conjunction), the maps show around 30 percent of the surface. Even though Venus appeared unavoidably distorted, rather like the view in a shaving mirror, the results were encouraging. More advanced Venusian studies were made in 1969 using the 305 metre stationary dish at Arecibo in Puerto Rico.
More radar-reflective areas which appeared bright were considered to be hilly or mountainous regions of "hard crust", and darker areas were thought to be smooth plains, perhaps like the lunar maria but not residing in circular basins. The nomenclature first chosen for features on Venus was a little unexciting - two bright spots were named Alpha Regio and Beta Regio.
Venus continued to be scrutinised by radar from the Earth into the 1970s and imaging techniques gradually became sophisticated enough to reveal fine detail. Circular crater-like formations began to present themselves, although it was not possible to say whether they were meteoritic impact scars or volcanic calderas.
In December 1978, the US Pioneer-Venus Orbiter began its detailed radar mapping with a far greater resolution than it was possible to achieve with Earth-based studies. Pioneer-Venus went thirteen years beyond its planned operational life of a year, and in October 1992 it burned-up in Venus' atmosphere as a result of inevitable orbital degradation.
Venus has two distinct types of terrain. A monotonous undulating landscape of low relief covers around 90 percent of the surface. The exciting parts of Venus are its three major upland plateaux. Aphrodite Terra, the largest, covers an area about equal to the entire Moon and stretches halfway around the planet south of the equator, occasionally rising to over seven kilometres. The Aphrodite plateau is split by the yawning trench of Diana Chasma, a valley which in places measures 280 km from rim to rim and reaches four kilometres deep. Compared to Aphrodite, Earth's Grand Canyon and the Moon's Alpine Valley are relegated to the league of "minor rifts". Venus' northern latitudes are blessed with Ishtar Terra a plateau somewhat smaller than Aphrodite but hosts the planet's highest mountain, the eleven kilometre high Maxwell Montes.
Hundreds of millions of years of intensive vulcanism in the absence of plate tectonics and plate margin activity have given rise to Venus' sprawling mountain plateaux. Magmatic hot-spots in the mantle have burrowed into the planet's relatively static crust and allowed huge accumulations of material to extrude on to the surface and accumulate across the local terrain.
In August 1990 the US Magellan probe entered orbit around Venus and started its high resolution radar mapping programme. Surface features as small as 100 metres across were resolved, and the height of the surface relief was measured to an accuracy of just 30 metres. By September 1992 Magellan had mapped 99 percent of Venus - better than the contemporary maps of our own Moon!
Venus' surface is randomly dotted with a sprinkling of craters, many of which have their own central peaks, terraced walls and systems of debris surrounding them. Some craters lie on high elevations and pronounced rounded domes, much like the appearance of summit craterlets on the smaller lunar domes. In some places these Venusian domes appear to have collapsed, creating sunken crater pits without raised rims, resembling the Moon's "dimple" craters but on a larger scale. Parts of Venus are crossed by parallel linear clefts comparable to those found in many parts of the Moon and probably forming as a result of tension in the crust. Magellan's radar exposed a strange world which has undergone (and is probably still experiencing) widespread volcanic activity.
Finally, the power of Earth-based radar was dramatically demonstrated in December 1992. A strong signal beamed by Goldstone Station's large dish towards the passing near-Earth asteroid 4179 Toutatis was received by the new 34 metre antenna. The signals were processed into remarkably detailed images which showed Toutatis to be twin lobed body around seven kilometres across. The asteroid's surface was shown to be heavily cratered, the largest scar measuring around a kilometre in diameter. It is in this area, rather than in the study of the Moon and planets, that radar imaging will provide exciting new results. In the absence of expensive dedicated spaceprobes we will continue to remotely feel our way around the solar system, listening to the echoes of nearby worlds in the hope that this will illuminate our understanding of our own cosmic back yard.
