Space Exploration, Fly-Past

Flying past a target in space is one of the oldest and cheapest means of space exploration and is still the method engineers choose for first investigations in space. It returns often surprisingly good science with a simple mission profile. At the beginning of space flight, rockets were too weak, navigation too unreliable, and probes too small to do anything but simply fly by.

Therefore most of the fly-pasts were in the first decade of the space age, carrying fields and particles experiments, and sometimes, primitive cameras. Later, fly-pasts were continued to reduce the cost and complexity of missions or to gain gravity boosting. Often the unknown nature of the exploration precludes anything but a flypast.

A simple variation on the fly-past mission is the ‘‘strike’’ mission. Such missions were limited to the moon at first because it was thought no life could exist there, and therefore bacteria carried on a spacecraft could not contaminate indigenous life. Also a strike mission would be an early navigational tour de force, and the probe would be in line- of-sight of the ground stations here on earth.

Earth’s moon was the first of the planets to be investigated by spacecraft. In 1959, the American Pioneer-4, with primitive instrumentation and camera, made the first flight to go near to the moon, if 60,050 kilometers is considered near for a strike mission. No pictures were recorded because the spacecraft did not come close enough to trigger the camera’s photoelectric sensor. Following successful fly-past, signals from the spacecraft were received from a record 655,300 kilometers away, amazing for an early mission that was planned to go half as far.

Later that year the Soviet Luna-3 swept by, carrying a primitive imaging system that returned a set of photographs of the hidden side of the moon, seen for the first time by inhabitants of the earth. Failed strike missions, Pioneer-3 and Luna-1 and -2, preceded both. The Soviets only sent one more intentional flyby mission. The Zond- 3 of 1965, which was actually a Mars mission that had missed its launch window and was retargeted, also imaged the far side of the moon, after several failed lander missions.

The U.S. sent a barrage of nine Rangers, primarily equipped with cameras, intended for impact on the moon, and several of these became inadvertent flybys when their navigation failed. Both countries quickly graduated to orbiters and landers due to their interest in the size and proximity of the moon.

While Mars and Venus held early fascination for space exploration, both planets are much smaller and farther away. Mariner-II flew past Venus in 1962 with a set of instruments including infrared and microwave sensors, radiometers, magnetometer for exploring space and the planet. The results established the magnitude of the inhospitable surface temperatures, measured at 425°C, and the absence of a magnetic field.

No cameras were used because opaque clouds hide the surface of Venus. The Soviets also had several craft fly by Venus, leading to the Venera-3 strike mission (which may have been a failed lander). There were no great discoveries, either because they were too far away for their instrumentation or because of radio failure. Mariner-IV passed Mars in 1964, shooting the first close-up photos of another planet, showing Martian craters, and gathering atmospheric data. The 21 pictures took one week to transmit to earth.

The last of the Mariner missions was Mariner- X, an intentional multiple fly-past of Mercury by way of a Venus near miss to gain gravity-boost, represented the pioneering use of this technique. It carried a high-resolution camera, and the probe imaged Mercury three times, using the planet’s gravity to gain the multiple fly-pasts.

During the 1970s, it became apparent that the outer planets would be lined up in such a way that a ‘‘grand tour’’ mission could be planned that would fly past all the gas giants—Jupiter, Saturn, Uranus and Neptune. Financially, this was deemed impossible, so a pair of Voyager spacecraft were prepared for only Jupiter and Saturn fly-pasts. Once launched, and with both probes operating, the grand tour would be deemed viable.

Since instrumentation was not intended to view Uranus and Neptune, new software was radioed to the spacecraft, for example, for image compression so that more pictures could be transmitted. Also, because of lower levels of light than expected, new techniques were developed to allow the cameras to track targets during longer exposures. Voyager-2 thus visited Jupiter, Saturn, Uranus and Neptune on one of the most valuable fly-past missions ever, returning spectacular images.

The Voyagers had, in addition to the high-resolution cameras, instruments for exploring areas of space such as magnetic fields. They also carried a message from earth on a disk on the spacecraft. The second probe to each of the giants, Galileo to Jupiter, and Cassini to Saturn, were orbiters carrying entry probes. Galileo’s entered the Jovan atmosphere, and Cassini’s is aimed at the Saturn moon Titan. Both missions depended on data gathered from the Voyager fly-pasts for mission planning.

The close approach of Halley’s Comet in the mid-l980s spawned a plethora of fly-past missions. The Soviets had two, Vega-I and -II, both of which dropped probes into the Venusian atmosphere on their journeys. The Japanese also had a pair, Sakigake and Suisei. Only Suisei had imaging equipment. They carried instruments to determine the composition of the comet. The European Space Agency sent one probe, Giotto, which went on to another comet later after a period of dormancy. These missions were the first visits to a comet, which has a very weak gravitational field, so flypasts were in order for both reasons of novelty and technical limitations.

The only planet remaining to be explored, Pluto, was also targeted for a fly-past, hopefully to arrive before the atmosphere of the planet begins to freeze as it moves farther from the sun. The earliest methods of space exploration will continue to be used, especially when exploring a new part of the universe.