Robotic Exploration of Space

In January 1959, only a little more than a year after the launching of Sputnik 1, the Soviet Union's Luna 1 flew 5,955 kilometers (3,692 miles) above the surface of the Moon, thus quickly heralding the age of planetary exploration. Since then, nearly 100 successful robotic missions to obtain closer looks at the planets, their moons, and asteroids have been launched, mainly by the United States and the Soviet Union. Among the planets, only Pluto, so far away that a signal from it would take five hours to reach Earth, has not had a spacecraft fly at least close by it. During the more than four decades of planetary exploration, continued improvements and miniaturization in electronics and computers, and in rocketry and instrument techniques in general, have been used to gain scientific knowledge.

The term "robotic" is used for any spacecraft without a human pilot. In addition to the obvious method of sending instructions directly from Earth, control of robotic spacecraft can also be programmed precisely in advance or programmed to react to the environment. Missions have included flybys , crash-landers , orbiters , atmospheric probes , and soft-landers . Some robotic spacecraft have multiple components, for instance an orbiter that released a lander (Viking 1 and Viking 2, at Mars) or an atmospheric probe (Galileo, at Jupiter). Robotic spacecraft have so far been essentially in the business of sending information or data about other planetary bodies back to Earth. Only spacecraft to the Moon have returned samples; in addition to the crewed Apollo missions, three Soviet spacecraft also brought back samples.

The Challenge of Robotic Planetary Exploration

Lunar and planetary exploration is a proposition far different from orbiting Earth. A launch vehicle can throw only about one-fifth as much mass out of Earth's gravitational field as it is capable of putting into Earth orbit. Time becomes more and more a factor with distance: To send a signal to and then from the Moon takes only two seconds. For Mars it takes between eighteen and forty-five minutes, depending on the relative positions of Mars and Earth. Everything has to be planned and programmed, well in advance.

Interplanetary spacecraft must last for years in space. It takes a few minutes to reach Earth orbit but almost a year to get to Mars. Galileo was launched in October 1989, and arrived at Jupiter through a complex route more than six years later. Once there, it began orbiting and sending back data, continuing to do so into the early twenty-first century. Spacecraft have to be remarkably reliable, because there is no means of replacing or repairing parts. Interplanetary space has dangers from solar and cosmic radiation , and there is the potential for damage from solar wind , dust, and even larger chunks of material. Spacecraft have to be resilient to the range of cold and hot temperatures, and the uneven temperatures, to which they are exposed.

Spacecraft: Getting There

The first lunar probe, Luna 1, went on to become the first artificial object to orbit the Sun. It was equipped to measure solar and cosmic radiation, interplanetary magnetic fields, the micrometeoroid flux , and the composition of gases. During the same year, Luna 2 became the first artificial object to crash into the Moon, and Luna 3 the first to circle behind the Moon. Luna 3 also sent back the first human glimpses of the farside of the Moon. These earliest missions illustrate most of the basic needs of robotic spacecraft: launch vehicle, power, communications, and scientific instruments. What they lacked that most subsequent missions (apart from the Ranger series) had was a propulsion system or a navigation system; they were just thrown at the Moon.

The earliest missions to any planets have been flybys, with some impacts. A flyby has the advantage of seeing more of the planet; an impact allows a closer look but there is then an immediate loss of the spacecraft. An orbiter allows for a longer, more complete look at a planet but requires an engine and fuel to slow the spacecraft and insert it into orbit. These engines have to be controllable and thus cannot be solid fuel rockets that cannot be turned off. If the planet has an atmosphere, it can be used for aerobraking to slow down the spacecraft, and this has been done at Mars. In many cases the engine is used several times to change the orbit. A controllable engine is also used for midcourse corrections, which are necessary to reach exactly the required destination.

An engine is needed for soft-landings, and in rare cases, for taking off again. Soft-landing techniques depend on whether the planet has an atmosphere or not. If it does, then parachutes, air bags, and other devices can be used to get hardware safely to the surface, after first slowing the spacecraft with engines and aerobraking. If there is no atmosphere, descent must be under the control of rocket engines.

The smallest spacecraft have weighed a little over 200 kilograms (440 pounds), the largest nearly 6,000 kilograms (13,230 pounds), as they left Earth orbit. For comparison, the mass of the Mercury spacecraft in orbit was about 1,400 kilograms (3,080 pounds), whereas a Gemini spacecraft weighed about 3,800 kilograms (8,370 pounds). A huge amount of mass on the launch pad is needed to get a few kilograms of scientific instruments to their destination. An Atlas-Agena space rocket and its load on the launch pad weighed about 125,000 kilograms (275,575 pounds)—about 90 percent of which was fuel—and at its destination the Ranger spacecraft was less than 400 kilograms (880 pounds). On the launch pad, the Luna 16 launch vehicle and its payload had a mass of about 1 million kilograms (2.2 million pounds). The spacecraft on the way to the Moon was about 5,600 kilograms (12,345 pounds); the empty lander, 1,900 kilograms (4,185 pounds); and the little sphere that eventually was parachuted back to Earth containing its precious cargo of only about 100 grams (3.5 ounces) of lunar soil, less than 40 kilograms (88 pounds).

Generally a spacecraft is first placed in an Earth parking orbit , and from there is given another boost to give it the appropriate interplanetary trajectory. In some cases, such as the Magellan mission to Venus and the Galileo mission to Jupiter, the spacecraft was carried first to low Earth orbit on a space shuttle. While early lunar trajectories were fairly direct, many later missions had more complex journeys. Mariner 2 to Venus was launched in the direction opposite of Earth's orbit, then gradually fell in towards the Sun, overtaking Earth, and catching up with its target. Voyager 2 used gravitational assists consecutively to fly past Jupiter, Saturn, Uranus, and Neptune, with ten years between the first and last encounters. Galileo, now in orbit around Jupiter, flew by Venus and twice by Earth to obtain gravitational assists, and also flew by two asteroids on its journey. Most of a spacecraft's flight is ballistic , that is, it is not powered but is pulled by gravity, with engines needed for course corrections.

All spacecraft must have power, to run instruments and controls, and communications systems, to receive and send information. In the inner solar system, including Mars, at least some of the power can be obtained from solar panels. At the outer planets there is insufficient sunlight for solar panels. For these, spacecraft are designed around large-dish antennae powered with radioisotope thermoelectric generators , which generate heat by natural radioactive decay. Spacecraft have been designed to operate on very little power. Cassini's generators produced 815 watts at launch and will still be producing over 600 watts at the end of its mission at Saturn (by comparison, a typical household light bulb is 100 watts).

Missions and Scientific Instruments

Spacecraft instruments have used much of the electromagnetic spectrum to observe the planets and their surroundings, including low-energy radar waves, infrared , visible light (with which we are most familiar), ultraviolet , and high-energy X rays . Most instruments are passive but some are active, including laser-pulsing to measure distance (and hence topography) and radar sounding. Particles and dust have also been measured directly. A wide variety of instruments have been carried on the nearly 100 missions undertaken through the early twenty-first century, but some have been more commonly used than others.

All spacecraft carry a radio transmitter, used for transmitting both data about the spacecraft itself and about the scientific measurements. As a spacecraft goes behind a planet with an atmosphere, the changes in the radio signal provide information on the atmosphere, such as its density and thickness. The signal is used to track the spacecraft, and variations in the spacecraft motion are informative. The overall gravitational tug provides information about the mass of the planet. Tiny measurable changes in the motion of the spacecraft tell about variation in the gravity field, which in turn tell about how mass is distributed in the planet. This standard technique of planetary study was used in the analysis of Lunar Orbiter tracking to determine mass concentrations beneath many of the large circular basins on the Moon.

A magnetometer has been carried on many spacecraft because they are small, reliable, and useful instruments. Even Luna 1 carried a magnetometer and found that the Moon produces no significant magnetic field, quite unlike Earth. The magnetic fields around planets and asteroids are informative, like gravity, about what the interior is like at the present time. They also provide information about what things may have been like in the past. The magnetometer carried on the Mars Global Surveyor has shown areas that were magnetized at some time in the past. Magnetometers discovered the powerful magnetic fields produced by Jupiter.

Many spacecraft carry a camera providing images that approximate what human eyes see. These come in many different varieties, some providing wide views, some having very high resolutions. The Ranger missions to the Moon carried cameras as the prime instrument, to see what the Moon was like as we saw it from closer and closer. With cameras we have seen the volcanoes, craters, and valleys of Mars, the cratered Moonlike surface of Mercury, the smooth but cratered surfaces of some asteroids, and the cracked icy crusts of some of Jupiter's moons. Images have provided huge amounts of geological information about the planets. Geologists are still using images from the Moon collected in the 1960s. For Venus, which has a thick atmosphere, such images are no good for surface studies, and instead radar techniques have been used to map and understand the surface features.

Chemical and mineralogical data about both surfaces and atmospheres have been obtained from orbit. Natural X-ray and gamma-ray sources provide direct chemical information but are blocked by thick atmospheres. They have been used extensively for the Moon, however, and relevant instruments are part of a planned mission to Mercury. The Martian atmosphere is thin enough that such instruments can be used there, but the spacecraft carrying them, Mars Observer, failed. X-ray fluorescence (carrying their own X-ray sources) or other chemistry instruments were carried by the Viking landers on Mars, the Surveyor landers on the Moon, and the Venera landers on Venus, to measure the composition of rocks and soils, as well as to search for traces of life. Spectral reflectance observations, in the ultraviolet through visible through infrared wavelengths , have been used extensively to understand mineralogy. Specific minerals absorb particular wavelengths, so measurements are made that show these absorptions, and in this way the mineralogy can be inferred. Such absorptions also show the presence of phases, such as water, in atmospheres.

Some missions are much more complicated, involving different kinds of instrumentation. Galileo is not only observing Jupiter and its moons from orbit, but it also launched a probe into Jupiter's atmosphere, to measure its chemical composition and physical properties (e.g., density). Atmospheric probes have also been used at Venus, and the Cassini mission to Saturn is carrying a probe to be launched into the atmosphere of Titan, that planet's largest moon. While the Soviet Union used primitive rovers on two lunar missions in the 1960s, the only other rover yet used to add to our knowledge of planets was flown on Mars Pathfinder. Its rover Sojourner made small forays to investigate rocks and soils near the lander.

Spacecraft: The Future

Spacecraft have flown by every major planet, and most of their important moons, in the solar system. We can hardly say that we know enough about any of them as yet, both from the point of view of types of mission or of instruments flown, or of how much has been seen before a mission ends. Mercury has only been flown by, as have Saturn and the more distant planets. Cassini is on a mission to orbit Saturn, while a Discovery-class mission will orbit Mercury. We have barely looked at comets; the small Discovery mission Stardust is on its way to comet Wild 2. It will meet its coma at 20,000 kilometers per hour (12,400 miles per hour), six times faster than a speeding bullet, collect small particles, and bring them to Earth. Because of the potential relationship with understanding the origin of life, many people think it highly desirable to find out more about the properties of Europa, one of Jupiter's large moons, and its potential sub-ice ocean.

Two types of mission are likely to become more common in the future. One is a sample return, particularly from Mars. These missions are inherently complex. The other category consists of rover missions. A fixed lander obviously has limited capabilities, and the extension of its senses by adding mobility has tremendous advantages. Sojourner demonstrated the usefulness of such machines, but in both a real as well as a metaphorical sense, it only scratched the surface.

see also Exploration Programs (volume 2); Government Space Programs (volume 2); NASA(volume 3); Pluto (volume 2); Robotics Technology (volume 2).

Graham Ryder

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