A1.    Among some of the reasons offered to justify the space program are: (1) to generate or maintain national prestige, for example, we can do something you cannot do, (2) to satisfy the challenge of exploration, as also motivates the exploration of remote locations on Earth, (3) to gain scientific knowledge of other places in th solar system, and (4) to develop applications and products which are useful for everyday life and ordinary people.

A2.    First, we should send a satellite to fly by the planet to study its general properties. Such a mission gives us enough general information to determine what aspects require more detailed investigation during the next phase of exploration. Such explorations are generally undertaken by orbiting spacecraft, short term landers, and atmospheric probes. Next comes intensive study with long term landers, often equipped with some kind of roving vehicle for studying other locations. Finally will come the utilization of the resources or properties of the planet for other purposes.

A3.    Many missions could be mentioned. Three examples are the Viking mission to Mars that sent 2 orbiters and 2 landers to map the surface, study the atmosphere, and analyze the surface; the Voyager mission that sent 2 fly-by spacecraft to visit Jupiter, Saturn, Uranus, and Neptune; and the Magellan mission that sent an orbiter to Venus to map its surface.

A4.    The primary motivation for the Apollo program was to maintain national prestige -- to beat the Russians to the Moon. But the program probably sent humans to the Moon before there was adequate scientific justification, and we stopped the scientific exploration of the Moon too soon to gain full benefit from the huge effort and expense of the Apollo program.

A5.    We have performed fly-by reconnaissance missions of all planets but Pluto. More detailed investigations have occurred at Venus, the Moon, and Mars. Only the Moon has been visited by humans.

A6.    Many spacecraft have been sent to the Moon, with many failure especially in the early days. These missions have included fly-bys, hard and soft landers, and ultimately humans. Once the Apollo mission ended, no spacecraft visited the Moon for nearly 20 years, until rather recently we began to study the Moon again.

A7.    Fly-by spacecraft are simple and cheap, but provide very limited opportunity to observe their target. Orbiting spacecraft provide long-term observations of their target, but still are capable only of remote observations. Spacecraft which land on their target can take much more complete observations of their target, but can observe only a limited region of the target.

A8.    Astronomy has traditionally been an observational science. We have been required to study astronomical bodies from a great distance, unable to probe and test them directly. Satellite exploration of the solar system has now changed that! We have visited all planets but one, along with several other smaller bodies in the solar system, and have returned samples of the Moon to Earth-based laboratories for further study. The original reasons for sending satellites to explore other bodies in the solar system had more to do with maintaining national pride than with scientific discovery. The development of technology for other uses, and the potential for exploitation of resources on other planets, have also sometimes motivated our exploration.

A9.    The first mission to explore a planet should be a brief reconnaissance by a fly-by spacecraft to determine the general properties of the planet so that further studies are properly directed. This mission can be followed by an orbiter, which conducts a long term, more detailed study of the planet. Then may come a lander or atmospheric probe to directly sample and measure the material of the planet. Rovers or long term exploration of the planet may follow, with utilization of the planet’s resources in the distant future.

A10.     1.     Fly-by the planet to see what it's like.

            2.      Orbit the planet for long term observations

            3.      Explore (e.g., with a roving vehicle on the surface) to gather detailed information

            4.      Utilize the planet for human gain

All planets but Pluto have been visited at least once, but we have not reached step 4 for any planets and have done only a little of step 3 at the Moon.