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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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To the Moon
Paul Spudis
Lunar Geologist, Lunar and Planetary Institute
In this image, which was mapped from Lunar Prospector, we see a global map of the element thorium, a radioactive trace element showing the location of KREEP, an important key to lunar igneous history.

What do you think are the most significant events that have occurred in the past fifty years of robotic planetary exploration? Why?

In rough order of importance, I would rank them:

1. The first reconnaissance of the entire solar system, including all of the planets from Mercury to Neptune, the principal satellites of the planets, including our own moon, and an inventory of the variety of minor planetary objects including meteorites, asteroids and comets. This reconnaissance began with the impact of the Soviet Luna 1 probe (first lunar mission) and the Mariner 2 flyby of Venus (first planetary mission). Over the next 50 years we continued to send impactors, flybys, orbiters, soft landers, rovers, fixed stations and people to various planetary locations near to and far from the Earth. The results of these robotic and human explorations changed our understanding of the origin and evolution of the solar system and established a new scientific discipline.

The sprawling Caloris basin on Mercury is one of the solar system's largest impact basins. Created during the early history of the solar system by the impact of a large asteroid-sized body, the basin spans about 1,500 km and is seen in yellowish hues in this enhanced color mosaic by MESSENGER.
This false-color image of Saturn's main rings was made by combining data from multiple star occultations using the Cassini ultraviolet imaging spectrograph.
Asteroid Vesta appears in a splendid rainbow-colored palette in images obtained by NASA's Dawn spacecraft.

2. The determination that the collision of solid bodies (impact) is the fundamental geological process involved in planetary origin and evolution. Such an assertion was literally unimaginable before the Space Age.

The complex lunar crater Tycho is 85 km in diameter and one of the youngest, most prominent impact craters on the moon. It formed from the collision of an asteroid roughly 10 km in diameter about 108 million years ago.
This is a computer-generated gravity map image of the Chicxulub Crater found on Mexico's Yucatan Peninsula. The buried impact structure has been implicated in the mass extinction of life 65 million years ago.

3. The characterization, sampling, global survey and documentation of the habitability of the moon, our nearest planetary neighbor.

Global map of the iron content of the lunar surface, mapped by Clementine.

4. An understanding of how the sun works and how it interacts with the planets and interplanetary space.

This composite SOHO image combines Extreme Ultraviolet Imaging Telescope (EIT) images from three wavelengths (171, 195 and 284 angstrom) into one that reveals solar features unique to each wavelength.

In your field of work, what are some examples of the great achievements and discoveries in planetary science and robotic exploration throughout the past 50 years?

Robotic spacecraft have flown by, hit, soft-landed on and orbited the moon, educating us in its processes and history in extensive detail.

The first complete mosaic of the lunar south pole from the Clementine mission, which led us to believe that water might be present in the permanently dark regions on the moon.

The moon is a differentiated, complex planetary object, with a crust, mantle and core. It underwent an extended impact and volcanic history. Its regolith contains a unique record of the impact and solar particle flux history of the Earth-moon system.

Also, the moon contains significant amounts of water in the polar areas, accumulated over the last two billion years. This water enables the moon to serve as a habitable logistics depot for the creation of a reusable, extensible space transportation system.

First radar map of the north pole of the moon from the U.S. Mini-SAR experiment on the Indian Chandrayaan-1 mission, showing anomalous craters indicative of the presence of water ice.

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