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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Nancy Grace Roman
Retired Chief of NASA's Astronomy and Relativity Programs, NASA Headquarters
While cruising around Saturn in early October 2004, Cassini captured a series of images that have been composed into this large global natural color view of Saturn and its rings.

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

I have found the Cassini-Huygens mission to be the most interesting.

This mission has given us a detailed picture of a terrestrial analog (Saturn's moon Titan) under quite different conditions and provided exciting observations of the moon Enceladus.

This mosaic from Cassini-Huygens showing three frames provides unprecedented detail of the high ridge area including the flow down into a major river channel from different sources on Saturn's moon Titan. Image Credit: ESA/NASA/JPL/University of Arizona

Pictured here is a high resolution Cassini image of Enceladus from a close flyby.

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?

I think that the greatest achievements of the past 50 years in planetary robotic exploration are the discoveries of a wide variety of conditions on the moons in the solar system, particularly that three of the Galilean moons have water layers and the high degree of volcanic activity on the fourth.

Jupiter's largest moon, Ganymede, is revealed in this natural color mosaic of images obtained by NASA's Voyager 1 spacecraft.

The dark spots are called "lenticulae," the Latin term for freckles. Their similar sizes and spacing suggest that Europa's icy shell may be churning away like a lava lamp, with warmer ice moving upward from the bottom of the ice shell while colder ice near the surface sinks downward. Other evidence has shown that Europa likely has a deep melted ocean under its icy shell. Ruddy ice erupting onto the surface to form the lenticulae may hold clues to the composition of the ocean and whether if it could support life.

This view of Jupiter's moon Callisto features the enormous, multi-ringed feature called Valhalla, which was likely created by an ancient impact.

One of the most surprising discoveries of the Voyager 1 mission was the violent volcanoes of Jupiter's moon Io.

The Enceladus fountains are related to these.

Giant plumes of ice were photographed in dramatic fashion by the Cassini spacecraft during a flyby of Saturn's moon Enceladus. Numerous plumes are seen rising from long tiger-stripe canyons across Enceladus' craggy surface.

The tiny moons of Mars are a contrast to these large moons. Although they have not been studied carefully, they are probably similar to the asteroids -- such as Vesta -- that have been investigated in more detail.

This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA's Mars Express spacecraft, is one of the highest-resolution pictures of the Martian moon Phobos.

This color-enhanced view of Deimos, the smaller of the two moons of Mars, were taken by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Deimos is about 7.5 miles in diameter.

NASA's Dawn spacecraft obtained this image with its framing camera on 18 July 2011. It was taken from a distance of about 6,500 miles (10,500 km) away from the protoplanet Vesta. The smallest detail visible is about 1.2 miles (2.0 km).

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