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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Neil Gehrels
Astrophysicist, Goddard Spaceflight Center
This image shows a portion of a crater. It is particularly interesting because it contains gullies. Gullies are slope features that are proposed to require some amount of liquid water to form.

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

I was awestruck by the first pictures of eroded slopes and gullies on Mars. The year was 2007 and the pictures came from Mars Reconnaissance Orbiter (MRO).

The above image of Martian dunes with large gullies was taken by Mars Reconnaissance Orbiter's HiRISE instrument.

The Viking lander showed a red desert with rocks, which was amazing at the time, but the slopes and gullies from MRO looked so much like typical scenes on Earth that a light-bulb clicked in my brain --- Mars and Earth are not that different.

The pictures from the Cassini-Huygens lander on Titan were also unforgettable.

This mosaic of 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.

They were fuzzy, but clearly showed lakes. We believe the lakes are filled with liquid ethane and methane instead of water. What a strange world that is -- we must get back for a closer look.

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 am interested in the regions around planets where their magnetic fields are important: the so-called magnetospheres. Every planet has one, and large planets like Jupiter and Saturn have enormous ones.

A magnetosphere is the region of space surrounding a planet, which is dominated by the planet's own magnetic field more so than by the solar wind.

They are nearly impossible to study from a distance, so the most important achievements for my field were the first fly-bys of the outer planets by the Pioneer and Voyager spacecraft.

From left to right: Dr. John A. Simpson and Dr. James Van Allen shown during a Pioneer 10/11 news conference held on 27 Jan. 1973.

As these spacecraft swooped past Jupiter and Saturn we got the first measurements from inside the magnetospheres of these two planets.

This dramatic view of Jupiter's Great Red Spot and its surroundings was obtained by Voyager 1 on 25 Feb. 1979 when the spacecraft was 9.2 million km (5.7 million miles) from Jupiter.

In this image of Saturn taken by Voyager 1 we also see Saturn's moons Tethys and Dione.

Both planets are amazing, but my favorite is Jupiter. The magnetic field is intense and traps large quantities of particles.

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

In fact, it was found that particles from the volcanoes on Jupiter's moon Io are picked up by Jupiter's magnetic field and eventually spiral into the planet and cause the aurorae.

This image shows X-ray auroras observed by the Chandra X-ray Observatory overlaid on a simultaneous optical image from the Hubble Space Telescope of Jupiter. Credit: X-ray: NASA/CXC/SwRI/R.Gladstone et al.; Optical: NASA/ESA/Hubble Heritage

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