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Solar System Exploration at 50
Exploration Stories: Favorite Historical Moments

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Tremendous Io
Amanda Hendrix
Planetary Scientist, Jet Propulsion Laboratory
Jupiter's faint ring system is shown in this color composite as two light orange lines protruding from the left toward Jupiter's limb. The Voyager 2 spacecraft was at a range of 1,450,000 km (900,000 miles) and about two degrees below the plane of the ring when it took this image.

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

So many great and significant events! I have listed some of my favorites; however these are not in priority order -- that would be very difficult!

Voyager: The Voyager flybys of the Jupiter system provided a glimpse into an absolutely fascinating and important system. The discovery of active volcanism on Io was definitely a highlight.

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

And of course the Voyager spacecraft are still going (and going ... )! Tremendous!

The boulder-strewn field of red rocks reaches to the horizon nearly two miles from Viking 2 on Mars' Utopia Plain. The salmon color of the sky is caused by dust particles suspended in the atmosphere.

Viking 2 landed 3 September 1976 -- about 4,600 miles from its twin, Viking 1, which touched down on 20 July.

Viking: The twin Mars Viking landers showed us images of an eerily Earth-like-looking world. As a kid from Pasadena, they reminded me of a Southern Californian desert. (They really made an impression on me as a seven-year old! Mars just seemed so accessible and sort of familiar -- not even that weird.)

The image above is the first photograph ever taken from the surface of Mars. It was taken by the Viking 1 lander shortly after it touched down on Mars on 20 July 1976.

What's that unusual looking spot on asteroid Itokawa? It's the shadow of the robot spacecraft Hayabusa that took the image. Hayabusa collected and returned to Earth the first samples of an asteroid.

Hayabusa: The Hayabusa mission by the Japan Aerospace Exploration Agency (JAXA) returned the first samples to Earth directly from an asteroid!

This is a spectacular NASA Hubble Space Telescope close-up view of an electric-blue aurora that is eerily glowing one half billion miles away on the giant planet Jupiter.

Hubble Space Telescope: The Hubble Space Telescope has been absolutely terrific for planetary exploration. While not up close like the other missions, it has provided critical views and coverage of solar system objects, of which ground-based observations are largely incapable (e.g., wavelength coverage, spatial resolution, etc.).

A Mariner 9 photo mosaic of Mars being assembled by hand.

Mariner 9: Mariner 9 was the first spacecraft to orbit another planet and provided fantastic initial (relatively) long-term coverage of Mars and its seasonal variations.

The Cassini spacecraft delivered this stunning vista showing small, battered Epimetheus and smog-enshrouded Titan, with Saturn's A and F rings stretching across the scene.

Cassini-Huygens: The Cassini mission at Saturn is such a outstanding mission. The science being returned is phenomenal, and continues to this day!

The existence of oceans or lakes of liquid methane on Saturn's moon Titan was predicted more than 20 years ago. But with a dense haze preventing a closer look it was not possible to confirm their presence; that is until the Cassini flyby on 22 July 2006.

To spend half a Saturn-year in the system, studying every aspect of it -- from the rings, to the planet, to the moons (including Titan), to the magnetosphere -- is truly an outstanding flagship mission. The Huygens probe was a terrific success and allowed unprecedented views and measurements on the surface of elusive Titan.

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?

Due to Io's volcanic activity, impact craters have disappeared on its surface. This image was taken by the Voyager 1 spacecraft.

Some of the great achievements are already listed above. However, in terms of scientific discoveries, I would have to say that the discovery of active volcanism on Io is a great achievement.

Io exhibits such tremendous amounts of heat and power -- it is really incredible. It is the most volcanically active body in the solar system -- and as a result, there are visible differences on the surface each time we observe it. Plus, Io has huge effects on the entire Jovian system: the volcanoes feed Io's atmosphere and also the torus (the donut-shaped ring of ionized material that circles Jupiter at Io's orbit). Sulfurous material from Io's volcanoes makes it out to the other moons of Jupiter, such as Europa and probably Callisto as well -- so the effects of Io are really wide-reaching.

Cassini imaging scientists used views like this one to help them identify the source locations for individual jets spurting ice particles, water vapor and trace organic compounds from the surface of Saturn's moon Enceladus.

The discovery of the active south-polar jets on Enceladus by the Cassini spacecraft was a great discovery. It is so exciting to find a body in the solar system that is currently geologically active. It is just so rare! For instance, Earth's Moon and so many other bodies are interesting in their own way, but are not doing too much. Io and Enceladus are actively losing material and evolving in an incredibly active way today. Plus, with Enceladus it was exciting to witness the discovery of activity, since we had known for so long that Enceladus' appearance was unique and that something special must be going on there ... though a giant plume at the south pole was not exactly expected.

This image shows the trailing hemisphere of Jupiter's moon Europa taken by the Galileo spacecraft at a distance of about 677,000 km.

The Galileo observations at Europa gave us clues about this fascinating world. Europa was the first body in the solar system to give us hints at a subsurface ocean. This was first indicated by the magnetometer data, with further indirect evidence on the surface. Now we know that other icy moons in the solar system harbor subsurface oceans as well. However, Europa's size and position in the Jupiter system makes it special. We need to go back to Europa and find out whether it is geologically active today!

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