With over 1450 votes, the results are interesting to compare to current mission objectives being considered by NASA mission planners.

First Stop
Europa was the clear winner for our readers, coming in with 40% of the total votes. This icy moon of Jupiter was first discovered by Galileo Galilei in 1610, and has since captivated scientists over the centuries. NASA first visited the Jupiter system in the early 1970s with the Pioneer 10 and Pioneer 11 missions.

Voyager 1 and Voyager 2 also captured important images of Europa as they flew past Jupiter on their way to the outer solar system. High resolution images from Voyager 2, in particular, showed a surface of cracks and intersecting linear features. The moon was obviously more active than scientists had previously thought.

More recently, the Galileo mission went into orbit around Jupiter and captured fantastic images of Europa on a number of flybys between 1995 and 2003. This mission provided some of the most important evidence that Europa contains a liquid ocean beneath its icy crust. Astrobiologists now believe that the subsurface ocean on Europa is able to exist thanks to a process of tidal heating.

As Europa orbits Jupiter, tidal forces from the giant planet pull and distort the tiny moon. This provides enough energy to keep the ocean from freezing.

Tidal heating could also provide energy for life deep below the ice that caps the ocean. Thanks to astrobiology research, we now know that life can thrive around hydrothermal vents at the bottom of Earth's oceans - where microorganisms can produce energy and food using heat from the Earth rather than sunlight.

This is why Europa has become such an exciting prospect for future astrobiology missions. In light of Europa's potential to support habitable environments, many proposals for future missions to the moon have been made. Unfortunately, budget constraints have prevented any Europa-dedicated missions from launching thus far. The Jupiter Icy Moons orbiter (JIMO), for instance, was axed in 2005.

However, in less than 500 days, NASA's Juno mission is scheduled to launch. If successful, Juno will arrive at Jupiter five years later. Juno is focused on studying the planet Jupiter itself, providing information about the planet's formation, atmosphere, magnetic and gravity fields, auroras and the planet's deep structure. Studying Jupiter will yield valuable information about the entire Jupiter System, and how the massive planet affects Europa and the potential for habitable environments in the moon's subsurface ocean.

A more Europa-specific mission, the joint NASA/ESA Europa Jupiter system Mission (EJSM), got a boost in early 2009 when the agencies announced that the mission would be given priority over the Titan Saturn System Mission (TSSM). EJSM will use two orbiters to study the Jupiter system together before settling into orbits around the planet's moons Ganymede and Europa. A main goal of this mission will be to study the habitability of these icy moons. Still in the planning phases, EJSM is currently scheduled for launch in 2020.

Runner up...Mars
Sitting solidly in second place with 24% of the votes in our readers' poll is the perennial favorite of many astrobiologists - Mars. Our little red neighbor toward the asteroid belt has long been the target for speculations about life in the solar system. In the late 1800s, astronomer Pervical Lowell observed what he believed to be extensive 'canal systems' on Mars.

He came to conclusion that the canals must have been constructed by intelligent beings. Science fiction writers weaved stories of advanced civilizations, both friendly and benign, that would one day make contact with Earth.

In 1965, hopes of contacting our martian cousins were truly dashed when NASA's Mariner 4 spacecraft orbited the planet and returned images of a dry, barren and lifeless surface. The Viking mission of the 1970s also made it clear that not even microbial life would be easy to detect on Mars.

Since Viking, Mars has been a regular stop for robotic missions, including Pathfinder, the Mars Exploration Rovers and the recent Phoenix lander. Currently, there are three satellites orbiting Mars and sending constant streams of data back to Earth.

Optimism about the potential for past life, and possibly even present life on Mars has increased with modern findings concerning the potential for water beneath the surface of the planet. Additionally, scientists are still debating whether or not features on Mars were carved by ancient bodies of water and rivers on the planet's surface.

Strange, seasonal occurrences of methane in the Mars atmosphere have also led some scientists to theorize about microbial communities deep below the frozen soil of present day Mars.

Many questions remain about the potential for past or present life on Mars. The Mars exploration programs at NASA and other space agencies around the world remain robust. Currently, NASA is preparing to send its largest ever rover, the Mars Science Laboratory, to the red planet. This massive robot carries with it a suite of instruments specifically designed to study the habitability of Mars.

Bottom of the pack
Rounding out our reader poll in the third, fourth and fifth places are the Moon (14%), Titan (13%) and 'an asteroid' (8%). Our readers seem to be fairly in line with current priorities at NASA. The Moon has been a relatively constant target of NASA science, with the most famous efforts being the Apollo missions of the 1960s and 1970s.

During the Bush administration, lunar exploration received a huge boost when it was announced that NASA was going to concentrate efforts on returning astronauts to the lunar surface. Recent developments, however, see NASA committing more resources to support commercial development of the launch technologies needed for human exploration.

The Constellation program, which was an umbrella program for lunar exploration that included the development of a new launch vehicle, was recently cancelled.

We're still planning on a return to the Moon, but NASA's role will now be much different. Even with schedule changes in access to space for human missions to the Moon, NASA is still forging ahead with developing technologies to support astronauts once they get there. NASA is developing robotic rovers and studying mission scenarios for exploring the lunar surface.

Missions like the Lunar Reconnaissance Orbiter and the recent LCROSS impactor are helping us understand our closest celestial neighbor in new detail. Future missions like LADEE and GRAIL are in the planning stages and will provide essential information about the lunar environment and the potential to support long-term human habitation.

By relying on commercial companies to develop human access to space, NASA can devote more time and energy to furthering scientific exploration of distant locations, such as Europa and Mars. Titan also remains a priority at NASA following the recent revelations of our first robotic visit to the surface of this mysterious Saturnian moon. F

ive years ago, Cassini released the European Space Agency's Huygens probe, which descended through the thick atmosphere of Titan and revealed a dynamic world with mountains, dunes and lakes of liquid methane. The Cassini mission has been extended, and is still providing information about the Saturn System and its unique moons like Titan and Enceladus.

NASA and ESA made the decision last year to focus on a mission to Europa with EJSM, but a return to Saturn, Titan and Enceladus is scheduled to follow. The Titan Saturn System Mission (TSSM) is another joint NASA/ESA program, and includes an orbiter, a lake lander and a 'Montgolfiere' hot-air balloon.

Last in our poll was a visit to an asteroid. Asteroids are not expected to harbor habitable environments, but they can provide important information about the history of the solar system, and may have delivered molecules essential for the origin of life to the early Earth.

The first spacecraft to perform a dedicated asteroid visit was NASA's Near Earth Asteroid Rendezvous (NEAR) Shoemaker mission, which arrived at the asteroid 433 Eros on Valentine's Day in 2000. Currently, asteroids remain a target for NASA and other space agencies around the world. The Japanese Space Agency (JAXA) sent a robotic explorer to visit the asteroid 25143 Itokawa in 2003.

The craft attempted to collect samples for a return to Earth, but scientists are unsure if the sampling procedure was successful. We'll find out later this year when the spacecraft returns home and ejects its sample capsule. Currently, NASA's Dawn mission is en route to Ceres and Vesta, two 'protoplanets' that lie in the asteroid belt between Mars and Jupiter.

Judging by the results of our poll, Astrobio.net readers seem to be interested in locations in the solar system that are believed to be the most likely to support life.

Mars and Europa could theoretically hold habitable niches for microorganisms beneath their soil or ice. Life on Titan seems to be a long shot, but some scientists believe it's possible - or that Titan might support life forms vastly different than those we're familiar with here on Earth.

The results of the poll may also reflect a response to gaps in the current exploration of the solar system. Europa was the one location on our list that is not being explored with current missions.

With Cassini still churning out observations of Titan, orbiters and rovers exploring Mars, satellites mapping the moon, and asteroids being sampled - Europa might just seem a little lonely under Jupiter's watchful red eye. In any case, the majority of our readers interested in Europa and its potential subsurface ocean should be happy with the recent decisions to focus on the icy moon and the Jupiter system before a return to Titan and Saturn.

A new poll
This year marks the 50th anniversary of Exobiology and Astrobiology research at NASA, and this will be the topic of our next reader poll. We'd like to know your opinions about the most 'important' discoveries that have come out of the science of astrobiology. Obviously, there are too many significant discoveries to list - but we'll be providing a shortlist to choose from.

Is the potential for life on moons of giant planets, such as Juipter's Europa and Saturn's Titan, the key scientific finding of the past 50 years? Or maybe it was the first discovery of a planet orbiting a distant star?

Results closer to home, such as realizations about the durability of extremophiles and the subsequent rewriting of the 'tree of life', are also important outcomes of astrobiology research. Stay tuned to Astrobio.net to see how the results pan out...

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