On Oct. 29, 1991, the Galileo spacecraft flew by the asteroid Gaspra on its way to Jupiter. Gaspra was the first asteroid to ever be visited by a spacecraft.

Gaspra orbits near the inner edge of the asteroid belt. The Galileo spacecraft had been travelling through the asteroid belt for about two months before it reached Gaspra. Galileo was 990 miles (1,593 kilometers) away from Gaspra when it whizzed by, and it was travelling at a relative speed of about 5 miles (8 km) per second.

The spacecraft took several photos of the asteroid and used an instrument called the Near-Infrared Mapping Spectrometer to study its chemical composition and other physical properties. Galileo determined that Gaspra has an irregular and elongated shape measuring about 12 miles long and 7 miles wide. The images revealed that Gaspra was also covered in craters. Because Galileo was too far away for Gaspra's gravity to affect it, the spacecraft was not able to determine the asteroid's mass.

Why it mattered

The Galileo spacecraft's flyby of Gaspra was a pitstop near the midpoint of its 6-year journey to Jupiter, the largest planet in our solar system. NASA launched Galileo toward Jupiter in October 1989, with the probe swinging by Venus and Earth in 1990 before its encounter with Gaspra.

The Gaspra flyby was the first asteroid flyby in history, with Galileo approaching to just under 1,000 miles (1,601 kilometers) while flying at a whopping 18,000 mph. Galileo's photos of Gaspra "revealed a cratered, complex, irregular body about 20 by 12 by 11 kilometers (12.4 by 7.4 by 6.8 miles), with a thin covering of dust and rubble," NASA wrote in an overview.

"The picture we get of Gaspra is one of something that has suffered a lot during its history," the UPI press service quoted Joseph Veverka, a Galileo imaging team member, as saying at the time. "We suspect that this is the survivor of a series of catastrophic collisions in the past."

Galileo would go on to fly by Earth a second time in 1992, then swing close by the asteroid Ida in 1993 before finally arriving at Jupiter in 1995. Along the way, the probe was in the right place at the right time to watch as the Comet Shoemaker-Levy 9 broke into pieces and crashed into Jupiter in July 1994.

Since Galileo's historic first asteroid flyby, NASA and other space agencies have gone on to land on asteroids (like NEAR on asteroid Eros), collect samples of asteroids and return them to Earth (like NASA's OSIRIS-REx and Japan's Hayabusa probes), and have even intentionally crashed probes into asteroids (like NASA's DART asteroid impact mission) to test planetary protection techniques.

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On Sept. 21, 2003, NASA's Galileo mission to Jupiter and its moons came to an end.

After 14 years of exploration, the Galileo spacecraft intentionally fell into Jupiter and disintegrated in the planet's dense atmosphere. Galileo launched in 1989 from the payload bay of the space shuttle Atlantis and arrived at Jupiter in 1995.

It was the first spacecraft to orbit Jupiter and the first to send a probe into its atmosphere. It discovered evidence of saltwater below the surfaces of three of Jupiter's moons — Europa, Ganymede and Callisto. It was purposely put on a collision course with Jupiter because it was running out of fuel, and NASA wanted to make sure that it wouldn't impact any of Jupiter's moons that could harbor life in their subsurface oceans.

Why it mattered

NASA's Galileo mission to Jupiter was a first-of-its-kind mission, a dedicated orbiter designed to survive the extreme harsh radiation environment around the largest planet in our solar system in an attempt to uncover its secrets.

Named in honor of Galileo Galilei, the Italian Renaissance scientist who discovered Jupiter's major moons in 1610, the Galileo spacecraft carried nine primary instruments to study the giant planet. It also carried a small "Atmospheric Probe," a module just 50 inches (127 centimeters) wide that weighed 750 pounds (339 kilograms) and carried its own instrument suite to measure Jupiter's atmosphere as the probe plunged into the planet.

Galileo made a host of Jupiter discoveries during its storied 14-year mission (which NASA extended three times before the end). The probe discovered that Ganymede, the largest moon in the solar system, has its own magnetic field. It revealed that Jupiter's volcanic moon Io has 100 times more volcanic activity that the Earth, made the first observations of ammonia clouds on Jupiter. And then there is Jupiter's moon Europa, where Galileo found evidence of a subsurface ocean buried beneath an icy crust. That's just a tiny bit of the many Jupiter revelations made by Galileo's science team.

Galileo completed its primary mission in December 1997, with its mission extensions stretching its flight through 2003. In all, Galileo made 35 flybys of Jupiter's moons including 11 of Europa, eight each of Callisto and Ganymede, seven of Io and one of Amalthea.

The Galileo spacecraft set the stage for dedicated orbiters to the outer planets, including NASA's Cassini spacecraft to Saturn and the Juno spacecraft at Jupiter today. Galileo's demise also demonstrated a bit of planetary protection for NASA, which would later recreate the death dive at Saturn with Cassini's Grand Finale as way to protect that planet's icy moons from contamination as well.

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On Aug. 28, 1993, NASA's Galileo spacecraft flew by the asteroid Ida while on its way to Jupiter.

Galileo was the first spacecraft to fly by an asteroid in the asteroid belt. Ida was actually the second one it visited after another flyby of the asteroid Gaspra in 1991.

Galileo whizzed by asteroid Ida going almost 28,000 mph and came within 1,500 miles of the asteroid's surface. During the encounter, Galileo actually discovered a tiny moon orbiting Ida! This was the first confirmed satellite of an asteroid. The International Astronomical Union named the moon Dactyl after a group of mythological beings who lived on a sacred mountain called Mount Ida.

Why it mattered

Asteroid 243 Ida was actually the second asteroid ever to be visited by a spacecraft (Galileo previously flew by the asteroid Gaspra in October 1991), but this flyby clinched the theory that asteroids could have moons.

Until the Galileo spacecraft photographed Ida and Dactyl, astronomers suspected some asteroids could have their own moons, but Galileo proved it once and for all. During the flyby, Galileo approached within 1,500 miles (2,400 kilometers) of Ida as it made its way to Jupiter.

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"It was previously thought that natural satellites of asteroids could form, but they probably weren't common," Torrence Johnson, Galileo's project scientist, once said of the flyby in a NASA statement. "Having found one fairly quickly, we can say that they're probably more common than previously thought."

Ida is an S-type asteroid made mostly of silicate rock and sits in the main asteroid belt. Astronomers suspect it is what's known as a Koronis asteroid, leftover debris from an ancient crash of two much larger objects, according to NASA. It was discovered on Sept. 29, 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory.

Galileo imaging team member Ann Harch actually discovered Ida's moon Dactyl on Feb. 17, 1994, months after the flyby, while reviewing data, NASA said.

Galileo did not stop to study Ida and Dactyl. As with its Gaspra flyby, the probe zoomed by to continue its trip to Jupiter. NASA launched the Galileo mission on Oct. 18, 1989 and it arrived at Jupiter on Dec. 7, 1995, orbiting the giant planet for nearly 8 years. The mission ended on Sept. 21, 2003, when Galileo intentionally crashed into Jupiter.

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If Jupiter's moon Europa has geysers, the natural engines that power them are well-hidden.

Scientists have re-examined data from NASA's Galileo mission in greater detail in search of regions on Europa warm enough to be linked to plumes of water vapor. If hotspots exist on this moon — which harbors a huge ocean of liquid water beneath its icy shell — they will most likely remain hidden until NASA's Europa Clipper spacecraft arrives at Jupiter in the late 2020s or early 2030s, the researchers determined.

In 2016, scientists announced the provisional detection of a faint plume on Europa using NASA's Hubble Space Telescope. A second unconfirmed plume was announced the following year. Since then, the moon has remained quiet, and scientists have debated whether the features spotted by the space telescope were actually caused by wafting water vapor. [Possible Water Plumes on Europa: The Discovery in Images]

In an effort to solve the mystery, Julie Rathbun and her colleagues turned to data collected by Galileo when it orbited Jupiter in the 1990s and early 2000s. Rathbun is a senior scientist at the Planetary Science Institute in Tucson, Arizona. The researchers had searched the moon for heat signals almost a decade ago, before the potential plumes were spotted. Now, they returned to examine the two potential source regions more closely, looking for signs of heat that might indicate geological activity. The results were less than encouraging.

"We didn't see any hotspots at those locations," Rathbun said yesterday (Oct. 22) during a news conference at the American Astronomical Society's annual Division for Planetary Sciences meeting in Knoxville, Tennessee.

But that doesn't necessarily mean Europa isn't spouting off, added Rathbun.

The case of the missing hotspot

Plumes are common throughout the solar system. Jupiter's moon Io is constantly shooting volcanic material into the air. Saturn's icy moon Enceladus famously blasts water vapor and other material from its subsurface ocean into space via a set of geysers near the south pole. And Earth is rich in geysers, from Yellowstone National Park's Old Faithful to Iceland's Great Geysir.

Firing off the gas that makes up these plumes requires an energy source, which usually heats up the ground around the plume source. Enceladus, Io and Earth all have hotspots around their geysers and volcanoes.

But not Europa, as far as we can tell. Rathbun and her colleagues saw no hotspots at either of the purported plume locations described in the Galileo data. 

There are four possible reasons for this result, Rathbun said. First, it's possible that the jets don't exist. It's also possible that the hotspots exist but are subtler than could be detected with Galileo or the Earth-based Atacama Millimeter/Submillimeter Array (ALMA). Another option is that the plumes are sporadic and just didn't fire off while Galileo was nearby. Finally, Europa's plumes may operate differently from the geysers on other worlds.

"Europa was expected to be active," Rathbun said. "Folks have been wondering where [the plumes] are and what happened to them." [Photos: Europa, Mysterious Icy Moon of Jupiter]

Another chance

Both Europa and Enceladus are icy moons, and each is thought to host a subsurface ocean. So, it's no surprise that the planetary scientists based their Europa search on the geysers confirmed on Enceladus. The geysers from Saturn's moon come from long, narrow "tiger-stripe" fissures with roughly the same area as that probed on Europa, and the expected temperatures are also similar.

It's possible, however, that Europa's plume regions have a different configuration than those of the Saturn satellite.

"I don't think it's a tiger stripe [that produced the plume], but there could be some small source that's there that we're just not detecting," Rathbun said.

Timing is another concern. Galileo explored Jupiter and its moons from 1995 to 2003, more than a decade before Hubble spotted the plumes. If Europa's fountains are sporadic, their hosting sites may not yet have become active while Galileo was looking. Similarly, ALMA peered at the moon several months after Hubble's observations, potentially giving the area time to cool off.

Sunlight further complicates the issue. At the beach, sand and rocks heat up and cool at different rates based on their composition. After the plumes were first spotted, ALMA identified a region near a young crater called Pwyll that seemed warmer than surrounding areas. However, Rathbun said that Galileo showed that the same region is coolerthan its environment on the nightside; ALMA's observations, like all those made from Earth, show only the moon's dayside. Rathbun said that the variations most likely originated from features with different compositions. 

"It's much more consistent with different thermal properties than there being a hotspot," she said.

Ideally, the same instrument would compare daytime and nighttime observations to rule out heat from the sun as an explanation for the temperature differences, but unfortunately, Galileo didn't gather daytime images of the puzzling region.

Rathbun said the final possibility is that the plume-generating mechanism on Europa doesn't heat things up the way familiar geyser features do. It's currently unknown what kind of mechanism could do that, but she said the possibility can't be ruled out.

"I am constantly amazed at how many things we find in the outer solar system that don't make any sense," she said.

While there isn't enough data to determine which of the four explanations likely represents reality, Rathbun said that her favorite option is the bizarre answer.

"I'd love it if there's something different going on," she said. "That's always the most fun."

Europa probably won't be able to keep such secrets for much longer. NASA's Europa Clipper spacecraft, which is currently scheduled to launch in 2022, will make 45 flybys of Europa at altitudes ranging from 1,700 miles to 16 miles (2,700 kilometers to 25 km).

One of the instruments Clipper will carry is the Europa Thermal Emission Imaging System (E-THEMIS), a heat detector that will take thermal images of the moon at various wavelengths. 

"If there is a hotspot somewhere on Europa, then E-THEMIS will see it," Rathbun said.

Follow Nola Taylor Redd on Twitter @NolaTRedd. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.

The case for a giant plume of water vapor wafting from Jupiter's potentially life-supporting moon Europa just got a lot stronger.

NASA's Hubble Space Telescope has spotted tantalizing signs of such a plume multiple times over the past half decade, but those measurements were near the limits of the powerful instrument's sensitivity. Now, researchers report in a new study that NASA's Galileo Jupiter probe, which orbited the planet from 1995 to 2003, also detected a likely Europa plume, during a close flyby of the icy moon in 1997.

The newly analyzed Galileo data provides "compelling independent evidence that there seems to be a plume on Europa," said study lead author Xianzhe Jia, an associate professor in the Department of Climate and Space Sciences and Engineering at the University of Michigan. [Possible Water Plumes on Europa: The Discovery in Images]

This is exciting news for astrobiologists: If the plume is indeed real, it could offer a way for a spacecraft to sample Europa's buried ocean of liquid water without even touching down on the moon. And NASA is working on a mission that could do just that.

Hints of a plume

At 1,900 miles (3,100 kilometers) wide, Europa is slightly smaller than Earth's moon. But scientists think the Jovian satellite harbors a huge amount of liquid water — perhaps twice as much water as Earth does, in fact — in a deep global ocean sloshing beneath the object's ice shell.

This ocean appears to be in contact with Europa's rocky core, making possible a variety of interesting and complex chemical reactions. So, many astrobiologists regard Europa as one of the solar system's best bets to host alien life, along with the icy Saturn moon Enceladus, which also features a subsurface ocean.

Near Enceladus' south pole, more than 100 individual geysers continuously blast water ice, organic molecules and other material far out into space — so far that this plume stuff forms Saturn's E ring. Researchers think the geyser material is coming from Enceladus' ocean, so flying through the moon's plume provides a possible way to search for signs of life on Enceladus without even touching down. (NASA's Saturn-orbiting Cassini probe flew through Enceladus' plume multiple times, but the spacecraft wasn't equipped with life-detection gear. Nobody knew about Enceladus' geysers until Cassini spotted them in 2005.)

Recently, evidence has been building that Europa may have a plume as well. In late 2012, Hubble spotted signs of such a feature near the moon's south pole. Further Hubble observations in 2014 and 2016 identified candidate plumes close to Europa's equator — in both cases, in the middle of a 200-mile-wide (320 km) Europan "hotspot" discovered by the Galileo probe. 

As suggestive as these results were, they didn't represent a definitive Europa plume discovery, researchers said at the time. That's because project team members couldn't conclusively rule out other possible explanations, such as instrument artifacts. [6 Most Likely Places for Alien Life in the Solar System]

A new look at old data

The newly analyzed Galileo data could help solidify evidence for the plume's existence. 

During its time at Jupiter, Galileo performed 11 flybys of Europa. Jia and his team took an in-depth look at information the probe gathered during the closest of these encounters — a 1997 flyby that brought Galileo within 128 miles (206 km) of the moon's frigid, fractured surface.

The researchers found that, during this flyby, Galileo detected a significant change in Europa's magnetic field, as well as a brief but big increase in the density of plasma, or ionized gas. Both of these observations provide strong evidence of a plume, Jia said. And these lines of evidence are independent of those gathered by Hubble. For example, in the 2014 and 2016 candidate detections, the possible plumes blocked some ultraviolet light emitted by Jupiter.  

Intriguingly, Jia and his colleagues also determined that the 1997 candidate plume emanated from the same general hotspot (or thermal anomaly) as the 2014 and 2016 phenomena.

Why did it take more than two decades to tease this result out of the Galileo data set? For starters, Jia said, the Galileo mission team wasn't specifically looking for plumes. 

In addition, "to make sense of the observations, we had to really go for sophisticated numerical modeling" techniques, he told Space.com. "And I don't think those were available back 20 years ago." 

Flying through the plume?

The new study, which was published online today (May 14) in the journal Nature Astronomy, is undoubtedly of great interest to NASA. The agency is developing a $2 billion Jupiter-orbiting mission called Europa Clipper, which is scheduled to launch in the early to mid-2020s.

If all goes according to plan, Clipper will make 40 to 45 flybys of Europa over the course of its mission, studying the moon's ice shell and subsurface ocean in an attempt to better understand Europa's potential to host life as we know it. Europa Clipper will also scout out locations for a future lander mission, which Congress has directed the space agency to develop as well.

NASA officials have also said they'd like Clipper to dive through Europa's putative plumes, if possible, to potentially grab fresh samples of the moon's ocean (if plume material is indeed coming directly from that ocean). 

The evidence gathered to date suggests that the processes producing Europa's plumes — if they do indeed exist — may not be continuous like the geysers of Enceladus but instead intermittent. Ephemeral plumes could make it tough to plan sample-snagging flybys. 

But the new results offer some optimism in this regard, Jia said. After all, scientists have now spotted possible plume activity in the same area of Europa multiple times over a 19-year span.

"The general region of the thermal anomaly might have long-lived plume activity," Jia said. 

Such activity may be caused by different individual jets or geysers turning on and off over time, he added. If NASA (and the rest of us who care about the search for alien life) get lucky, those jets may be on when the Europa Clipper reaches its destination and starts its science work.

Follow Mike Wall on Twitter @michaeldwall and Google+. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.