A stunning time-lapse video of Earth captured from the International Space Station shows a lightning storm flashing over the U.S. and "possible satellites" orbiting overhead.

The European Space Agency (ESA) created the video using images taken by French astronaut Thomas Pesquet, a member of the Expedition 51 crew on the orbiting complex.

"Time lapse over California with a thunderstorm on the horizon," Pesquet wrote in a caption posted with the video on Flickr. "These time lapses are made on Earth by taking many pictures and playing them one after the other. There are usually around 25 pictures for a second of video." [Photos: Earth's Lightning Seen from Space]

Mystery objects

About halfway through the video, you may notice some small, bright objects streaking through the sky. ESA officials told Space.com that these are "probably functioning satellites," though scientists were unable to confirm which satellites they were. "The giveaway is the fact that the lights are not tumbling, which indicates they are actively controlled," ESA communications officer Daniel Scuka said in an email.

ESA's Space Debris Office determined that the objects are most likely not space junk, because "the objects' brightness in the video is consistent with intact objects," officials said. And they're probably not meteors, either, said Detlef Koschny, a scientist in ESA's Space Situational Awareness Program office who studies near-Earth objects.

Koschny explained that a bright meteor burning up in the atmosphere typically has a duration of a second or less, possibly 2 or 3 seconds for larger objects. This movie, however, runs 25 times faster than real time, he said, meaning the objects are bright for several tens of seconds.

The objects' altitude doesn't fit that of meteors, either, he added. "The typical altitude of a meteor is around 80 to 110 kilometers [50 to 68 miles]. This corresponds to the height of the airglow, which is visible curving above the Earth as a brightish band," Koschny said. "These objects are higher, at least 300 km [186 miles]. Meteors would not be visible in that height."

Watching storms from space

When lightning illuminates the sky, it is considered an indication of strong updrafts before or during a thunderstorm, according to the National Weather Service. As air swirls around in a turbulent, stormy atmosphere, the friction generates electrical charges in the clouds. As those charges build up, it leads to electrification and lightning that can be seen both from the ground and in space.

Looking through the windows of the International Space Station (ISS) may seem like a convenient way to monitor storms from space, but astronauts at the orbiting lab don't spend much time storm-watching. Also, their vantage point is limited. The ISS flies roughly 250 miles (400 km) above Earth, and astronauts on board cannot see Earth's north or south poles due to the station's orbit. And because it travels at about 17,500 mph (28,000 km/h), the ISS doesn't stay over the same place for very long.

Last year, however, NASA launched the most powerful lightning mapper yet; it's on the GOES-16 satellite (previously known as GOES-R), which is in geostationary orbit above the Americas. The instrument, called the Geostationary Lightning Mapper (GLM), can view lightning beneath it to a resolution of about 6.2 miles (10 km).GLM beamed back its first photo of lightning from space in March.

Space.com senior producer Steve Spaleta contributed to this report. You can follow Elizabeth Howell @howellspace, or Space.com @Spacedotcom. We're also on Facebook and Google+. Original article on Space.com.  

What's a planet? What's a dwarf planet? Should we make a distinction? Should we really care about these definitions in the first place?

As we learn more about the outer solar system, the boundaries begin to blur.

A tiny celestial body called 2014 UZ224 and informally known as DeeDee (for “distant dwarf”) is a distant world about 92 astronomical units, or Earth-sun distances, from our sun. Recent observations from the Atacama Large Millimeter/submillimeter Array (ALMA) revealed that DeeDee is roughly 395 miles (635 kilometers) across, which would give it enough mass to be spherical.

Why does it matter if DeeDee is round? In 2006, a controversial vote by the International Astronomical Union defined three parameters for a planet. Simply speaking, the IAU says a planet must be in orbit around the sun, have enough mass to be round, and have cleared the neighborhood around its orbit — meaning it needs to be gravitationally dominant and hold any nearby bodies within its orbit.

It's the last part of the definition that most aggravates those who argue that Pluto – redefined as a "dwarf" planet under the IAU – is more planet than not. The argument is that the rocky planets of Earth, Mars, Venus, and Mercury have also not cleared their neighborhoods, as many asteroids co-orbit along with them.

The planetary geologist Kirby Runyon, a Ph.D. student at the Johns Hopkins University Applied Physics Laboratory, wrote a paper this year proposing a geophysical-based definition of a planet that would dispense with the orbital criterion and basically include any round celestial body that isn’t a star. The idea hatched by Runyon and his co-authors — which include Alan Stern, the principal investigator of the New Horizons mission to Pluto — would increase the number of purported planets in our solar system to over 110, including Earth’s moon and DeeDee.

RELATED: Behind the Push to Get Pluto Its Planetary Groove Back

"What it's really showing is the diversity of planets in our solar system,” said Runyon of the DeeDee news, “and giving us a better understanding of planets in the rest of the galaxy.”

"DeeDee is almost certainly made out of ices — water ices, methane, and carbon dioxide — which is similar to what Pluto is made of," he added. "These are very soft materials, compared with rocky silicate. It's more easily pulled into a sphere than rock or metal."

Adding more fodder to the debate over the definition, when the New Horizons spacecraft flew by Pluto in 2015, it unveiled a world of surprising complexity, ranging from mountainous areas to vast nitrogen-ice lakes.

"We call Pluto a 'dwarf' planet, but it's just an adjective for 'planet,’” Runyon said. “It's still a planet, and that's where we take umbrage with the IAU.”

"Astronomers aren't experts in planetary science, and they basically passed a bunch of B.S. off on the public back in 2006 with a planet classification so flawed that it rules the Earth out as a planet, too," Stern remarked in 2016. "A week later, hundreds of planetary scientists, more people than at the IAU vote, signed a petition that rejects the new definition. If you go to planetary science meetings and hear technical talks on Pluto, you will hear experts calling it a planet every day."

RELATED: Remind Me Again, Why Isn't Pluto a Planet?

As a geomorphologist, Runyon is most particularly interested in the shape of landforms on worlds around the solar system. His dissertation deals with wind-blown sand geology on Mars, as mapped by the Mars Reconnaissance Orbiter's HiRISE (High Resolution Imaging Science Experiment) camera.

But he'll still keep his eye on the outer solar system. New Horizons is set to fly by a tiny world called 2014 MU69 on Jan. 1, 2019. While MU69 is too small to be even a dwarf planet, Runyon will be looking for features on MU69 such as ripples, which were also seen during the Rosetta mission during observations of Comet 67P/Churyumov–Gerasimenko. He'll also look at the shape of craters, among other things.

"MU69 has never warmed up since the formation of the solar system," he said. "It will be really interesting to see how different or complex it might be. There's a laundry list of things we'll be looking for, and hopefully we'll be surprised."

Originally published on Seeker.

A relocatable lander could explore the hazy skies of Saturn's intriguing moon Titan, according a new mission proposal. As the eight-bladed whirlybird travels across the moon, it could investigate some of the most promising potentially habitable sites on the Saturn satellite, where methane and ethane fall from the sky and flow as rivers and lakes.

The lander-size instrument, known as Dragonfly, would take advantage of Titan's low gravity and thick atmosphere to visit multiple sites over several years, moving from one promising site to the next and recharging between the brief flights.

"It's such a rich place to be able to explore in situ, and then it hands us the way to explore it," the project's principal investigator, Elizabeth Turtle, told Space.com. Turtle, a planetary scientist at the Johns Hopkins Applied Research Laboratory in Maryland, is leading the team that's proposing an in-depth exploration of Titan as part of NASA's New Frontiers mission program, which generally funds midsize missions to explore the solar system. She presented the Dragonfly concept last month at the Lunar and Planetary Sciences Conference in The Woodlands, Texas. [Amazing Photos of Titan: Saturn's Biggest Moon]

On Titan, flowing methane and ethane rivers and seas provide a unique opportunity to explore the chemistry that could lead to the rise of life. But it's the thick atmosphere that would make the mission possible.

"The atmosphere is what is giving us this ability to travel on Titan," Turtle said.

"Just waiting for us"

When the Cassini-Huygens mission, a joint initiative between NASA and the European Space Agency, arrived at Titan in 2004, it discovered a world where methane rained down onto the surface into organic-rich lakes and seas. It dropped the Huygens probe onto Titan's surface, providing a tantalizing peek at some of the chemistry beneath the clouds. Over the past decade, the orbiter revealed even more details about Titan's surface, including a variety of environments with the potential to have chemical evolution similar to Earth's, Turtle said.

"The kind of prebiotic chemistry that we're looking at, these are things we can't do in the lab — the timescales are too long to do these experiments in the lab — but Titan has been doing them for ages," Turtle said.

"The results are just sitting on the surface," she added. "If we can get to these different places on the surface of Titan, we can pick up the results of the experiments. They're just waiting for us."

When it came to exploring various locations on Mars, that meant rovers. Each rover dropped at a promising location could trek for tens of miles over its lifetime. As of April 2017, NASA's Opportunity rover had traveled a total of more than 27 miles (43 kilometers), and Curiosity had traveled nearly 10 miles (16 km).

But instead of sending multiple rovers to explore Titan, Turtle wants to use the moon's thick atmosphere to travel more efficiently. Titan's atmosphere is about four times as dense as Earth's, while its gravity is about a tenth as strong. 

"Heavier-than-air flight is substantially easier [on Titan]," Turtle said. "That means we can take a really capable lander and move it by a few tens of kilometers in a single flight, and hundreds of kilometers over the time of the mission."

In the past, Titan mission proposals have included balloons and airships that took advantage of the thick atmosphere to travel. But these missions required these vehicles to be constantly in the air, which consumed a great deal of power, Turtle said. They also provided only cursory exploration of the surface.

Instead, Dragonfly would use two rotors positioned at each of its four corners to fly from one region of the moon to the next, then recharge while landed using the multimission radioisotope thermoelectric generator (MMRTG) it would carry with it, which converts the heat from decaying plutonium-238 into electricity. This could mean Dragonfly could fly around Titan for years, or even decades, Turtle said. At the same time, the thick atmosphere would block damaging radiation, providing a welcoming environment for a long-lived mission, she added.

If NASA selects Dragonfly, the spacecraft would be ready for launch in the mid-2020s and should arrive in the 2030s, Turtle said. Cassini's data would provide potential landing sites, but once Dragonfly arrived, it would be able to scout them out and, using the same type of program that Mars rovers use to land safely, decide which one would be the best landing location. After landing, the quadcopter could launch and map several potential sites, and then return to its original spot to continue investigating while scientists decide where it should go next. [How Humans Could Live on Saturn's Moon Titan (Infographic)]

Flying isn't the only task Dragonfly would excel at, Turtle said: A drill and a sampling system would allow it to examine the surface up close, while a spectrometer would let it study the surface composition in larger patches. Meteorology and remote sensing would help characterize the atmosphere and weather of Titan, where a methane cycle stands in for Earth's water cycle, she added. 

The spacecraft's ability to move would help it keep Earth in its line of sight, as it will be communicating directly with the planet. Dragonfly would arrive during Titan's northern winter, so it would start out in the southern hemisphere because Earth won't be in the sky in the north, Turtle said. But as the seasons shift, the quadcopter could move, too, heading up north when our planet rises again.

Begging us to visit

With methane and ethane falling as raindrops from the sky, Titan boasts a hydration cycle both similar to and different from Earth's. The moon is covered with organic materials that make it a potential home for a different sort of life to evolve. That's just one reason many scientists are eager to visit the Saturn satellite. [Life on Titan? Saturn's Cold Moon Fascinates Scientists (Video)]

"Titan is the ideal destination to do prebiotic chemistry," Turtle said. "It has incredibly rich organic material all over the surface."

The giant sand dunes of organic material that stretch for thousands of kilometers across Titan's equator are a potentially intriguing target. Although scientists aren't certain how these dunes form, they may represent what Turtle called a "grab bag" of materials from across the surface.

Impact craters provide another interesting region to explore. The impacts should have melted ice in the crust, thus putting liquid water in close contact with organic material for extended periods, Turtle said. 

"We can start to look at how the organic chemistry progressed," she added. 

Along the way, Dragonfly could hunt for very basic signs of life on the methane-rich world.

"If we're taking the instrumentation to measure the details of the chemistry, we can also look for biosignatures, because it's the same measurements," Turtle said.

With a $1 billion price tag, NASA's New Frontiers missions are exploring some of the most intriguing places in the universe. Previous selections included the New Horizons mission to Pluto, the Juno spacecraft to Jupiter and the OSIRIS-REx asteroid sample return mission. The current competition lists six mission themes, including sample-return missions from comets or the moon, an ocean worlds explorer, a probe to Saturn, exploration of the Trojan asteroids and a Venus in-situ explorer. NASA plans to select a new mission every five years.

The deadline for the latest round of New Frontiers proposals is April 28, and Dragonfly will be one of the candidates. In November 2017, NASA will select a subset of the proposals for further study and will make its final selection in July 2019. That means there will be a long wait for Turtle, who is hoping Dragonfly comes out on top.

"Titan is just begging us to do this," she said.

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