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A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth).

There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and between 2004 and 2017, it also had an artificial one - the Cassini spacecraft, which explored the ringed planet and its moons.

Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial satellite was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.

Brief history of artificial satellites

Following that feat, on Nov. 3, 1957 the Soviets launched an even more massive satellite - Sputnik 2 - which carried a dog, Laika. The United States' first satellite was Explorer 1 on Jan. 31, 1958. The satellite was only 2 percent the mass of Sputnik 2, however, at 30 pounds (13 kg).

The Sputniks and Explorer 1 became the opening shots in a space race between the United States and the Soviet Union that lasted until at least the late 1960s. The focus on satellites as political tools began to give way to people as both countries sent humans into space in 1961. Later in the decade, however, the aims of both countries began to split. While the United States went on to land people on the moon and create the space shuttle, the Soviet Union constructed the world's first space station, Salyut 1, which launched in 1971. (Other stations followed, such as the United States' Skylab and the Soviet Union's Mir.)

(Image credit: NASA/Jet Propulsion Laboratory)

Other countries began to send their own satellites into space as the benefits rippled through society. Weather satellites improved forecasts, even for remote areas. Land-watching satellites such as the Landsat series (on its ninth generation now) tracked changes in forests, water and other parts of Earth's surface over time. Telecommunications satellites made long-distance telephone calls and eventually, live television broadcasts from across the world a normal part of life. Later generations helped with Internet connections.

With the miniaturization of computers and other hardware, it's now possible to send up much smaller satellites that can do science, telecommunications or other functions in orbit. It's common now for companies and universities to create " CubeSats", or cube-shaped satellites that frequently populate low-Earth orbit.

These can be lofted on a rocket along with a bigger payload, or sent from a mobile launcher on the International Space Station (ISS). NASA is now considering sending CubeSats to Mars or to the moon Europa (near Jupiter) for future missions, although the CubeSats aren't confirmed for inclusion.

The ISS is the biggest satellite in orbit, and took over a decade to construct. Piece by piece, 15 nations contributed financial and physical infrastructure to the orbiting complex, which was put together between 1998 and 2011. Program officials expect the ISS to keep running until at least 2024.

Parts of a satellite

Every usable artificial satellite - whether it's a human or robotic one - has four main parts to it: a power system (which could be solar or nuclear, for example), a way to control its attitude, an antenna to transmit and receive information, and a payload to collect information (such as a camera or particle detector).

As will be seen below, however, not all artificial satellites are necessarily workable ones. Even a screw or a bit of paint is considered an "artificial" satellite, even though these are missing these parts.

What keeps a satellite from falling to Earth?

A satellite is best understood as a projectile, or an object that has only one force acting on it - gravity. Technically speaking, anything that crosses the Karman Line at an altitude of 100 kilometers (62 miles) is considered in space. However, a satellite needs to be going fast - at least 8 km (5 miles) a second - to stop from falling back down to Earth immediately.

If a satellite is traveling fast enough, it will perpetually "fall" toward Earth, but the Earth's curvature means that the satellite will fall around our planet instead of crashing back on the surface. Satellites that travel closer to Earth are at risk of falling because the drag of atmospheric molecules will slow the satellites down. Those that orbit farther away from Earth have fewer molecules to contend with.

There are several accepted "zones" of orbits around the Earth. One is called low-Earth-orbit, which extends from about 160 to 2,000 km (about 100 to 1,250 miles). This is the zone where the ISS orbits and where the space shuttle used to do its work. In fact, all human missions except for the Apollo flights to the moon took place in this zone. Most satellites also work in this zone.

Geostationary or geosynchronous orbit is the best spot for communications satellites to use, however. This is a zone above Earth's equator at an altitude of 35,786 km (22,236 mi). At this altitude, the rate of "fall" around the Earth is about the same as Earth's rotation, which allows the satellite to stay above the same spot on Earth almost constantly. The satellite thus keeps a perpetual connection with a fixed antenna on the ground, allowing for reliable communications. When geostationary satellites reach the end of their life, protocol dictates they're moved out of the way for a new satellite to take their place. That's because there is only so much room, or so many "slots" in that orbit, to allow the satellites to operate without interference.

While some satellites are best used around the equator, others are better suited to more polar orbits - those that circle the Earth from pole to pole so that their coverage zones include the north and south poles. Examples of polar-orbiting satellites include weather satellites and reconnaissance satellites.

(Image credit: Rick Mastracchio ‏(via Twitter as @AstroRM))

What stops a satellite from crashing into another satellite?

There are an estimated half-million artificial objects in Earth orbit today, ranging in size from paint flecks up to full-fledged satellites - each traveling at speeds of thousands of miles an hour. Only a fraction of these satellites are useable, meaning that there is a lot of "space junk" floating around out there. With everything that is lobbed into orbit, the chance of a collision increases.

Space agencies have to consider orbital trajectories carefully when launching something into space. Agencies such as the United States Space Surveillance Network keep an eye on orbital debris from the ground, and alert NASA and other entities if an errant piece is in danger of hitting something vital. This means that from time to time, the ISS needs to perform evasive maneuvers to get out of the way.

Collisions still occur, however. One of the biggest culprits of space debris was the leftovers of a 2007 anti-satellite test performed by the Chinese, which generated debris that destroyed a Russian satellite in 2013. Also that year, the Iridium 33 and Cosmos 2251 satellites smashed into each other, generating a cloud of debris.

NASA, the European Space Agency and many other entities are considering measures to reduce the amount of orbital debris. Some suggest bringing down dead satellites in some way, perhaps using a net or air bursts to disturb the debris from its orbit and bring it closer to Earth. Others are thinking about refueling dead satellites for reuse, a technology that has been demonstrated robotically on the ISS.

Moons around other worlds

Most planets in our solar system have natural satellites, which we also call moons. For the inner planets: Mercury and Venus each have no moons. Earth has one relatively large moon, while Mars has two asteroid-sized small moons called Phobos and Deimos. ( Phobos is slowly spiralling into Mars and will likely break apart or fall into the surface in a few thousand years.)

Beyond the asteroid belt, are four gas giant planets that each have a pantheon of moons. As of late 2018, Jupiter has 79 confirmed moons, Saturn has 53, Uranus has 27 and Neptune has 14. New moons are occasionally discovered - mainly by missions (either past or present, as we can analyze old pictures) or by performing fresh observations by telescope.

Saturn is a special example because it is surrounded by thousands of small objects that form a ring visible even in small telescopes from Earth. Scientists watching the rings close-up over 13 years, during the Cassini mission, saw conditions in which new moons might be born. Scientists were particularly interested in propellers, which are wakes in the rings created by fragments in the rings. Just after Cassini's mission ended in 2017, NASA said it's possible the propellers share elements of planet formation that takes place around young stars' gassy discs.

Even smaller objects have moons, however. Pluto is technically a dwarf planet. However, the people behind the New Horizons mission, which flew by Pluto in 2015, argue its diverse geography makes it more planet-like. One thing that isn't argued, however, is the number of moons around Pluto. Pluto has five known moons, most of which were discovered when New Horizons was in development or en route to the dwarf planet.

A lot of asteroids have moons, too. These small worlds sometimes fly close to the Earth, and the moons pop out in observations with radar. A few famous examples of asteroids with moons include 4 Vesta (which was visited by NASA's Dawn mission), 243 Ida, 433 Eros, and 951 Gaspra. There are also examples of asteroids with rings, such as 10199 Chariklo and 2060 Chiron.

Many planets and worlds in our solar system have human-made "moons" as well, particularly around Mars - where several probes orbit the planet doing observations of its surface and environment. The planets Mercury, Venus, Mars, Jupiter and Saturn all had artificial satellites observing them at some point in history. Other objects had artificial satellites as well, such as Comet 67P/Churyumov-Gerasimenko (visited by the European Space Agency's Rosetta mission) or Vesta and Ceres (both visited by NASA's Dawn mission.) Technically speaking, during the Apollo missions, humans flew in artificial "moons" (spacecraft) around our own moon between 1968 and 1972. NASA may even build a " Deep Space Gateway" space station near the moon in the coming decades, as a launching point for human Mars missions.

Fans of the movie "Avatar" (2009) will remember that the humans visited Pandora, the habitable moon of a gas giant named Polyphemus. We don't know yet if there are moons for exoplanets, but we suspect - given that the solar system planets have so many moons - that exoplanets have moons as well. In 2014, scientists made an observation of an object that could be interpreted as an exomoon circling an exoplanet, but the observation can't be repeated as it took place as the object moved in front of a star. However, a second exomoon might have been found very recently.

Additional resources

Bibliography

Joukowsky Institute, Brown University, " 13 Things - space"

Amanda Barnett, NASA's Jet Propulsion Laboratory for NASA's Science Mission Directorate, " Basics of Space Flight - Section 1: Environment, Chapter 5: Planetary Orbits"

Astromaterials Research & Exploration Science, NASA, " The Orbital Debris Issue"

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A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth).

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If you want to protect a species, you have to know where it is. That can be difficult in the ocean, where animals are often hidden beneath the waves. To find out where ocean animals go and the routes they take to get there, scientists attach electronic tags to them. Those tags collect data on temperature, depth, and location, then transmit their data via satellite.

Jeffrey Polovina is a scientist with NOAA's Pacific Islands Fisheries Science Center in Honolulu, and he and his colleagues have tagged hundreds of loggerhead sea turtles, bigeye tuna, swordfish, moonfish, and whale sharks. "The tags allow us to see tracks in the ocean," Polovina said, "but we're also interested in the bigger picture."

As Polovina explains in this interview, the bigger picture comes from combining tag data with data from ocean-observing satellites. That oceanographic data shows large-scale features in the ocean such as currents, fronts, and eddies. The tracking data from the tags show how animals navigate through and around these features.

Your research involves combining tag data with data from ocean-observing satellites. What do you learn by doing this?

We look out on the ocean and see this homogenous expanse. But animals see it very differently, and that's why it's useful to send fleets of animals out with tags on them. By combining tag data with oceanographic data, we get to see how animals perceive the ocean, and that provides some amazing insights.

It's clear that these animals aren't just wandering randomly over the ocean. They're following well-defined paths. Sometimes they'll get into an eddy, which is a big circulation pattern that can be 100 km across, and they'll ride around the edge, maybe once or twice, each time taking maybe a month to go around. And then they'll get off right where they got on, and continue on their way. They get pushed around by strong currents, but then they'll travel back to resume their trajectory. They're able to stay within different features of the ocean, something that for a ship would require GPS and satellite imagery. You get a sense that these animals not only know where they are but they know where they're going in this big ocean.
The tracks of a loggerhead sea turtle that spent four months traveling the edge of an eddy in the Pacific Ocean (A is Oct 2003, B Nov 2003, C Dec 2003, and D Jan 2004). Currents and sea surface height are shown as background. Maps: Jeffrey Polovina et al. 2006. The Kuroshio Extension. Deepsea Res.II (53)326-339.
What was the biggest surprise you've had upon downloading data from an electronic tag?

Loggerhead sea turtles showed us a new aspect of the ocean that we didn't expect. The loggerheads we tagged nest on beaches in Japan, and they also turn up off the Baja Peninsula in Mexico. So we assumed that after hatching they swam across the Pacific to Mexico. We tagged about 400 loggerheads, and we found that quite a number of them spend years and even decades in the open ocean far from shore, migrating back and forth along a well-defined oceanographic front.

That front separates a nutrient-rich mass of water to the north from nutrient-poor waters to the south, and the turtles were moving back and forth across that boundary. People had studied these regions before, but until the turtles showed us that it was important, no one had paid attention to the boundary between them, which stretches most of the way across the Pacific Ocean. So they showed us an important feature of the ocean that we didn't know was out there.

What was the most challenging tagging situation you've encountered?
Fishermen in Fiji help Jeffrey Polovina (front right) carry a loggerhead sea turtle that is about to be outfitted with a satellite tag. This tagged turtle will help scientists identify habitats and migration routes that are important to loggerhead sea turtles. Photo: NOAA Fisheries
Each animal has its peculiar challenges and problems. We've tagged moonfish-they're also known as opah-which are these big, slow-swimming, deep-diving animals. We were getting a lot of data from opah that showed standard dive distributions of 300 to 400 meters depth.

Then after a couple weeks one of the tags started showing completely different behavior. They were going deeper, the water temperature was much warmer, and the light sensors recorded total darkness. [Editor's note: data from the light sensors are used to calculate location, since GPS doesn't work underwater.]

We thought the tag had malfunctioned and sent it back to the manufacturer. But it turned out that a shark ate the tag, and maybe the opah with it. The tag was in the stomach of an endothermic shark, so it was warmer than the ambient water temperature, and of course it was totally dark. After a couple of weeks in the shark it came out one way or another, popped up to the surface, and we got tracks of two different species from the same tag.

So the tags are also collecting information on the animals' diving behavior. What are we learning about the three-dimensional aspect of the ocean from tagging data?

It seems that every time we put a tag on an animal, it's diving a lot deeper than we had previously imagined. For instance, we used to think that whale sharks mainly used the shallow surface layer of the ocean. But our tagging showed that they often dive to at least 980 meters. We don't really know how deep they're going-the tags only register up to 980 meters. They might be going a lot deeper.
An opah, a.k.a moonfish. Photo: NOAA Fisheries
These deep dives are a puzzle. It may be that they're finding prey organisms at those depths. Maybe they go there to escape from predators, or maybe they're just going down there to cool off after building up heat at the surface layer.

We've also seen swordfish in the Central Pacific going down to 800 meters and feeding all day in that cold dark water and then coming up at night to warm up. Bigeye tuna shoot down to around 300 meters, but they only stay for an hour or two then shoot back up to warm up and oxygenate.

Different species seem to be partitioning the vertical depth structure in the ocean, each foraging on different species in a different depth range, so that they're not all at the same depth competing for the same resources.

What's the next step for tagging research?

Longer-lasting tags so we can collect multi-year data and smaller tags so we can tag smaller animals. For example, longer-lasting tags would allow us to analyze how the movements and habitats of equatorial tuna change between El Niño and La Niña conditions. Smaller tags would allow us to learn about movements of hatchling sea turtles.

We're also looking forward to tags with new types of sensors. Currently most tags just provide temperature, depth, and some measure of location. Tag sensors that provide swimming velocity, or that take physiological readings such as heartbeat for bioenergetic models or stomach temperature to indicate prey ingestion will allow us to ask a lot of interesting questions.
A whale shark. Photo: Wikimedia

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Satellite Tags Reveal How Animals See the Ocean

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If you want to protect a species, you have to know where it is. That can be difficult in the ocean, where animals are often hidden beneath the waves. To find out where ocean animals go and the routes they take to get there, scientists attach electronic tags to them.

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A tag used on animals who usually do not reach the surface very often; programmed to detach from an animal after a certain time; only broadcasts its data when it reaches the surface of the water

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a bulky, boxy tag which compresses the most data of any tag, measures the salinity, temperature and depth; usually used on beeg marine life

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Smart Position or Temperature Transmitting Tags (SPOTs)

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Smart Position or Temperature Transmitting Tags (SPOTs)

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A type of satellite transmitter tag that is primarily used for the type of animals that breathe from the surface of the ocean; it only uplinks when the antenna out of the water; usually used on whales, turtles and such

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a planet or moon that orbits around another celestial body

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man-made orbital systems used to gather data, whether scientific, militaristic, navigational or related to weather