Questions and Answers

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The definition of a planet continues to be debated. The debate about whether Pluto is a planet was generated by recent detections of hundreds of planetary objects in the outer solar system. The International Astronomical Union classifies Pluto as a dwarf planet. Most people call Pluto a planet because it orbits the Sun and it is large enough that its own gravity has pulled it into a spherical shape.

Yes — it has five! Pluto's largest moon, named Charon, is half the size of Pluto. Side by side Pluto and Charon would fit across the diameter of our Moon. Click here for more information on Charon. The other moons are named Hydra, Nix, Kerberos and Styx.

Very, very cold. The temperature on Pluto is minus 387 degrees Fahrenheit (which is minus 233 degrees Celsius or 40 Kelvin).

The surface of Pluto is extremely cold, roughly 40 degrees above absolute zero (minus 387 Fahrenheit or minus 233 Celsius), so it seems unlikely that life could exist there. At such cold temperatures, water, which is vital for life as we know it, is essentially rock-like. Pluto's interior is warmer, however, and some think there could even be an ocean deep inside.

Visit the "Why Pluto?" page for more discussion on the possibility of life on Pluto.

Yes, though Pluto's atmosphere is not very thick. The pressure at the surface of Pluto is about 3 to 100 microbars or 3 to 100 millionths of the surface pressure of the Earth's atmosphere. The main constituent is molecular nitrogen, (N2) the same as on Earth. Molecules of methane and carbon monoxide have also been detected at Pluto. But no oxygen has been detected at Pluto yet.

Click here for further discussion on Pluto's atmosphere.

A typical scene on Pluto would probably be an icy, frosty, dimly lit landscape. It might be similar to a view on Earth in the winter in the arctic, lit by a full moon. Go here for further discussion of views from Pluto.

Before New Horizons, Pluto was the only planet in our solar system unexplored by space probes. A mission was needed to go to the Pluto system and the Kuiper Belt to explore the mysterious, icy worlds at the edge of our solar system and tell us about the origin and evolution of our planetary neighbors.

Check out the "Why Pluto?" page for more on New Horizons' mission objectives.

Traveling to Pluto using the minimum amount of fuel would take longer than 30 years. NASA's Voyager mission demonstrated the advantages of using the gravity of the giant planets, particularly Jupiter, to "boost" a spacecraft and reduce travel times to the outer solar system. Travel time to Pluto would be a bit longer with other vehicles:

Bike:47,600 years
Car:6,660 years
Jet plane:700 years
Space shuttle:25 years

New Horizons, using a flyby of Jupiter in February 2007, arrived at Pluto in 2015. The journey took 9½ years.

Pluto's gravity is weak so that it takes a large amount of fuel to go into orbit around the planet- and when New Horizons zipped past Pluto at nearly 14 kilometers per second (more than 30,000 miles per hour), there was no practical way to store the tremendous amount of fuel the spacecraft would need to slow down enough to begin an orbit mission. A flyby mission provides many images and other kinds of information about Pluto and its moons, as well as an opportunity to fly on to another Kuiper Belt Object.

The cost of the mission, including the launch vehicle and operations through the Pluto-Charon encounter, is roughly $700 million. Divided among the population of the United States (according to the U.S. Census clock at https://www.census.gov/popclock/world) over the 10-year duration of the mission, this comes out to about 25 cents per person, per year.

Fun Facts

Yes. During daylight on Pluto, the Sun would be almost 300 times as bright as the full Moon on Earth (1/900 times dimmer than full daylight on Earth).

The Sun would still be the brightest object in the sky, by far. Although a mere fraction of the brightness of sunlight on Earth, the Sun seen from Pluto would still be about 20 million times brighter than the brightest star.

Pluto is 14th magnitude as seen from Earth - meaning it's hundreds of times fainter than the naked eye can see. Earth has 36 times the surface area, 1,000 times the illumination and a similar reflectivity as Pluto, so, Earth should be about 3rd magnitude (as bright as an easily visible star) as seen from Pluto. The biggest problem with seeing Earth from Pluto is that it would be located close to the Sun, but it might be briefly visible to the naked eye when Charon eclipses the Sun.

Just below 7% of your Earthly weight on Pluto, and just over 3% of your terrestrial weight on Charon. To be a little more accurate, every 100 pounds of weight on your bathroom scale on Earth would weigh just 6.7 pounds on Pluto and 3.4 pounds on Charon.

No. To escape Charon's gravity you need to get up a lot of speed. The escape velocity from Charon is near 1,351 miles per hour (610 meters per second). Not even the fastest runner or the strongest person could reach those speeds on Charon.

Yes, in a fashion. Let's look at some of the key questions below . . .

New Horizons discovered huge ridges and mountains of water ice on Pluto, towering up to more than 6 km (nearly 4 miles or 20,000 feet) above the surface.

On Earth, a skier heading straight down a 30-degree slope will accelerate to 11 miles per hour in one second if his wax is really good and he's got a good tuck. Gravity on Pluto is about 15 times less than on the Earth, so the same skier on Pluto would "tear" down the same hill at 0.7 mph after one second.

Pluto's surface is mainly covered with nitrogen ice at about minus 387 Fahrenheit (or minus 233 Celsius). It's not known whether solid nitrogen ice is as slippery as water ice, but a skier/boarder on Pluto may want to take advantage of another property of nitrogen ice - its vapor pressure.

All ices are slowly sublimating, which means that some of their surface molecules escape into the atmosphere. You've seen puddles of water evaporate on a sunny day, and sublimation is the same, except the molecules jump directly from the solid ice to the atmosphere. Carbon dioxide ice (dry ice) sublimates when you have a chunk of it in a room, for example.

The sublimation rate increases rapidly when the ice temperature gets little warmer. This is very important on Pluto, where the entire atmosphere depends on the average temperature of the nitrogen ice. A small increase in the nitrogen ice temperature of 2 degrees Fahrenheit will double Pluto's atmosphere!

With heated skis, our Pluto downhiller skier would float on a cushion of nitrogen gas. Since the nitrogen condenses right onto the surface (as opposed to falling as snow), the surface is probably hard and icy, not champagne powder.

The idea that we slide on water ice (either on skates or skis) because the pressure of the skate or ski causes melting is a myth - see the San Francisco Exploratorium's site on the science of hockey for more on that topic. We will have to wait for laboratory data before we can discuss the slipperiness of nitrogen ice at temperatures typical of Pluto's surface.

An astronaut (Plutonaut?) stepping from their spaceship onto Pluto's surface would quickly notice many unusual qualities of this alien environment. Perhaps the first impression would be the overall sense of darkness. The Sun, just a bright pinpoint in the sky, provides only a thousandth as much illumination to Pluto's surface as it does to Earth's, making daytime on the distant planet much darker than a cloudy, stormy day here at home. But Pluto's sky is strikingly clear, and in addition to the Sun, thousands of stars are visible, even in daytime. There are no clouds and Pluto's air is far too thin to cause the sky to be bright and blue, as it is on Earth. Depending on which part of the planet the astronaut landed, they might see Pluto's largest moon, Charon, looming in the sky some seven times larger than our own Moon appears in Earth's sky. Charon is smaller than Earth's Moon, but it is much closer to the planet, making it appear far larger. Remarkably, from a given location on Pluto, Charon remains motionless in the sky, going through its cycle of phases in 6.4 days without rising or setting.

If the astronaut landed on the opposite side of Pluto, they wouldn't see Charon no matter how long they waited, because Pluto and Charon keep their same sides toward one another all the time. On this side of Pluto, the moon never rises above the horizon. For more on this, click here.

Pluto's solid surface, with its hills, valleys, craters and other topographic features, is primarily made of ice, perhaps similar to environments near the North and South poles of our own planet. But the ice on Pluto's surface is primarily frozen nitrogen, not water. At Pluto's extremely low daytime temperature of about minus 380 degrees Fahrenheit (minus 233 Celsius or 40 degrees above absolute zero), nearly everything familiar to us, even gas, is frozen solid. When frozen, nitrogen (which is also the most abundant gas in Earth's atmosphere) forms large, transparent crystals several inches across. Much of Pluto's surface must be an amazing and fantasy-like crystalline world, unlike any other place except Neptune 's largest moon, Triton, where frozen nitrogen also makes up most of the landscape. On both Pluto and Triton, small amounts of methane (CH4, natural gas on Earth) are frozen into the nitrogen crystals. Some regions of Pluto's surface have exposures of ordinary water ice and small amounts of frozen carbon monoxide (CO).

In the dim Pluto daylight, the astronaut may be able to see that the landscape has a yellowish or pinkish color caused by particles of haze that slowly fall from the thin, cloudless atmosphere. The feeble sunlight falling on the thin envelope of nitrogen, methane and carbon monoxide gases that make up Pluto's atmosphere causes chemical reactions that form a thin layer of smog over the entire planet. These smog particles are a mixture of complex organic compounds fashioned by Nature from carbon, nitrogen, hydrogen, and oxygen atoms. These atoms are broken apart from the molecules of gas in Pluto's atmosphere by ultraviolet light from the Sun. Over vast expanses of time, some of the particles making up the smog accumulate on the surface and are incorporated into the ices, giving them a faint yellow or pink color that can be clearly seen from Earth.

Some regions on Pluto have a dark gray tone, as seen from pictures and other observations made with large telescopes. These regions may hold a concentration of carbon-rich materials, or perhaps rocky minerals similar to those found on Earth, the Moon and other rocky planets. If the regions turn out to be icy landscapes tinged with carbon-rich materials, the carbon may have come from collisions with comets, which are rich in complex molecules inherited from the giant molecular cloud of dust and gas from which they (and the Sun and planets) formed. Similarly, if the darker gray regions consist of a rocky mineral coating on the ice, those minerals may also have come from the impacts of comets, which are known to be rich in silicate minerals.


There are two reasons. The first is an engineering reason. To get to Pluto (which is 5 billion kilometers or 3 billion miles from Earth) in just 9.5 years, as New Horizons will, the spacecraft must travel very, very quickly. As a result, New Horizons will speed by Pluto at a velocity of about 43,000 kilometers per hour(27,000 miles per hour). To get into orbit, operators would have to reduce that speed by over 90%, which would require more than 1,000 times the fuel that New Horizons can carry.

The second reason is scientific: If we did stop to go into orbit, we wouldn't be able to go on to explore the Kuiper Belt!

The huge distance of Pluto from the Sun, about 32 times farther than the Earth-Sun distance, creates "extreme" conditions in several respects. First, it takes a long time to get to Pluto (9.5 years with a gravity boost from Jupiter), which means that the spacecraft and instruments must have long lifetimes (i.e., their "warranties" must be good for over a decade!). The large distance from the Sun also means that solar cells could not be used to power the spacecraft, and the onboard systems must be designed to operate in a cold environment. Finally, sending a spacecraft all the way to Pluto required a powerful rocket and places severe constraints on the weights of the instruments. The more mass you want to carry, the larger the rocket needs to be. Even using the Lockheed-Martin Atlas V rocket, one of the most powerful in the U.S. fleet, New Horizons had to weigh about 1,050 pounds (480 kilograms).

All of the above posed major technical challenges, but the New Horizons team has found ways to overcome all of them and delivered a spacecraft that can meet all of NASA's primary scientific objectives at Pluto.

The New Horizons spacecraft is roughly 8 feet (2.5 meters) across and weighed approximately 1,050 pounds(480 kilograms) — about half a ton — when first fueled. It's about the size (and shape) of a baby grand piano.

It carries seven scientific instruments:

  1. Alice is an ultraviolet spectrometer used for measuring gas composition
  2. Ralph combines an infrared spectrometer (LEISA) for mapping surface composition with a color optical imager (MVIC) for mapping surface structure and composition
  3. REX is a radio experiment for measuring atmospheric composition and temperature
  4. LORRI is an optical telescope that provides the highest resolution imaging of the surface
  5. PEPSSI is a plasma-sensing instrument for measuring particles escaping from Pluto's atmosphere
  6. SWAP is a plasma-sensing instrument for measuring the properties of the solar wind at Pluto, Pluto's atmospheric escape rate, and for searching for a magnetosphere around Pluto. The "solar wind" is a stream of charged particles streaming away from the Sun at high speed.
  7. SDC, an instrument used to measure dust impacts at the New Horizons spacecraft during its entire trajectory, was built by students at the University of Colorado!


The cost of the mission, including the launch vehicle and operations through the Pluto-Charon encounter, will be roughly $700 million. Divided among the population of the United States at launch, over the 10-year duration of the mission, this comes out to about 20 cents per person, per year.


The surface of Pluto is extremely cold, roughly 40 degrees above absolute zero (minus 387° Fahrenheit or minus 233° Celsius), so it seems unlikely that life could exist there. At such cold temperatures, water, which is vital for life as we know it, is essentially rock-like. Pluto's interior is warmer, however, and some think there could even be an ocean deep inside.

Life as we know it requires three things:

  • Water
  • Biogenic elements such as carbon, phosphorus and sulfur, in addition to the oxygen and hydrogen in water
  • A source of energy (light, heat, chemical potential) that a living organism can use

Pluto's surface is far too cold for liquid water, but its interior is probably warm and maintained that way by the slow decay of naturally occurring elements such as uranium, potassium-40 and thorium.

Enough heat is released that a water ocean may exist between the rocky core of Pluto and its thick outer layer of ice. Planetary scientists have long thought that icy satellites might possess oceanic layers underneath their surface ice layers. The discovery by the Galileo orbiter that Europa, Callisto and possibly Ganymede possess interior oceans greatly increases our expectation that Pluto also possesses an ocean. Pluto's ocean is also likely to contain biogenic elements in a solution, especially if it is in contact with an organic-rich layer.

Where Pluto probably does not pass astrobiological muster is in the matter of sufficient energy to power life. Pluto's ocean would be dark and cold - near-freezing. Even if in contact with a rock core, it is almost certainly true that this modest core is today insufficiently hot to be volcanically active or even to drive circulations. So it is difficult to argue for a deep biosphere on Pluto today. On the other hand, it is also true that Pluto's rock core was much hotter and probably active in the geological past, so it is not utter lunacy to speculate that some form of primitive, microbial life may have evolved long ago and just might have once plied the "Styxian seas" of Pluto.

A special panel of the National Academy of Sciences that was formed to advise NASA on a planetary science strategy every 10 years (the so-called "Decadal Survey") ranked the exploration of Kuiper Belt Objects, including Pluto, as its highest scientific priority. The New Horizons mission is NASA's way to implement that recommendation.

The key scientific objectives of the New Horizons mission were:

  • Characterize the global geology and morphology of Pluto and Charon - What do Pluto and Charon look like close up?
  • Map the surface composition of Pluto and Charon - What are Pluto and Charon made of, and how are those materials distributed over the surface of each object?
  • Characterize the neutral atmosphere of Pluto and its escape rate - Why does Pluto have an atmosphere and how long will it last?

Generally, New Horizons seeks to understand where Pluto and Charon "fit in" with the other objects in the solar system. We currently classify the planets into groups. Earth, Mars, Venus and Mercury are the "terrestrial" planets, which are mostly rocky objects. In contrast, the "gas giant" planets, which include Jupiter, Saturn, Uranus and Neptune, are dominated by thick, molecular hydrogen atmospheres. Pluto and Charon belong to a third category that could be called "ice dwarfs." They have solid surfaces but, unlike the terrestrial planets, a significant portion of their mass is icy material (such as frozen water, carbon dioxide, molecular nitrogen, methane and carbon monoxide).

Pluto and Charon are also widely considered to be among the largest objects in the Kuiper Belt, a vast reservoir of icy objects located just outside of Neptune's orbit and extending out to about 50 astronomical units from the Sun. The Kuiper Belt is thought to be the source of most short-period comets - those with orbits shorter than 200 years - so scientists really want to compare the composition and surface properties of Pluto and Charon to those of cometary nuclei.

Pluto and its moons are truly part of the current "frontier" in planetary science. No spacecraft had ever explored them before New Horizons, and the results from the mission promise to tell us much about the origins and outskirts of our solar system.

After flying by Pluto, the New Horizons spacecraft flew through the planet's shadow. As the spacecraft moved in and out of the shadow, sunlight passed through the planet's atmosphere before reaching the spacecraft. Absorption of sunlight by Pluto's atmosphere is detected as characteristic "dips" in the ultraviolet part of the spectrum of light measured by New Horizons' Alice instrument. This technique is a very powerful method for measuring even trace amounts of atmospheric gas.

In addition, radio waves sent from Earth to New Horizons bent as they passed through Pluto's atmosphere. The amount of bending of the radio waves was detected by the New Horizons Radio Science Experiment (called REX) and is related to both the average molecular mass and the temperature of the atmosphere.

Together, these ultraviolet and radio "occultation experiments" provided powerful probes of Pluto's tenuous atmosphere.

Pluto has 5 moons! Charon, Hydra, Nix, Styx, and Kerberos. These moons were discovered from Earth, but New Horizons searched for even smaller moons when it flew past Pluto, but no more were found.

Pluto is currently moving away from the Sun, having reached its closest approach distance in 1989. Generally, the closer an object is to the Sun, the warmer it should be and the more rapidly its surface ice should sublime into space. The sublimation of ices on the surface of Pluto is responsible for its tenuous atmosphere. As Pluto moves away from the Sun it will get colder and, eventually, its atmosphere will almost completely condense back onto the surface.

The actual situation is a bit more complicated than the simple illustration discussed above. Because of "thermal lag," the time of Pluto's closest approach to the Sun in 1989 was probably not when its surface temperature was greatest, just as the temperature on Earth is hottest at mid-afternoon rather than noon. In the case of Pluto, the latest observations reveal that the atmosphere has thickened during the past decade. But this trend will definitely reverse as Pluto continues moving away from the Sun. Scientists don't know exactly when the condensation will start to dominate sublimation - which is why they wanted to get to Pluto as soon as possible!

Unfortunately, New Horizons can't reach either object. Quaoar is located far away from the trajectory of any spacecraft that travels toward Pluto during the next several decades. The outer solar system is a big place with a lot of volume! New Horizons is just our first attempt to probe this region, but scientists are sure Quaoar and Sedna will be high on the list of candidate targets as they contemplate other missions to explore the outer solar system during the next several decades.

In the 1970s technological advances in telescopes and other instruments brought tiny, faint Pluto within range for Earth-bound observers. In 1975, using the technique of light analysis called spectroscopy, astronomers Dale Cruikshank, David Morrison and Carl Pilcher measured a portion of Pluto's infrared spectrum using one of the then-largest telescopes on Earth.

They aimed the 4-meter Mayall Telescope and its powerful spectrometer (located at Kitt Peak National Observatory in Arizona) at Pluto and recorded the signature, or spectral fingerprint, of frozen methane. This discovery gave the first indication that Pluto's surface is icy rather than rocky, and opened a new era of investigations of the realm of small icy objects in the outer solar system that continues today. Working with other colleagues, Cruikshank (now a New Horizons team member) later discovered the frozen nitrogen and carbon monoxide, as well as evidence for the colored organic chemicals that make up Pluto's surface.

From our understanding of the properties of the ices of Pluto, we know that some of them slowly evaporate from the surface and enter the atmosphere as gases, much in the way that ice cubes evaporate in the freezer of the refrigerator. Ices of carbon monoxide and methane have also been detected on Pluto's surface using telescope measurements of reflected sunlight (at near infrared wavelengths). Scientists therefore believe that the atmosphere also contains trace amounts of carbon monoxide and methane gases supplied by sublimation of their ices.

Thus, even frigid, distant and tiny Pluto is a dynamic world where the processes of nature continuously change the surface and the atmosphere, creating an alien and exotic world that beckons us from Earth to visit, explore and learn.