Pluto
is so far from Earth that even powerful telescopes reveal little detail of its
surface. The Hubble Space Telescope gathered the light for the pictures of Pluto
shown here. Image credit: NASA
Pluto is named for the Roman god
of the underworld. Venetia Burney, an 11-year-old girl from Oxford, England,
suggested the name.
This
is the clearest view yet of the distant planet Pluto and its moon, Charon, as
revealed by NASA's Hubble Space Telescope (HST). The image was taken by the
European Space Agency's Faint Object Camera on February 21, 1994 when the planet
was 2.6 billion miles (4.4 billion kilometers) from Earth; or nearly 30 times
the separation between Earth and the sun. Hubble's corrected optics show the two
objects as clearly separate and sharp disks. This now allows astronomers to
measure directly (to within about 1 percent) Pluto's diameter of 1440 miles
(2320 kilometers) and Charon's diameter of 790 miles (1270 kilometers). The
Hubble observations show that Charon is bluer than Pluto. This means that both
worlds have different surface composition and structure. A bright highlight on
Pluto suggests it has a smoothly reflecting surface layer. A detailed analysis
of the Hubble image also suggests there is a bright area parallel to the equator
on Pluto. This result is consistent with surface brightness models based on
previous ground-based photometric observations. However, subsequent HST
observations will be required to confirm whether the feature is real. Though
Pluto was discovered in 1930, Charon wasn't detected until 1978. That is because
the moon is so close to Pluto that the two worlds are typically blurred together
when viewed through ground-based telescopes. (If our moon were as close to
Earth, it would be as big in the night sky as an apple held at arm's length).
The new HST image was taken when Charon was near its maximum elongation from
Pluto of .9 arc seconds. The two worlds are 12,200 miles apart (19,640
kilometers). Hubble's ability to distinguish Pluto's disk at a distance of 2.6
billion miles (4.4 billion kilometers) is equivalent to seeing a baseball at a
distance of 40 miles (64 kilometers). Pluto typically is called the double
planet because Charon is half the diameter of Pluto (our Moon is one-quarter the
diameter of Earth).
Once known as the smallest,
coldest, and most distant planet from the Sun, Pluto has a dual identity, not to
mention being enshrouded in controversy since its discovery in 1930. On August
24, 2006, the International Astronomical Union (IAU) formally downgraded Pluto
from an official planet to a dwarf planet. According to the new rules a planet
meets three criteria: it must orbit the Sun, it must be big enough for gravity
to squash it into a round ball, and it must have cleared other things out of the
way in its orbital neighborhood. The latter measure knocks out Pluto and Eris,
which orbit among the icy wrecks of the Kuiper Belt, and Ceres, which is in the
asteroid belt.
(1) A "planet" is a
celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for
its self-gravity to overcome rigid body forces so that it assumes a hydrostatic
equilibrium (nearly round) shape, and (c) has cleared the neighborhood around
its orbit.
(2) A "dwarf planet" is
a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass
for its self-gravity to overcome rigid body forces so that it assumes a
hydrostatic equilibrium (nearly round) shape, (c) has not cleared the
neighborhood around its orbit, and (d) is not a satellite.
(3) All other objects except
satellites orbiting the Sun shall be referred to collectively as "Small
Solar-System Bodies".
Pluto is also a member of a group
of objects that orbit in a disc-like zone beyond the orbit of Neptune called the
Kuiper Belt. This distant region consists of thousands of miniature icy worlds
with diameters of at least 1,000 km and is also believed to be the source of
some comets.
Discovered by American astronomer
Clyde Tombaugh in 1930, Pluto takes 248 years to orbit the Sun. Pluto's most
recent close approach to the Sun was in 1989. Between 1979 and 1999, Pluto's
highly elliptical orbit brought it closer to the Sun than Neptune, providing
rare opportunities to study Charon.
Most of what we know about Pluto
we have learned since the late 1970s from Earth-based observations, the Infrared
Astronomical Satellite (IRAS), and the Hubble Space Telescope. Many of the key
questions about Pluto, Charon, and the outer fringes of our solar system await
close-up observations by a robotic space flight mission.
No spacecraft have yet visited
Pluto. However, NASA launched a mission called New Horizons that will explore
both Pluto and the Kuiper Belt region.
Pluto and and Charon orbit the
Sun in a region where there may be a population of hundreds or thousands of
similar bodies that were formed early in solar system history. These objects are
referred to interchangeably as trans-Neptunian objects, Edgeworth-Kuiper Disk
objects or ice dwarves.
Pluto is about two-thirds the
diameter of Earth's Moon and may have a rocky core surrounded by a mantle of
water ice. Due to its lower density, its mass is about one-sixth that of the
Moon. Pluto appears to have a bright layer of frozen methane, nitrogen, and
carbon monoxide on its surface. While it is close to the Sun, these ices thaw,
rise, and temporarily form a thin atmosphere, with a pressure one one-millionth
that of Earth's atmosphere. Pluto's low gravity (about 6 percent of
Earth's)causes the atmosphere to be much more extended in altitude than our
planet's. Because Pluto's orbit is so elliptical, Pluto grows much colder during
the part of each orbit when it is traveling away from the Sun. During this time,
the bulk of the planet's atmosphere freezes.
In 1978, American astronomers
James Christy and Robert Harrington discovered that Pluto has a satellite
(moon), which they named Charon. Charon is almost half the size of Pluto and
shares the same orbit. Pluto and Charon are thus essentially a double planet.
Charon's surface is covered with dirty water ice and doesn't reflect as much
light as Pluto's surface.
Because Pluto is so small and far
away, it is difficult to observe from Earth. In the late 1980s, Pluto and Charon
passed in front of each other repeatedly for several years. Observations of
these rare events allowed astronomers to make crude maps of each body. From
these maps it was learned that Pluto has polar caps, as well as large, dark
spots nearer its equator.
Pluto: Facts &
Figures
Discovered By
Clyde Tombaugh
Date of Discovery
1930
Average Distance from the Sun
Metric: 5,906,380,000 km
English: 3,670,050,000
miles
Scientific Notation:
5.90638 x 109 km (39.482 A.U.)
By Comparison: 39.482 x
Earth
Perihelion (closest)
Metric: 4,436,820,000 km
English: 2,756,902,000
miles
Scientific Notation:
4.43682 x 109 km (29.658 A.U.)
By Comparison: 30.171 x
Earth
Aphelion (farthest)
Metric: 7,375,930,000 km
English: 4,583,190,000
miles
Scientific Notation:
7.37593 x 109 km (49.305 A.U.)
By Comparison: 48.481 x
Earth
Equatorial Radius
Metric: 1,151 km
English: 715 miles
Scientific Notation:
1.151 x 103 km
By Comparison: 0.180 x
Earth
Equatorial Circumference
Metric: 7,232 km
English: 4,494 miles
Scientific Notation:
7.232 x 103 km
Volume
Metric: 6,390,000,000 km3
English: 1,530,000,000 mi3
Scientific Notation: 6.39
x 109 km3
By Comparison: 0.0059 x
Earth
Mass
Metric:
13,000,000,000,000,000,000,000 kg
Scientific Notation: 1.3
x 1022 kg
By Comparison: 0.0022 x
Earth
Density
Metric: 2 g/cm3
By Comparison: ~ 0.4 x
Earth
Surface Area
Metric: 16,650,000 km2
English: 6,430,000 square miles
Scientific Notation:
1.665 x 107 km2
By Comparison: 0.033 x
Earth
Equatorial Surface Gravity
Metric: 0.81 m/s2
English: 2.7 ft/s2
By Comparison: If you
weigh 100 pounds on Earth, you would weigh 8 pounds on
Pluto.
Escape Velocity
Metric: 4,570 km/h
English: 2,840 mph
Scientific Notation:
1,270 m/s
By Comparison: Escape
velocity of Earth is 25,022 mph.
Sidereal Rotation Period (Length
of Day)
- 6.387 Earth days (retrograde)
- 153.3 hours (retrograde)
By Comparison: One Earth
day is 24 hours.
Sidereal Orbit Period (Length of
Year)
247.92 Earth years
90,553 Earth days
Mean Orbit Velocity
Metric: 17,096 km/h
English: 10,623 mph
Scientific Notation:
4,749.0 m/s
By Comparison: 0.425 x
Earth
Orbital Eccentricity
0.2488
By Comparison: 14.9 x
Earth
Orbital Inclination to Ecliptic
17.14 degrees
Equatorial Inclination to Orbit
119.61 degrees
By Comparison: 5.10 x
Earth
Orbital Circumference
Metric: 32,820,000,000 km
English: 20,390,000,000
miles
Scientific Notation:
3.282 x 1010 km
By Comparison: 35.505 x
Earth
Minimum/Maximum Surface
Temperature
Metric: -233/-223 °C
English: -387/-369 °F
Scientific Notation:
40/50 K
Atmospheric Constituents
By Comparison: Earth's
atmosphere consists mostly of N2 and O2.
Pluto/Earth
Comparison
Bulk parameters
Pluto Earth Ratio (Pluto/Earth)
Mass (1024 kg) 0.0125 5.9736 0.0021
Volume (1010 km3) 0.715 108.321 0.0066
Equatorial radius (km) 1195 6378.1 0.187
Polar radius (km) 1195 6356.8 0.188
Volumetric mean radius (km) 1195 6371.0 0.188
Ellipticity (Flattening) 0.0000 0.00335 0.0
Mean density (kg/m3) 1750 5515 0.317
Surface gravity (m/s2) 0.58 9.80 0.059
Surface acceleration (m/s2) 0.58 9.78 0.059
Escape velocity (km/s) 1.2 11.19 0.107
GM (x 106 km3/s2) 0.00083 0.3986 0.0021
Bond albedo 0.4 - 0.6 0.306 1.3 - 2.0
Visual geometric albedo 0.5 - 0.7 0.367 1.4 - 1.9
Visual magnitude V(1,0) -1.0 -3.86 -
Solar irradiance (W/m2) 0.89 1367.6 0.0007
Black-body temperature (K) 37.5 254.3 0.147
Number of natural satellites 3* 1
Planetary ring system No No
Orbital parameters
Pluto Earth Ratio (Pluto/Earth)
Semimajor axis (106 km) 5906.38 149.60 39.482
Sidereal orbit period (days) 90,465 365.256 247.68
Tropical orbit period (days) 90,588 365.242 248.02
Perihelion (106 km) 4436.82 147.09 30.164
Aphelion (106 km) 7375.93 152.10 48.494
Synodic period (days) 366.73 - -
Mean orbital velocity (km/s) 4.72 29.78 0.158
Max. orbital velocity (km/s) 6.10 30.29 0.201
Min. orbital velocity (km/s) 3.71 29.29 0.127
Orbit inclination (deg) 17.16 0.000 -
Orbit eccentricity 0.2488 0.0167 14.899
Sidereal rotation period (hrs) -153.2928 23.9345 6.405
Length of day (hrs) 153.2820 24.0000 6.387
Obliquity to orbit (deg) 122.53 23.45 (2.451)
Pluto Observational Parameters
Discoverer: Clyde Tombaugh
Discovery Date: 18 February 1930
Distance from Earth
Minimum (106 km) 4284.7
Maximum (106 km) 7528.0
Apparent diameter from Earth
Maximum (seconds of arc) 0.11
Minimum (seconds of arc) 0.06
Mean values at opposition from Earth
Distance from Earth (106 km) 5750.54
Apparent diameter (seconds of arc) 0.08
Apparent visual magnitude 15.1
Maximum apparent visual magnitude 13.65
Pluto Mean Orbital Elements
(J2000)
Semimajor axis (AU) 39.48168677
Orbital eccentricity 0.24880766
Orbital inclination (deg) 17.14175
Longitude of ascending node (deg) 110.30347
Longitude of perihelion (deg) 224.06676
Mean longitude (deg) 238.92881
On 11 February 1999 at 11:22 UT (6:22 a.m. EST), Pluto passed Neptune as the
furthest planet from the Sun once again and will remain so until 5 April 2231.
North Pole of Rotation
Right Ascension: 313.02
Declination : 9.09
Reference Date : 12:00 UT 1 Jan 2000 (JD 2451545.0)
Pluto Atmosphere
Surface Pressure: ~3 microbar
Average temperature: ~50 K (-223 C)
Scale height: ~60 km
Mean molecular weight: ~16-25 g/mole
Atmospheric composition: Methane (CH4), Nitrogen (N2)
Charon
Mean distance from Pluto (km) 19,600
Sidereal orbit period (days) 6.38725
Sidereal rotation period (days) 6.38725
Orbital inclination to Pluto (deg) 0.0
Orbital eccentricity 0.0
Equatorial radius (km) 593
Mass (1021 kg) 1.62
Mean density (kg/m3) 1850
Surface gravity (m/s2) 0.31
Escape velocity (km/s) 0.60
Geometric albedo 0.38
Apparent visual magnitude 16.8
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and other sources as stated and credited Researched
by Charles Welch-Updated dailyThis Website is a project of the The
Ozone Hole Inc. a 501(c)(3) Nonprofit Organization
