Dwarf planet Ceres (1 Ceres)

Discovered by:                 Giuseppe Piazzi
Discovery date:                January 1, 1801     
Minor planet category:      Dwarf planet, Main belt
Magnitude range:              +6.6 to +9.3
Aphelion:                         2.9765 AU (445.28 million kms or 276.68 million miles)
Perihelion:                       2.5570 AU (382.52 million kms or 237.69 million miles)
Semi-major axis:              2.7668 AU (413.91 million kms or 257.19 million miles)

Equatorial radius:             487 kms or 303 miles
Polar radius:                    455 kms or 283 miles
Mass:                              9.43×10^20 kg or 0.00015 Earths
Mean density:                  2.08 g/cm3
Escape velocity:               0.51 km/s or 0.32 miles/s
Sidereal rotation period:     9.07 hours
Axial tilt:                          3 degrees

Eccentricity:                     0.075797
Orbital period:                   4.6 years
Inclination:                        10.6 degrees
Longitude of ascending node: 80.328 degrees
Albedo:                             0.09
Mean surface temp:          168 K (-105 C or -157 F)
Max. surface temp:           235 K (-38 C or -37 F)

Ceres as imaged by the Hubble Space Telescope (NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park))

Ceres was discovered on January 1, 1801 the first day of the 19th century. Ceres is located at an average distance of 2.77 AU (413.9 million kilometres or 257.2 million miles) from the Sun, in the main asteroid belt between the orbits of Mars and Jupiter.

It was suggested by Johann Elert Bode in 1772 that an undiscovered planet could exist between the orbits of Mars and Jupiter. This was based on a now discredited hypothesis proposed by Bode and Johann Daniel Titius in 1766. The theory known as Titius-Bode law, predicted to a reasonable accuracy the distances of the then known planets from the Sun, with one exception, a missing planet at a distance of 2.8 AU from the Sun.

Titius-Bode law (or sometime simply just Bodes law) once again fit well with the orbit of Ceres and was only discredited following the later discovery of Neptune, which didn't fit the pattern.

Originally classified as a planet (along with 2 Pallas, 3 Juno and 4 Vesta), it was 50 years later re-classified as an asteroid and then re-classified again on August 24, 2006 as a dwarf planet, the only one in the inner Solar System.

The other known dwarf planets are Eris, Haumea, Makemake and Pluto, all of which are located in the outer Solar System.

Ceres is a rocky body approx. 950 kilometres (590 miles) in diameter. It's the largest object in the asteroid belt but the smallest identified dwarf planet.

It contains about one-third of the mass of the asteroid belt.

The mass of Ceres comprises is about 3.0×10^20 kg, which is about 4% of the mass of the Moon.

The apparent magnitude of Ceres ranges from +6.7 to +9.3. Even at its brightest it's not visible to the naked eye but is an easy binocular object.

The surface of Ceres is probably a mixture of water ice and various hydrated minerals such as carbonates and clay minerals.

It's possible that Ceres may have a thin atmosphere with water frost on its surface.

The maximum temperature on the surface of Ceres is estimated to be 235 K (-38 C or -37 F). The average temperature is 168 K (-105 °C or -157 °F).

Orbital period is 4.6 years with the orbit inclined at 10.6 degrees. This compares to 1.58 degrees for the Earth, 7 degrees for Mercury and 17 degrees for Pluto. Orbital eccentricity is 0.08. This is very similar to 0.09 for Mars but much greater than 0.017 for the Earth (almost circular orbit).

NASA's unmanned Dawn spacecraft is currently en-route to Ceres and will arrive in March or April 2015. The spacecraft was previously in orbit around asteroid Vesta between July 2011 and September 2012.

Comet Hyakutake (C/1996 B2)

Discovered by:                Yuji Hyakutake
Discovery date:               January 31, 1996
Also known as:               The Great Comet of 1996
Peak magnitude:             0.0

Aphelion:                         3410 AU (510 billion kilometres or 317 billion miles)
Perihelion:                       0.2302 AU (34.44 million kilometres or 21.40 million miles)
Semi-major axis:             1700 AU (254 billion kilometres or 158 billion miles)
Eccentricity:                    0.9998946
Orbital period:                  70,000 years (estimate)
Inclination:                      124.92246 degrees

Last perihelion:                 May 1, 1996
Next perihelion:                72000 (estimate)

Comet Hyakutake photgraphed on March 25, 1996 (E. Kolmhofer, H. Raab - Johannes-Kepler-Observatory, Linz, Austria - http://www.sternwarte.at)

Comet Hyakutake is also known the Great Comet of 1996.

Discovered on January 31, 1996 by Japanese amateur astronomer while using 25×150 binoculars.

Apparent magnitude at discovery was +11.0. The comet was 2 AU from the Sun, beyond the orbit of Mars, and exhibited a small coma.

On March 25th, Hyakutake passed only 0.1 AU (14.96 million kilometres or 9.30 million miles) from the Earth. In the previous 100 years only four comets have passed closer.  

At the time Comet Hale-Bopp had recently been discovered and was touted as a potential “Great Comet” in 1997. The discovery of Hyakutake threw the possibility of second spectacular Comet into the mix.

Reached naked eye visibility in early March 1996. 

As it then approach the Earth, Hyakutake brightened significantly with its tail growing in length.

At closest approach to Earth, the comet reached magnitude 0.0 with a tail stretching up to 80 degrees in length. Even the coma was 2 degrees in diameter, 4 times the diameter of the full Moon.

Hyakutake was moving so rapidly across the night sky at this time that its movement could be detected against the stars in just a few minutes. It appeared close to the North Pole Star and consequently circumpolar (visible all night) from northern hemisphere based observers.

The comet had a notably bluish-green colour, which was noticeable even to the naked eye.

SInce Hyakutake was at its brightest for only a few days, it did not have the time to capture the public imagination in the same way that Comet Hale–Bopp did the following year.

Because of unfavourable weather conditions, many observers (particular European) missed the chance to see the comet at its peak.

Hyakutake reached perihelion on May 1, 1996 but was too close to the Sun to be easily seen.

After its perihelion passage, Hyakutake faded rapidly and was no longer a naked-eye target by June.

The last known observation of the comet was on November 2, 1996.

It has been calculated that Hyakutake passed through the inner Solar System approximately 17,000 years ago. Changes to its orbit during the 1996 passage resulting from gas-giant gravitational interactions means that it will not return to the inner Solar System for approximately 70,000 years.

Comet Lovejoy (C/2011 W3)

Discovered by:         Terry Lovejoy
Discovery date:        November 27, 2011
Also known as:        The Great Christmas Comet of 2011
Peak magnitude:      -4.0
Aphelion:                 157.4 AU (23.6 billion kilometres or 14.6 billion miles)
Perihelion:                0.00555 AU (830,000 kilometers or 516,000 miles)
Semi-major axis:       78.7 AU (11.8 billion kilometres or 7.3 billion miles)
Eccentricity:             0.99993
Orbital period:           622 years
Inclination:               134.4 degrees

Last perihelion:        December 16, 2011
Next perihelion:       2633 (estimate)

Comet Lovejoy visible near Earth's horizon from the International Space Station on December 22, 2011 (NASA/Dan Burbank)

Comet Lovejoy is also known the Great Christmas Comet of 2011.

Discovered on November 27, 2011 by Australian amateur observer Terry Lovejoy of Thornlands, Queensland using a 200mm (8-inch) Schmidt–Cassegrain telescope.

Apparent magnitude at discovery was +13.

Lovejoy is a Kreutz-group (sun-grazing) comet that was the first of its kind discovered by ground-based observations in 40 years.

As it approached the Sun it was observed in detail by the STEREO-A, STEREO-B, SOHO, SDO, Hinode and PROBA2 satellites and is the brightest sungrazing comet ever observed by SOHO.

It reached perihelion on December 16, 2011 at 00:17 UT. At this time, the comet was only 140,000 kilometres (87,000 miles) above the Sun's surface, traveling at a speed of 535 km/sec (333 miles/sec).

Amazingly it survived the close flyby of the Sun.

Peaked at magnitude –4.0 (about the same as Venus) although was generally invisible to the naked eye at this time due to closeness to the Sun.

After perihelion it became a fine morning object for Southern Hemisphere observers around December 21/22, although it had faded to 4th magnitude.

Spectacular images of Lovejoy were obtained by Dan Burbank from the International Space Station.

It put on a similar show to another sun-grazer, Comet Ikeya-Seki in 1965.

Lovejoy was the brightest comet to appear since McNaught hit magnitude -5.5 in 2007.

The comet appears to be fragmented. Parts of it could be the same object as comets observed in 467, 1106 and 1329.

Expected to return sometime around 2633.

Comet Ikeya Seki (C/1965 S1)

Discovered by:             Kaoru Ikeya and Tsutomu Seki
Discovery date:            September 18, 1965
Also known as:            The Great Comet of 1965
Peak magnitude:         -10.0
Aphelion (Part A):        183.2 AU (27.4 billion kilometres or 17.0 billion miles)
Aphelion (Part B):         207.4 AU (31.0 billion kilometres or 19.3 billion miles)
Perihelion (A):              0.00778 AU (1.16 million kilometers or 0.72 million miles)
Perihelion (B):              0.00778 AU (1.16 million kilometers or 0.72 million miles)
Semi-major axis (A):     91.6 AU (13.7 billion kilometres or 8.5 billion miles)
Semi-major axis (B):    103.7 AU (15.5 billion kilometres or 9.6 billion miles)
Eccentricity (A):           0.999915
Eccentricity (B):           0.999925
Orbital period (A):         876.7 years
Orbital period (B):         1056.1 years
Inclination (A):              141.9 degrees
Inclination (B):              141.9 degrees

Last perihelion:             October 21, 1965
Next perihelion:             2565 (estimate)

Comet Ikeya Seki photographed on October 30, 1965 (James W. Young - TMO/JPL/NASA)

Comet Ikeya–Seki is also known as the Great Comet of 1965.

Discovered independently on September 18, 1965 by Japanese astronomers Kaoru Ikeya and Tsutomu Seki.

Ikeya–Seki is a member of the Kreutz Sungrazers group of comets. These are believed to be fragments of a large comet that broke up in 1106.

At perihelion on October 21, 1965 it passed only 450,000 kilometres (280,000 miles) above the Sun's surface.

Reached magnitude –10 just before perihelion and was visible in daylight, appearing close to the Sun. 

Comet Ikeya–Seki was one of the brightest comets in recorded history and the brightest in the last millennium.

It broke into three pieces just before perihelion. The remains of the comet continued in almost identical orbits with it re-appearing in the morning sky in late October.

Spectacular naked-eye sight in late October with a beautiful long tail 60 degrees in length.

At its maximum length, Ikeya-Seki's tail extended for 113 million kilometers (70 million miles), ranking it as the fourth largest ever recorded. Only the Great Comets of 1680, 1811 and 1843 had longer tails.

Finally disappeared from view in early 1966.

Due to return around 2565 but with a shattered nucleus impossible to predict a repeat performance.

Comet Hale-Bopp (C/1995 O1)

Discovered by:         Alan Hale and Thomas Bopp
Discovery date:        July 23, 1995
Also known as:        The Great Comet of 1997
Peak magnitude:      -1.0
Aphelion:                 370.8 AU (55.47 billion kilometres or 34.47 billion miles)
Perihelion:               0.914 AU (136.73 million kilometres or 84.17 million miles)
Semi-major axis:      186 AU (27.83 billion kilometres or 17.51 billion miles)
Eccentricity:            0.995086
Orbital period:          2533 years
Inclination:               89.4 degrees

Last perihelion:        April 1, 1997
Next perihelion:       4385 (estimate)
 

Comet Hale-Bopp photographed on April 4, 1997 (E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria)

Comet Hale–Bopp is also known as the Great Comet of 1997.

Discovered on July 23, 1995 by amateur astronomers Alan Hale and Thomas Bopp when located 7.2 AU from the Sun. At this great distance it was between the orbits of Jupiter and Saturn.

Apparent magnitude at discovery was +10.5.

Holds the record for farthest comet from the Sun discovered by amateurs.

Images obtained at discovery showed a coma which is unusual for comets so far from the Sun.

Although comets are notoriously unpredictable, Hale-Bopp attained or even exceeded initial expectations.

It was visible to the naked eye for a record 18 months - twice as long as the previous record holder, the Great Comet of 1811.

The comet was brighter than magnitude 0.0 for eight weeks, which is longer than any other recorded comet.

Closest approach to Earth occurred on March 22, 1997 at a distance of 1.315 AU (196.7 million kilometres or 122.2 million miles). The comet reached perihelion a few days later on April 1, 1997.

After passing perihelion Hale-Bopp was a spectacular sight in the evening sky, especially for Northern Hemisphere observers.

At peak magnitude, it was brighter than all nighttime stars except for Sirius and is probably the most viewed comet in recorded history.

Its dust tail stretched 45 degrees.

The nucleus of Hale Bopp is large. It's estimated to be at least 40 kilometres (25 miles) in diameter, approx. 6 times the size of comet Halley.

As a consequence of orbital perturbations, the comet is predicted to next return around 4385. 

Comet Bennett (C/1969 Y1)

Discovered by:        John Caister Bennett
Discovery date:       December 28, 1969
Peak magnitude:     0.0
Aphelion:                282 AU (42.0 billion kilometres or 26.1 billion miles)
Perihelion:              0.538 AU (80.48 million kilometers or 50.01 million miles)
Semi-major axis:     141 AU (21.0 billion kilometres or 13.0 billion miles)
Eccentricity:            0.9962
Orbital period:         1678 years
Inclination:              90.0 degrees

Last perihelion:       March 20, 1970
Next perihelion:       3600 (estimate)

Comet Bennett in 1970 (Pretoria Centre of the ASSA)

Discovered on December 28, 1969 from Pretoria by renowned South African amateur astronomer and comet hunter John Caister Bennett.

Was at magnitude +8.5 when discovered, appearing small and diffuse and with no sign of tail. At the time Bennett was situated 1.70 AU from Earth and 1.68 AU from the Sun, just outside the orbit of Mars.

Perihelion was reached on March 20, 1970 with Bennett peaking at magnitude 0.0 and displaying a tail of 15 degrees in length.

Closest approach to Earth occurred 6 days later on March 26 (0.69 AU distant).

Bennett was widely observed in April in the morning sky as its position improved. Although now moving away from the Earth and the Sun and therefore fading the comet remained a superb sight especially during the first part of April.

By the beginning of May it was down to the limit of naked eye visibility.

With an orbital period of 1678 years, Bennett is expected to next return sometime around 3600.

Comet Arend-Roland (C/1956 R1)

Discovered by:        Sylvain Arend and Georges Roland
Discovery date:       November 8, 1956            
Peak magnitude:     -0.5
Aphelion:                N/A
Perihelion:               0.316 AU (47.27 million kilometres or 29.37 million miles)
Semi-major axis:     45,000 AU (6730 billion kilometres or 4180 billion miles)
Eccentricity:           1.00024
Orbital period:         N/A
Inclination:              119.9 degrees

Last perihelion:        April 8, 1957
Next perihelion:       N/A (ejection from Solar System)

Comet Arend-Roland photographed on April 26, 1957 with the 48-inch Palomar Schmidt telescope (Palomar Observatory).

Discovered on November 8, 1956 by Belgian astronomers Sylvain Arend and Georges Roland from photographic plates taken at the Uccle Observatory (Royal Observatory of Belgium) in Brussels.

The comet was magnitude +10 when discovered exhibiting a prominent central condensation and a short tail.

Perihelion occurred on April 8, 1957. It was well seen from the Northern Hemisphere during April, peaking at magnitude -0.5.

The tail of Arend–Roland reached a length of 15 degrees and for a few days in April a prominent anomalous tail (or antitail) about 5 degrees in length was visible. This remarkable event resulted in a rarely spectacle of a "spiked" comet.

For the first time attempts were made to detect the comet at radio frequencies. However, these efforts were unsuccessful.

This was the one and only appearance of this comet from Earth. With a hyperbolic orbit, Arend–Roland is traveling fast enough to completely escape from the Solar System and therefore will never return to our vicinity.

Comet Arend–Roland was the subject of the very first edition of the BBC's long running astronomy program The Sky at Night on April 24, 1957.

Comet Halley (1P/Halley)

Discovered by:          Prehistoric, Edmond Halley first recognised the periodicity
Discovery date:         Prehistoric       
Peak magnitude:      +2.4 (1986 return)
Aphelion:                  35.08 AU (5.248 billion kilometres or 3.261 billion miles)
Perihelion:                0.5860 AU (87.66 million kilometres or 54.83 million miles)
Semi-major axis:       17.83 AU (2.667 billion kilometres or 1.657 billion miles)
Eccentricity:              0.9671
Orbital period:            75.32 years
Inclination:                162.3 degrees

Last perihelion:          February 9th, 1986
Next perihelion:         July 28th, 2061

Halley's Comet (ESA)

Comet Halley or Halley's Comet is the best known and most famous comet of all.

It's a short period comet that's visible from Earth every 75–76 years and has been observed by astronomers since at least 240 BC.

In 1066, Halley's appearance was recorded on the Bayeux Tapestry.

Usually comets are named after the person or persons who discovered them. Halley (along with a few other comets) is different in that it was named after the person who first computed its orbit; English astronomer Edmond Halley in 1705.

Before this, the comet was recorded many times by Chinese, Babylonian, and medieval Europeans, but never recognised as being the same object.

Holds the distinction of being the only naked-eye comet that can be seen twice in a human lifetime.

Was first photographed and spectroscopic data obtained during the 1910 approach. On this occasion the comet passed close to Earth and was a spectacular sight; the Earth even passing through part of the comet tail.

Last appearance was in 1986 when Halley was best seen from the Southern Hemisphere. Sadly, the apparition was a poor one; Halley was on the opposite side of the Sun to the Earth and although visible to the naked-eye it reached at best only 2nd magnitude.

An armada of spacecraft visited Halley in 1986, including the Giotto and Vega missions that gave planetary scientists their first view of its surface and structure.

Nucleus of Halley's Comet (Halley Multicolor Camera Team, Giotto Project, ESA)

The comet nucleus is irregular, measuring about 15 x 8 x 8 kilometers (9.4 x 5 x 5 miles).

About 25% of the nucleus has been imaged by spacecraft flybys, revealing an extremely varied surface with hills, mountains, ridges, depressions and at least one crater.

Will next return in 2061.

Of all periodic comets, this one is the most active.

Halley's Comet is the parent body of two annual meteor showers, the Eta Aquariids (in May) and the richer Orionids (in October).