Deep Impact (spacecraft)

From Wikipedia, the free encyclopedia

Deep Impact

Artist's impression of the Deep Impact space probe after deployment of the Impactor.
Mission type	Flyby · impactor (9P/Tempel)
Operator	NASA · JPL
COSPAR ID	2005-001A
SATCAT no.	28517
Website	www.jpl.nasa.gov/missions/deep-impact/
Mission duration	Final: 8 years, 6 months, 26 days
Spacecraft properties
Manufacturer	Ball Aerospace · University of Maryland
Launch mass	Spacecraft: 601 kg (1,325 lb) Impactor: 372 kg (820 lb)
Dimensions	3.3 × 1.7 × 2.3 m (10.8 × 5.6 × 7.5 ft)
Power	92 W (solar array / NiH 2 battery)
Start of mission
Launch date	January 12, 2005, 18:47:08 UTC
Rocket	Delta II 7925
Launch site	Cape Canaveral SLC-17B
Contractor	Boeing
End of mission
Disposal	Contact lost
Last contact	August 8, 2013
Flyby of Tempel 1
Closest approach	July 4, 2005, 06:05 UTC
Distance	~500 km (310 mi)
Tempel 1 impactor
Impact date	July 4, 2005, 05:52 UTC

Deep Impact was a NASA space probe launched from Cape Canaveral Air Force Station on January 12, 2005. It was designed to study the interior composition of the comet Tempel 1 (9P/Tempel), by releasing an impactor into the comet. At 05:52 UTC on July 4, 2005, the Impactor successfully collided with the comet's nucleus. The impact excavated debris from the interior of the nucleus, forming an impact crater. Photographs taken by the spacecraft showed the comet to be more dusty and less icy than had been expected. The impact generated an unexpectedly large and bright dust cloud, obscuring the view of the impact crater.

Previous space missions to comets, such as Giotto, Deep Space 1, and Stardust, were fly-by missions. These missions were able to photograph and examine only the surfaces of cometary nuclei, and even then from considerable distances. The Deep Impact mission was the first to eject material from a comet's surface, and the mission garnered considerable publicity from the media, international scientists, and amateur astronomers alike.

Upon the completion of its primary mission, proposals were made to further utilize the spacecraft. Consequently, Deep Impact flew by Earth on December 31, 2007 on its way to an extended mission, designated EPOXI, with a dual purpose to study extrasolar planets and comet Hartley 2 (103P/Hartley). Communication was unexpectedly lost in September 2013 while the craft was heading for another asteroid flyby.

Scientific goals

The Deep Impact mission was planned to help answer fundamental questions about comets, which included what makes up the composition of the comet's nucleus, what depth the crater would reach from the impact, and where the comet originated in its formation. By observing the composition of the comet, astronomers hoped to determine how comets form based on the differences between the interior and exterior makeup of the comet. Observations of the impact and its aftermath would allow astronomers to attempt to determine the answers to these questions. 

The mission's Principal Investigator was Michael A'Hearn, an astronomer at the University of Maryland. He led the science team, which included members from Cornell University, University of Maryland, University of Arizona, Brown University, Belton Space Exploration Initiatives, JPL, University of Hawaii, SAIC, Ball Aerospace, and Max-Planck-Institut für extraterrestrische Physik.

Spacecraft design and instrumentation

Spacecraft overview

The spacecraft consists of two main sections, the 372-kilogram (820 lb) copper-core "Smart Impactor" that impacted the comet, and the 601 kg (1,325 lb) "Flyby" section, which imaged the comet from a safe distance during the encounter with Tempel 1.

The Flyby spacecraft is about 3.3 meters (10.8 ft) long, 1.7 meters (5.6 ft) wide and 2.3 meters (7.5 ft) high. It includes two solar panels, a debris shield, and several science instruments for imaging, infrared spectroscopy, and optical navigation to its destination near the comet. The spacecraft also carried two cameras, the High Resolution Imager (HRI), and the Medium Resolution Imager (MRI). The HRI is an imaging device that combines a visible-light camera with a filter wheel, and an imaging infrared spectrometer called the "Spectral Imaging Module" or SIM that operates on a spectral band from 1.05 to 4.8 micrometres. It has been optimized for observing the comet's nucleus. The MRI is the backup device, and was used primarily for navigation during the final 10-day approach. It also has a filter wheel, with a slightly different set of filters. 

The Impactor section of the spacecraft contains an instrument that is optically identical to the MRI, called the Impactor Targeting Sensor (ITS), but without the filter wheel. Its dual purpose was to sense the Impactor's trajectory, which could then be adjusted up to four times between release and impact, and to image the comet from close range. As the Impactor neared the comet's surface, this camera took high-resolution pictures of the nucleus (as good as 0.2 meters per pixel [7.9 in/px]) that were transmitted in real-time to the Flyby spacecraft before it and the Impactor were destroyed. The final image taken by the Impactor was snapped only 3.7 seconds before impact.

The Impactor's payload, dubbed the "Cratering Mass", was 100% copper, with a weight of 100 kg. Including this cratering mass, copper formed 49% of total mass of the Impactor (with aluminium at 24% of the total mass); this was to minimize interference with scientific measurements. Since copper was not expected to be found on a comet, scientists could ignore copper's signature in any spectrometer readings. Instead of using explosives, it was also cheaper to use copper as the payload.

Explosives would also have been superfluous. At its closing velocity of 10.2 km/s, the Impactor's kinetic energy was equivalent to 4.8 metric tons of TNT, considerably more than its actual mass of only 372 kg.

The mission coincidentally shared its name with the 1998 film, Deep Impact, in which a comet strikes the Earth.

Mission profile

Cameras of the Flyby spacecraft, HRI at right, MRI at left

 

Deep Impact prior to launch on a Delta II rocket

Following its launch from Cape Canaveral Air Force Station pad SLC-17B at 18:47 UTC on January 12, 2005, the Deep Impact spacecraft traveled 429 million km (267 million mi) in 174 days to reach comet Tempel 1 at a cruising speed of 28.6 km/s (103,000 km/h; 64,000 mph). Once the spacecraft reached the vicinity of the comet on July 3, 2005, it separated into the Impactor and Flyby sections. The Impactor used its thrusters to move into the path of the comet, impacting 24 hours later at a relative speed of 10.3 km/s (37,000 km/h; 23,000 mph). The Impactor delivered 1.96×10¹⁰ joules of kinetic energy—the equivalent of 4.7 tons of TNT. Scientists believed that the energy of the high-velocity collision would be sufficient to excavate a crater up to 100 m (330 ft) wide, larger than the bowl of the Roman Colosseum. The size of the crater was still not known one year after the impact. The 2007 Stardust spacecraft's NExT mission determined the crater's diameter to be 150 meters (490 ft). 

Just minutes after the impact, the Flyby probe passed by the nucleus at a close distance of 500 km (310 mi), taking pictures of the crater position, the ejecta plume, and the entire cometary nucleus. The entire event was also photographed by Earth-based telescopes and orbital observatories, including Hubble, Chandra, Spitzer, and XMM-Newton. The impact was also observed by cameras and spectroscopes on board Europe's Rosetta spacecraft, which was about 80 million km (50 million mi) from the comet at the time of impact. Rosetta determined the composition of the gas and dust cloud that was kicked up by the impact.

Mission events

Animation of Deep Impact's trajectory from January 12, 2005, to August 8, 2013
  Deep Impact ·   Tempel 1 ·   Earth ·   103P/Hartley

Before launch

A comet-impact mission was first proposed to NASA in 1996, but at the time, NASA engineers were skeptical that the target could be hit. In 1999, a revised and technologically upgraded mission proposal, dubbed Deep Impact, was accepted and funded as part of NASA's Discovery Program of low-cost spacecraft. The two spacecraft (Impactor and Flyby) and the three main instruments were built and integrated by Ball Aerospace & Technologies in Boulder, Colorado. Developing the software for the spacecraft took 18 months and the application code consisted of 20,000 lines and 19 different application threads. The total cost of developing the spacecraft and completing its mission reached US$330 million.

Launch and commissioning phase

The probe was originally scheduled for launch on December 30, 2004, but NASA officials delayed its launch, in order to allow more time for testing the software. It was successfully launched from Cape Canaveral on January 12, 2005 at 1:47 pm EST (1847 UTC) by a Delta II rocket.

Deep Impact's state of health was uncertain during the first day after launch. Shortly after entering orbit around the Sun and deploying its solar panels, the probe switched itself to safe mode. The cause of the problem was simply an incorrect temperature limit in the fault protection logic for the spacecraft's RCS thruster catalyst beds. The spacecraft's thrusters were used to detumble the spacecraft following third stage separation. On January 13, 2005, NASA announced that the probe was out of safe mode and healthy.

On February 11, 2005, Deep Impact's rockets were fired as planned to correct the spacecraft's course. This correction was so precise that the next planned correction maneuver on March 31, 2005, was unnecessary and canceled. The "commissioning phase" verified that all instruments were activated and checked out. During these tests it was found that the HRI images were not in focus after it underwent a bake-out period. After mission members investigated the problem, on June 9, 2005, it was announced that by using image processing software and the mathematical technique of deconvolution, the HRI images could be corrected to restore much of the resolution anticipated.

Cruise phase

Comet Tempel 1 imaged on April 25 by the Deep Impact spacecraft

The "cruise phase" began on March 25, 2005, immediately after the commissioning phase was completed. This phase continued until about 60 days before the encounter with comet Tempel 1. On April 25, 2005, the probe acquired the first image of its target at a distance of 64 million km (40 million mi).

On May 4, 2005, the spacecraft executed its second trajectory correction maneuver. Burning its rocket engine for 95 seconds, the spacecraft speed was changed by 18.2 km/h (11.3 mph). Rick Grammier, the project manager for the mission at NASA's Jet Propulsion Laboratory, reacted to the maneuver stating that "spacecraft performance has been excellent, and this burn was no different... it was a textbook maneuver that placed us right on the money."

Approach phase

The approach phase extended from 60 days before encounter (May 5, 2005) until five days before encounter. Sixty days out was the earliest time that the Deep Impact spacecraft was expected to detect the comet with its MRI camera. In fact, the comet was spotted ahead of schedule, 69 days before impact (see Cruise phase above). This milestone marks the beginning of an intensive period of observations to refine knowledge of the comet's orbit and study the comet's rotation, activity, and dust environment. 

On June 14 and 22, 2005, Deep Impact observed two outbursts of activity from the comet, the latter being six times larger than the former. The spacecraft studied the images of various distant stars to determine its current trajectory and position. Don Yeomans, a mission co-investigator for JPL pointed out that "it takes 7½ minutes for the signal to get back to Earth, so you cannot joystick this thing. You have to rely on the fact that the Impactor is a smart spacecraft as is the Flyby spacecraft. So you have to build in the intelligence ahead of time and let it do its thing." On June 23, 2005, the first of the two final trajectory correct maneuvers (targeting maneuver) was successfully executed. A 6 m/s (20 ft/s) velocity change was needed to adjust the flight path towards the comet and target the Impactor at a window in space about 100 kilometers (62 mi) wide.

• 

  May 30, 2005, 35 days from impact 
  
  
• 

  June 15, 19 days from impact 
  
  
• 

  June 21, 13 days from impact 
  
  
• 

  June 27, 7 days from impact, near end of approach phase
  

Impact phase

Deep Impact comet encounter sequence

 

Impact phase began nominally on June 29, 2005, five days before impact. The Impactor successfully separated from the Flyby spacecraft on July 3 at 6:00 UTC (6:07 UTC ERT). The first images from the instrumented Impactor were seen two hours after separation.

The Flyby spacecraft performed one of two divert maneuvers to avoid damage. A 14-minute burn was executed which slowed down the spacecraft. It was also reported that the communication link between the Flyby and the Impactor was functioning as expected. The Impactor executed three correction maneuvers in the final two hours before impact.

The Impactor was maneuvered to plant itself in front of the comet, so that Tempel 1 would collide with it. Impact occurred at 05:45 UTC (05:52 UTC ERT, +/- up to three minutes, one-way light time = 7m 26s) on the morning of July 4, 2005, within one second of the expected time for impact. 

The impactor returned images as late as three seconds before impact. Most of the data captured was stored on board the Flyby spacecraft, which radioed approximately 4,500 images from the HRI, MRI, and ITS cameras to Earth over the next few days. The energy from the collision was similar in size to exploding five tons of dynamite and the comet shone six times brighter than normal.

A mission timeline is located at Impact Phase Timeline (NASA).

• 

  Comet Tempel 1, imaged from 4.2 million km at the start of Impact phase 
  
  
• 

  Impactor imaged by Flyby spacecraft shortly after separation 
  
  
• 

  Image from Impactor 
  
  
• 

  Impactor close-up image, taken shortly before impact 
  
  
• 

  The moment of impact, as shown on NASA TV
   
  
• 

  HRI movie of impact 
  
  
• 

  High Resolution Instrument, Visual CCD (HRIV) during encounter (video) 
  
  
• 

  Impactor Targeting Sensor, Visual CCD (ITS) during encounter (video)
  

Results

Mission team members celebrate after the impact with the comet

Mission control did not become aware of the Impactor's success until five minutes later at 05:57 UTC. Don Yeomans confirmed the results for the press, "We hit it just exactly where we wanted to" and JPL Director Charles Elachi stated "The success exceeded our expectations."

In the post-impact briefing on July 4, 2005, at 08:00 UTC, the first processed images revealed existing craters on the comet. NASA scientists stated they could not see the new crater that had formed from the Impactor, but it was later discovered to be about 100 meters wide and up to 30 meters (98 ft) deep. Lucy McFadden, one of the co-investigators of the impact, stated "We didn't expect the success of one part of the mission [bright dust cloud] to affect a second part [seeing the resultant crater]. But that is part of the fun of science, to meet with the unexpected." Analysis of data from the Swift X-ray telescope showed that the comet continued outgassing from the impact for 13 days, with a peak five days after impact. A total of 5 million kg (11 million lb) of water and between 10 and 25 million kg (22 and 55 million lb) of dust were lost from the impact.

Initial results were surprising as the material excavated by the impact contained more dust and less ice than had been expected. The only models of cometary structure astronomers could positively rule out were the very porous ones which had comets as loose aggregates of material. In addition, the material was finer than expected; scientists compared it to talcum powder rather than sand. Other materials found while studying the impact included clays, carbonates, sodium, and crystalline silicates which were found by studying the spectroscopy of the impact. Clays and carbonates usually require liquid water to form and sodium is rare in space. Observations also revealed that the comet was about 75% empty space, and one astronomer compared the outer layers of the comet to the same makeup of a snow bank. Astronomers have expressed interest in more missions to different comets to determine if they share similar compositions or if there are different materials found deeper within comets that were produced at the time of the Solar System's formation.

'Before and after' comparison images from Deep Impact and Stardust, showing the crater formed by Deep Impact on the right hand image.

Astronomers hypothesized, based on its interior chemistry, that the comet formed in the Uranus and Neptune Oort cloud region of the Solar System. A comet which forms farther from the Sun is expected to have greater amounts of ices with low freezing temperatures, such as ethane, which was present in Tempel 1. Astronomers believe that other comets with compositions similar to Tempel 1 are likely to have formed in the same region.

Crater

Because the quality of the images of the crater formed during the Deep Impact collision was not satisfactory, on July 3, 2007, NASA approved the New Exploration of Tempel 1 (or NExT) mission. The mission utilized the already existing Stardust spacecraft, which had studied Comet Wild 2 in 2004. Stardust was placed into a new orbit so that it passed by Tempel 1 at a distance of approximately 200 km (120 mi) on February 15, 2011, at 04:42 UTC. This was the first time that a comet was visited by two probes on separate occasions (1P/Halley had been visited by several probes within a few weeks in 1986), and it provided an opportunity to better observe the crater that was created by Deep Impact as well as observing the changes caused by the comet's latest close approach to the Sun. 

On February 15, NASA scientists identified the crater formed by Deep Impact in images from Stardust. The crater is estimated to be 150 meters (490 ft) in diameter, and has a bright mound in the center likely created when material from the impact fell back into the crater.

Public interest

Media coverage

The image of the impact which was widely circulated in the media

The impact was a substantial news event reported and discussed online, in print, and on television. There was a genuine suspense because experts held widely differing opinions over the result of the impact. Various experts debated whether the Impactor would go straight through the comet and out the other side, would create an impact crater, would open up a hole in the interior of the comet, and other theories. However, twenty-four hours before impact, the flight team at JPL began privately expressing a high level of confidence that, barring any unforeseen technical glitches, the spacecraft would intercept Tempel 1. One senior personnel member stated "All we can do now is sit back and wait. Everything we can technically do to ensure impact has been done." In the final minutes as the Impactor hit the comet, more than 10,000 people watched the collision on a giant movie screen at Hawaii's Waikīkī Beach.

Experts came up with a range of soundbites to summarize the mission to the public. Iwan Williams of Queen Mary University of London, said "It was like a mosquito hitting a 747. What we've found is that the mosquito didn't splat on the surface; it's actually gone through the windscreen."

One day after the impact Marina Bay, a Russian astrologer, sued NASA for US$300 million for the impact which "ruin[ed] the natural balance of forces in the universe." Her lawyer asked the public to volunteer to help in the claim by declaring "The impact changed the magnetic properties of the comet, and this could have affected mobile telephony here on Earth. If your phone went down this morning, ask yourself Why? and then get in touch with us." On August 9, 2005 the Presnensky Court of Moscow ruled against Bay, although she did attempt to appeal the result. One Russian physicist said that the impact had no effect on Earth and "the change to the orbit of the comet after the collision was only about 10 cm (3.9 in)."

Send Your Name To A Comet campaign

The CD containing the 625,000 names is added to the Impactor

The mission was notable for one of its promotional campaigns, "Send Your Name To A Comet!". Visitors to the Jet Propulsion Laboratory's website were invited to submit their name between May 2003 and January 2004, and the names gathered—some 625,000 in all—were then burnt onto a mini-CD, which was attached to the Impactor. Dr. Don Yeomans, a member of the spacecraft's scientific team, stated "this is an opportunity to become part of an extraordinary space mission ... when the craft is launched in December 2004, yours and the names of your loved-ones can hitch along for the ride and be part of what may be the best space fireworks show in history." The idea was credited with driving interest in the mission.

Reaction from China

Chinese researchers used the Deep Impact mission as an opportunity to highlight the efficiency of American science because public support ensured the possibility of funding long-term research. By contrast, "in China, the public usually has no idea what our scientists are doing, and limited funding for the promotion of science weakens people's enthusiasm for research."

Two days after the U.S. mission succeeded in having a probe collide with a comet, China revealed a plan for what it called a "more clever" version of the mission: landing a probe on a small comet or asteroid to push it off course. China said it would begin the mission after sending a probe to the Moon.

Contributions from amateur astronomers

Deep Impact participation certificate of Maciej Szczepańczyk

 

Since observing time on large, professional telescopes such as Keck or Hubble is always scarce, the Deep Impact scientists called upon "advanced amateur, student, and professional astronomers" to use small telescopes to make long-term observations of the target comet before and after impact. The purpose of these observations was to look for "volatile outgassing, dust coma development and dust production rates, dust tail development, and jet activity and outbursts." By mid-2007, amateur astronomers had submitted over a thousand CCD images of the comet.

One notable amateur observation was by students from schools in Hawaii, working with US and UK scientists, who during the press conference took live images using the Faulkes Automatic Telescope in Hawaii (the students operated the telescope over the Internet) and were one of the first groups to get images of the impact. One amateur astronomer reported seeing a structureless bright cloud around the comet, and an estimated 2 magnitude increase in brightness after the impact. Another amateur published a map of the crash area from NASA images.

Musical tribute

The Deep Impact mission coincided with celebrations in the Los Angeles area marking the 50th anniversary of "Rock Around the Clock" by Bill Haley & His Comets becoming the first rock and roll single to reach No. 1 on the recording sales charts. Within 24 hours of the mission's success, a 2-minute music video produced by Martin Lewis had been created using images of the impact itself combined with computer animation of the Deep Impact probe in flight, interspersed with footage of Bill Haley & His Comets performing in 1955 and the surviving original members of The Comets performing in March 2005. The video was posted to NASA's website for a couple of weeks afterwards. 

On July 5, 2005, the surviving original members of The Comets (ranging in age from 71–84) performed a free concert for hundreds of employees of the Jet Propulsion Laboratory to help them celebrate the mission's success. This event received worldwide press attention. In February 2006, the International Astronomical Union citation that officially named asteroid 79896 Billhaley included a reference to the JPL concert.

Extended mission

Deep Impact embarked on an extended mission designated EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) to visit other comets, after being put to sleep in 2005 upon completion of the Tempel 1 mission.

Comet Boethin plan

Its first extended visit was to do a flyby of Comet Boethin, but with some complications. On July 21, 2005, Deep Impact executed a trajectory correction maneuver that allows the spacecraft to use Earth's gravity to begin a new mission in a path towards another comet.

The original plan was for a December 5, 2008, flyby of Comet Boethin, coming within 700 kilometers (430 mi) of the comet. Michael A'Hearn, the Deep Impact team leader, explained "We propose to direct the spacecraft for a flyby of Comet Boethin to investigate whether the results found at Comet Tempel 1 are unique or are also found on other comets." The $40 million mission would provide about half of the information as the collision of Tempel 1 but at a fraction of the cost. Deep Impact would use its spectrometer to study the comet's surface composition and its telescope for viewing the surface features.

However, as the December 2007 Earth gravity assist approached, astronomers were unable to locate Comet Boethin, which may have broken up into pieces too faint to be observed. Consequently, its orbit could not be calculated with sufficient precision to permit a flyby.

Flyby of Comet Hartley 2

Comet Hartley 2 on November 4, 2010

In November 2007 the JPL team targeted Deep Impact toward Comet Hartley 2. However, this would require an extra two years of travel for Deep Impact (including earth gravity assists in December 2007 and December 2008). On May 28, 2010, a burn of 11.3 seconds was conducted, to enable the June 27 Earth fly-by to be optimized for the transit to Hartley 2 and fly-by on November 4. The velocity change was 0.1 m/s (0.33 ft/s).

On November 4, 2010, the Deep Impact extended mission (EPOXI) returned images from comet Hartley 2. EPOXI came within 700 kilometers (430 mi) of the comet, returning detailed photographs of the "peanut" shaped cometary nucleus and several bright jets. The probe's medium-resolution instrument captured the photographs.

Comet Garradd (C/2009 P1)

Deep Impact observed Comet Garradd (C/2009 P1) from February 20 to April 8, 2012, using its Medium Resolution Instrument, through a variety of filters. The comet was 1.75–2.11 AU (262–316 million km) from the Sun and 1.87–1.30 AU (280–194 million km) from the spacecraft. It was found that the outgassing from the comet varies with a period of 10.4 hours, which is presumed to be due to the rotation of its nucleus. The dry ice content of the comet was measured and found to be about ten percent of its water ice content by number of molecules.

Possible mission to asteroid (163249) 2002 GT

At the end of 2011, Deep Impact was re-targeted towards asteroid (163249) 2002 GT which it would reach in January 2020. At the time of re-targeting, whether or not a related science mission would be carried out in 2020 was yet to be determined, based on NASA's budget and the health of the probe. A 71-second engine burn on October 4, 2012, changed the probe's velocity by 2 m/s (6.6 ft/s) to keep the mission on track.

Comet C/2012 S1 (ISON)

In February 2013, Deep Impact observed Comet ISON. The comet remained observable until March 2013.

Contact lost and end of mission

On September 3, 2013, a mission update was posted to the EPOXI mission status website, stating "Communication with the spacecraft was lost some time between August 11 and August 14 ... The last communication was on August 8. ... the team on August 30 determined the cause of the problem. The team is now trying to determine how best to try to recover communication."

On September 10, 2013, a Deep Impact mission status report explained that mission controllers believe the computers on the spacecraft are continuously rebooting themselves and so are unable to issue any commands to the vehicle's thrusters. As a result of this problem, communication with the spacecraft was explained to be more difficult, as the orientation of the vehicle's antennas is unknown. Additionally, the solar panels on the vehicle may no longer be positioned correctly for generating power.

On September 20, 2013, NASA abandoned further attempts to contact the craft. According to A'Hearn, the most probable reason of software malfunction was a Y2K-like problem. August 11, 2013, 00:38:49, was 2³² of one-tenth seconds from January 1, 2000, leading to speculation that a system on the craft tracked time in one-tenth second increments since January 1, 2000, and stored it in a signed 32-bit integer, which then overflowed at this time, similar to the Year 2038 problem.

Comet (updated)

From Wikipedia, the free encyclopedia

Comets – nucleus, coma and tail:

• Top: 9P/Tempel (impactor collision: Deep Impact), and 67P/Churyumov–Gerasimenko (Rosetta)
• Middle: 17P/Holmes and its blue ionized tail, and 81P/Wild (Wild 2) visited by Stardust
• Bottom: Hale–Bopp seen from Earth in 1997, and C/2011 W3 (Lovejoy) imaged from Earth orbit

A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process called outgassing. This produces a visible atmosphere or coma, and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind acting upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma may be up to 15 times the Earth's diameter, while the tail may stretch one astronomical unit. If sufficiently bright, a comet may be seen from the Earth without the aid of a telescope and may subtend an arc of 30° (60 Moons) across the sky. Comets have been observed and recorded since ancient times by many cultures.

Comets usually have highly eccentric elliptical orbits, and they have a wide range of orbital periods, ranging from several years to potentially several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. Long-period comets are set in motion towards the Sun from the Oort cloud by gravitational perturbations caused by passing stars and the galactic tide. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central part immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. Recent years, the discovery of some minor bodies that has a long-period comet orbit but has the characteristics of a inner solar system asteroid sometimes is called Manx Object (It will still be classified as Comet, such as C/2014 S3 (PANSTARRS)). 27 Manxes were found from 2013-2017.

As of July 2018 there are 6,339 known comets, a number that is steadily increasing as they are discovered. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System (in the Oort cloud) is estimated to be one trillion. Roughly one comet per year is visible to the naked eye, though many of those are faint and unspectacular. Particularly bright examples are called "great comets". Comets have been visited by unmanned probes such as the European Space Agency's Rosetta, which became the first ever to land a robotic spacecraft on a comet, and NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior.

Etymology

A comet was mentioned in the Anglo-Saxon Chronicle that allegedly made an appearance in 729 AD.

The word comet derives from the Old English cometa from the Latin comēta or comētēs. That, in turn, is a latinisation of the Greek κομήτης ("wearing long hair"), and the Oxford English Dictionary notes that the term (ἀστὴρ) κομήτης already meant "long-haired star, comet" in Greek. Κομήτης was derived from κομᾶν ("to wear the hair long"), which was itself derived from κόμη ("the hair of the head") and was used to mean "the tail of a comet".

The astronomical symbol for comets is ☄ (in Unicode U+2604), consisting of a small disc with three hairlike extensions.

Physical characteristics

Nucleus

Nucleus of 103P/Hartley as imaged during a spacecraft flyby. The nucleus is about 2 km in length.

 

Comet 81P/Wild exhibits jets on light side and dark side, stark relief, and is dry.

The solid, core structure of a comet is known as the nucleus. Cometary nuclei are composed of an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. As such, they are popularly described as "dirty snowballs" after Fred Whipple's model. However, some comets may have a higher dust content, leading them to be called "icy dirtballs". Research conducted in 2014 suggests that comets are like "deep fried ice cream", in that their surfaces are formed of dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.

Comet Borrelly exhibits jets, but has no surface ice.

The surface of the nucleus is generally dry, dusty or rocky, suggesting that the ices are hidden beneath a surface crust several metres thick. In addition to the gases already mentioned, the nuclei contain a variety of organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane and perhaps more complex molecules such as long-chain hydrocarbons and amino acids. In 2009, it was confirmed that the amino acid glycine had been found in the comet dust recovered by NASA's Stardust mission. In August 2011, a report, based on NASA studies of meteorites found on Earth, was published suggesting DNA and RNA components (adenine, guanine, and related organic molecules) may have been formed on asteroids and comets.

The outer surfaces of cometary nuclei have a very low albedo, making them among the least reflective objects found in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects less than 3.0%; by comparison, asphalt reflects seven percent. The dark surface material of the nucleus may consist of complex organic compounds. Solar heating drives off lighter volatile compounds, leaving behind larger organic compounds that tend to be very dark, like tar or crude oil. The low reflectivity of cometary surfaces causes them to absorb the heat that drives their outgassing processes.

Comet nuclei with radii of up to 30 kilometres (19 mi) have been observed, but ascertaining their exact size is difficult. The nucleus of 322P/SOHO is probably only 100–200 metres (330–660 ft) in diameter. A lack of smaller comets being detected despite the increased sensitivity of instruments has led some to suggest that there is a real lack of comets smaller than 100 metres (330 ft) across. Known comets have been estimated to have an average density of 0.6 g/cm³ (0.35 oz/cu in). Because of their low mass, comet nuclei do not become spherical under their own gravity and therefore have irregular shapes.

Roughly six percent of the near-Earth asteroids are thought to be extinct nuclei of comets that no longer experience outgassing, including 14827 Hypnos and 3552 Don Quixote. 

Results from the Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov–Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons (within 1 km (0.62 mi) above the comet nucleus) produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which (acetamide, acetone, methyl isocyanate and propionaldehyde) have been detected for the first time on a comet.

Properties of some comets

Name	Dimensions (km)	Density (g/cm3)	Mass (kg)
Halley's Comet	15 × 8 × 8	0.6	3×1014
Tempel 1	7.6 × 4.9	0.62	7.9×1013
19P/Borrelly	8 × 4 × 4	0.3	2.0×1013
81P/Wild	5.5 × 4.0 × 3.3	0.6	2.3×1013
67P/Churyumov–Gerasimenko	4.1 × 3.3 × 1.8	0.47	1.0×1013

Coma

Hubble image of Comet ISON shortly before perihelion.

 

The streams of dust and gas thus released form a huge and extremely thin atmosphere around the comet called the "coma". The force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous "tail" to form pointing away from the Sun.

The coma is generally made of  H₂O and dust, with water making up to 90% of the volatiles that outflow from the nucleus when the comet is within 3 to 4 astronomical units (450,000,000 to 600,000,000 km; 280,000,000 to 370,000,000 mi) of the Sun. The  H₂O parent molecule is destroyed primarily through photodissociation and to a much smaller extent photoionization, with the solar wind playing a minor role in the destruction of water compared to photochemistry. Larger dust particles are left along the comet's orbital path whereas smaller particles are pushed away from the Sun into the comet's tail by light pressure.

C/2006 W3 (Chistensen) emitting carbon gas (IR image)

Although the solid nucleus of comets is generally less than 60 kilometres (37 mi) across, the coma may be thousands or millions of kilometres across, sometimes becoming larger than the Sun. For example, about a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 also had a coma roughly the diameter of the Sun. Even though the coma can become quite large, its size can decrease about the time it crosses the orbit of Mars around 1.5 astronomical units (220,000,000 km; 140,000,000 mi) from the Sun. At this distance the solar wind becomes strong enough to blow the gas and dust away from the coma, and in doing so enlarging the tail. Ion tails have been observed to extend one astronomical unit (150 million km) or more.

Both the coma and tail are illuminated by the Sun and may become visible when a comet passes through the inner Solar System, the dust reflects sunlight directly while the gases glow from ionisation. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye. Occasionally a comet may experience a huge and sudden outburst of gas and dust, during which the size of the coma greatly increases for a period of time. This happened in 2007 to Comet Holmes.

In 1996, comets were found to emit X-rays. This greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. The X-rays are generated by the interaction between comets and the solar wind: when highly charged solar wind ions fly through a cometary atmosphere, they collide with cometary atoms and molecules, "stealing" one or more electrons from the atom in a process called "charge exchange". This exchange or transfer of an electron to the solar wind ion is followed by its de-excitation into the ground state of the ion by the emission of X-rays and far ultraviolet photons.

Bow shock

Bow shocks form at as a result of the interaction between the solar wind and the cometary ionosphere, which is created by ionization of gases in the coma. As the comet approaches the Sun, increasing outgassing rates cause the coma to expand, and the sunlight ionizes gases in the coma. When the solar wind passes through this ion coma, the bow shock appears. 

The first observations were made in the 1980s and 90s as several spacecraft flew by comets 21P/Giacobini–Zinner, 1P/Halley, and 26P/Grigg–Skjellerup. It was then found that the bow shocks at comets are wider and more gradual than the sharp planetary bow shocks seen at, for example, Earth. These observations were all made near perihelion when the bow shocks already were fully developed. 

The Rosetta spacecraft observed the bow shock at comet 67P/Churyumov–Gerasimenko at an early stage of bow shock development when the outgassing increased during the comet's journey toward the Sun. This young bow shock was called the "infant bow shock". The infant bow shock is asymmetric and, relative to the distance to the nucleus, wider than fully developed bow shocks.

Tails

Diagram of a comet showing the dust trail (or antitail), the dust tail, and the ion gas tail, which is formed by the solar wind flow.

 

In the outer Solar System, comets remain frozen and inactive and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from observations by the Hubble Space Telescope but these detections have been questioned. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. 

Typical direction of tails over a comet's orbit near the Sun

 

The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail called the type II or dust tail. At the same time, the ion or type I tail, made of gases, always points directly away from the Sun because this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. On occasions - such as when the Earth passes through a comet's orbital plane, a tail pointing in the opposite direction to the ion and dust tails called the antitail may be seen.

The observation of antitails contributed significantly to the discovery of solar wind. The ion tail is formed as a result of the ionisation by solar ultra-violet radiation of particles in the coma. Once the particles have been ionized, they attain a net positive electrical charge, which in turn gives rise to an "induced magnetosphere" around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles. Because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet in the flow direction of the solar wind. In this bow shock, large concentrations of cometary ions (called "pick-up ions") congregate and act to "load" the solar magnetic field with plasma, such that the field lines "drape" around the comet forming the ion tail.

If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs. This leads to a "tail disconnection event". This has been observed on a number of occasions, one notable event being recorded on 20 April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. This event was observed by the STEREO space probe.

In 2013, ESA scientists reported that the ionosphere of the planet Venus streams outwards in a manner similar to the ion tail seen streaming from a comet under similar conditions."

Jets

Gas and snow jets of 103P/Hartley

Uneven heating can cause newly generated gases to break out of a weak spot on the surface of comet's nucleus, like a geyser. These streams of gas and dust can cause the nucleus to spin, and even split apart. In 2010 it was revealed dry ice (frozen carbon dioxide) can power jets of material flowing out of a comet nucleus. Infrared imaging of Hartley 2 shows such jets exiting and carrying with it dust grains into the coma.

Orbital characteristics

Most comets are small Solar System bodies with elongated elliptical orbits that take them close to the Sun for a part of their orbit and then out into the further reaches of the Solar System for the remainder. Comets are often classified according to the length of their orbital periods: The longer the period the more elongated the ellipse.

Short period

Periodic comets or short-period comets are generally defined as those having orbital periods of less than 200 years. They usually orbit more-or-less in the ecliptic plane in the same direction as the planets. Their orbits typically take them out to the region of the outer planets (Jupiter and beyond) at aphelion; for example, the aphelion of Halley's Comet is a little beyond the orbit of Neptune. Comets whose aphelia are near a major planet's orbit are called its "family". Such families are thought to arise from the planet capturing formerly long-period comets into shorter orbits.

At the shorter orbital period extreme, Encke's Comet has an orbit that does not reach the orbit of Jupiter, and is known as an Encke-type comet. Short-period comets with orbital periods less than 20 years and low inclinations (up to 30 degrees) to the ecliptic are called traditional Jupiter-family comets (JFCs). Those like Halley, with orbital periods of between 20 and 200 years and inclinations extending from zero to more than 90 degrees, are called Halley-type comets (HTCs). As of 2018, only 83 HTCs have been observed, compared with 660 identified JFCs.

Recently discovered main-belt comets form a distinct class, orbiting in more circular orbits within the asteroid belt.

Because their elliptical orbits frequently take them close to the giant planets, comets are subject to further gravitational perturbations. Short-period comets have a tendency for their aphelia to coincide with a giant planet's semi-major axis, with the JFCs being the largest group. It is clear that comets coming in from the Oort cloud often have their orbits strongly influenced by the gravity of giant planets as a result of a close encounter. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. These perturbations can deflect long-period comets into shorter orbital periods.

Based on their orbital characteristics, short-period comets are thought to originate from the centaurs and the Kuiper belt/scattered disc —a disk of objects in the trans-Neptunian region—whereas the source of long-period comets is thought to be the far more distant spherical Oort cloud (after the Dutch astronomer Jan Hendrik Oort who hypothesised its existence). Vast swarms of comet-like bodies are thought to orbit the Sun in these distant regions in roughly circular orbits. Occasionally the gravitational influence of the outer planets (in the case of Kuiper belt objects) or nearby stars (in the case of Oort cloud objects) may throw one of these bodies into an elliptical orbit that takes it inwards toward the Sun to form a visible comet. Unlike the return of periodic comets, whose orbits have been established by previous observations, the appearance of new comets by this mechanism is unpredictable.

Long period

Orbits of Comet Kohoutek (red) and the Earth (blue), illustrating the high eccentricity of its orbit and its rapid motion when close to the Sun.

Long-period comets have highly eccentric orbits and periods ranging from 200 years to thousands of years. An eccentricity greater than 1 when near perihelion does not necessarily mean that a comet will leave the Solar System. For example, Comet McNaught had a heliocentric osculating eccentricity of 1.000019 near its perihelion passage epoch in January 2007 but is bound to the Sun with roughly a 92,600-year orbit because the eccentricity drops below 1 as it moves farther from the Sun. The future orbit of a long-period comet is properly obtained when the osculating orbit is computed at an epoch after leaving the planetary region and is calculated with respect to the center of mass of the Solar System. By definition long-period comets remain gravitationally bound to the Sun; those comets that are ejected from the Solar System due to close passes by major planets are no longer properly considered as having "periods". The orbits of long-period comets take them far beyond the outer planets at aphelia, and the plane of their orbits need not lie near the ecliptic. Long-period comets such as Comet West and C/1999 F1 can have aphelion distances of nearly 70,000 AU with orbital periods estimated around 6 million years. 

Single-apparition or non-periodic comets are similar to long-period comets because they also have parabolic or slightly hyperbolic trajectories when near perihelion in the inner Solar System. However, gravitational perturbations from giant planets cause their orbits to change. Single-apparition comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's Hill sphere has an unstable maximum boundary of 230,000 AU (1.1 parsecs (3.6 light-years)). Only a few hundred comets have been seen to reach a hyperbolic orbit (e > 1) when near perihelion that using a heliocentric unperturbed two-body best-fit suggests they may escape the Solar System. 

As of 2018, 1I/ʻOumuamua is the only object with an eccentricity significantly greater than one that has been detected, indicating an origin outside the Solar System. While ʻOumuamua showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory—which suggests outgassing—indicate that it is indeed a comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity (1.057) of any known hyperbolic comet. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4 (NEAT), C/2009 R1, C/1956 R1, and C/2007 F1 (LONEOS). 

Some authorities use the term "periodic comet" to refer to any comet with a periodic orbit (that is, all short-period comets plus all long-period comets), whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of "non-periodic comet" is the same as "single-apparition comet", some use it to mean all comets that are not "periodic" in the second sense (that is, to also include all comets with a period greater than 200 years). 

Early observations have revealed a few genuinely hyperbolic (i.e. non-periodic) trajectories, but no more than could be accounted for by perturbations from Jupiter. If comets pervaded interstellar space, they would be moving with velocities of the same order as the relative velocities of stars near the Sun (a few tens of km per second). If such objects entered the Solar System, they would have positive specific orbital energy and would be observed to have genuinely hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.

 

Hyperbolic comet discoveries

Year	2007	2008	2009	2010	2011	2012	2013	2014	2015	2016	2017	2018
Number	12	7	8	4	13	10	16	9	16	5	18	3

Oort cloud and Hills cloud

The Oort cloud thought to surround the Solar System

The Oort cloud is thought to occupy a vast space starting from between 2,000 and 5,000 AU (0.03 and 0.08 ly) to as far as 50,000 AU (0.79 ly) from the Sun. Some estimates place the outer edge at between 100,000 and 200,000 AU (1.58 and 3.16 ly). The region can be subdivided into a spherical outer Oort cloud of 20,000–50,000 AU (0.32–0.79 ly), and a doughnut-shaped inner cloud, the Hills cloud, of 2,000–20,000 AU (0.03–0.32 ly). The outer cloud is only weakly bound to the Sun and supplies the long-period (and possibly Halley-type) comets that fall to inside the orbit of Neptune. The inner Oort cloud is also known as the Hills cloud, named after J. G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo; it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years.

Exocomets

Exocomets beyond the Solar System have also been detected and may be common in the Milky Way. The first exocomet system detected was around Beta Pictoris, a very young A-type main-sequence star, in 1987. A total of 10 such exocomet systems have been identified as of 2013, using the absorption spectrum caused by the large clouds of gas emitted by comets when passing close to their star.

Effects of comets

Connection to meteor showers

Diagram of Perseids meteors

As a result of outgassing, comets leave in their wake a trail of solid debris too large to be swept away by radiation pressure and the solar wind. If the Earth's orbit sends it through that debris, there are likely to be meteor showers as Earth passes through. The Perseid meteor shower, for example, occurs every year between 9 and 13 August, when Earth passes through the orbit of Comet Swift–Tuttle. Halley's Comet is the source of the Orionid shower in October.

Comets and impact on life

Many comets and asteroids collided with Earth in its early stages. Many scientists think that comets bombarding the young Earth about 4 billion years ago brought the vast quantities of water that now fill the Earth's oceans, or at least a significant portion of it. Others have cast doubt on this idea. The detection of organic molecules, including polycyclic aromatic hydrocarbons, in significant quantities in comets has led to speculation that comets or meteorites may have brought the precursors of life—or even life itself—to Earth. In 2013 it was suggested that impacts between rocky and icy surfaces, such as comets, had the potential to create the amino acids that make up proteins through shock synthesis. In 2015, scientists found significant amounts of molecular oxygen in the outgassings of comet 67P, suggesting that the molecule may occur more often than had been thought, and thus less an indicator of life as has been supposed.

It is suspected that comet impacts have, over long timescales, also delivered significant quantities of water to the Earth's Moon, some of which may have survived as lunar ice. Comet and meteoroid impacts are also thought to be responsible for the existence of tektites and australites.

Fear of comets

Fear of comets as acts of God and signs of impending doom was highest in Europe from AD 1200 to 1650. The year after the Great Comet of 1618, for example, Gotthard Arthusius published a pamphlet stating that it was a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices". By 1700 most scholars concluded that such events occurred whether a comet was seen or not. Using Edmund Halley's records of comet sightings, however, William Whiston in 1711 wrote that the Great Comet of 1680 had a periodicity of 574 years and was responsible for the worldwide flood in the Book of Genesis, by pouring water on the Earth. His announcement revived for another century fear of comets, now as direct threats to the world instead of signs of disasters. Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet, causing panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas" by the public.

Fate of comets

Departure (ejection) from Solar System

If a comet is traveling fast enough, it may leave the Solar System. Such comets follow the open path of a hyperbola, and as such they are called hyperbolic comets. To date, comets are only known to be ejected by interacting with another object in the Solar System, such as Jupiter. An example of this is thought to be Comet C/1980 E1, which was shifted from a predicted orbit of 7.1 million years around the Sun, to a hyperbolic trajectory, after a 1980 close pass by the planet Jupiter.

Volatiles exhausted

Jupiter-family comets and long-period comets appear to follow very different fading laws. The JFCs are active over a lifetime of about 10,000 years or ~1,000 orbits whereas long-period comets fade much faster. Only 10% of the long-period comets survive more than 50 passages to small perihelion and only 1% of them survive more than 2,000 passages. Eventually most of the volatile material contained in a comet nucleus evaporates, and the comet becomes a small, dark, inert lump of rock or rubble that can resemble an asteroid. Some asteroids in elliptical orbits are now identified as extinct comets. Roughly six percent of the near-Earth asteroids are thought to be extinct comet nuclei.

Breakup and collisions

The nucleus of some comets may be fragile, a conclusion supported by the observation of comets splitting apart. A significant cometary disruption was that of Comet Shoemaker–Levy 9, which was discovered in 1993. A close encounter in July 1992 had broken it into pieces, and over a period of six days in July 1994, these pieces fell into Jupiter's atmosphere—the first time astronomers had observed a collision between two objects in the Solar System. Other splitting comets include 3D/Biela in 1846 and 73P/Schwassmann–Wachmann from 1995 to 2006. Greek historian Ephorus reported that a comet split apart as far back as the winter of 372–373 BC. Comets are suspected of splitting due to thermal stress, internal gas pressure, or impact.

Comets 42P/Neujmin and 53P/Van Biesbroeck appear to be fragments of a parent comet. Numerical integrations have shown that both comets had a rather close approach to Jupiter in January 1850, and that, before 1850, the two orbits were nearly identical.

Some comets have been observed to break up during their perihelion passage, including great comets West and Ikeya–Seki. Biela's Comet was one significant example, when it broke into two pieces during its passage through the perihelion in 1846. These two comets were seen separately in 1852, but never again afterward. Instead, spectacular meteor showers were seen in 1872 and 1885 when the comet should have been visible. A minor meteor shower, the Andromedids, occurs annually in November, and it is caused when the Earth crosses the orbit of Biela's Comet.

Some comets meet a more spectacular end – either falling into the Sun or smashing into a planet or other body. Collisions between comets and planets or moons were common in the early Solar System: some of the many craters on the Moon, for example, may have been caused by comets. A recent collision of a comet with a planet occurred in July 1994 when Comet Shoemaker–Levy 9 broke up into pieces and collided with Jupiter.

Brown spots mark impact sites of Comet Shoemaker–Levy 9 on Jupiter

 

The break up of 73P/Schwassmann–Wachmann within three days (1995)

 

Ghost tail of C/2015 D1 (SOHO) after passage at the sun

 

Disintegration of P/2013 R3 (2014)

Nomenclature

Halley's Comet in 1910

The names given to comets have followed several different conventions over the past two centuries. Prior to the early 20th century, most comets were simply referred to by the year when they appeared, sometimes with additional adjectives for particularly bright comets; thus, the "Great Comet of 1680", the "Great Comet of 1882", and the "Great January Comet of 1910". 

After Edmund Halley demonstrated that the comets of 1531, 1607, and 1682 were the same body and successfully predicted its return in 1759 by calculating its orbit, that comet became known as Halley's Comet. Similarly, the second and third known periodic comets, Encke's Comet and Biela's Comet, were named after the astronomers who calculated their orbits rather than their original discoverers. Later, periodic comets were usually named after their discoverers, but comets that had appeared only once continued to be referred to by the year of their appearance.

In the early 20th century, the convention of naming comets after their discoverers became common, and this remains so today. A comet can be named after its discoverers, or an instrument or program that helped to find it.

History of study

Early observations and thought

Halley's Comet appeared in 1066, prior to the Battle of Hastings (Bayeux Tapestry).

From ancient sources, such as Chinese oracle bones, it is known that comets have been noticed by humans for millennia. Until the sixteenth century, comets were usually considered bad omens of deaths of kings or noble men, or coming catastrophes, or even interpreted as attacks by heavenly beings against terrestrial inhabitants.

Aristotle believed that comets were atmospheric phenomena, due to the fact that they could appear outside of the Zodiac and vary in brightness over the course of a few days. Pliny the Elder believed that comets were connected with political unrest and death.

In India, by the 6th century astronomers believed that comets were celestial bodies that re-appeared periodically. This was the view expressed in the 6th century by the astronomers Varāhamihira and Bhadrabahu, and the 10th-century astronomer Bhaṭṭotpala listed the names and estimated periods of certain comets, but it is not known how these figures were calculated or how accurate they were.

In the 16th century Tycho Brahe demonstrated that comets must exist outside the Earth's atmosphere by measuring the parallax of the Great Comet of 1577 from observations collected by geographically separated observers. Within the precision of the measurements, this implied the comet must be at least four times more distant than from the Earth to the Moon.

Orbital studies

The orbit of the comet of 1680, fitted to a parabola, as shown in Isaac Newton's Principia

Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example.

In 1705, Edmond Halley (1656–1742) applied Newton's method to twenty-three cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758–9. Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. When the comet returned as predicted, it became known as Halley's Comet (with the latter-day designation of 1P/Halley). It will next appear in 2061.

Studies of physical characteristics

From his huge vapouring train perhaps to shake
Reviving moisture on the numerous orbs,
Thro' which his long ellipsis winds; perhaps
To lend new fuel to declining suns,
To light up worlds, and feed th' ethereal fire.

James Thomson The Seasons (1730; 1748)

Isaac Newton described comets as compact and durable solid bodies moving in oblique orbit and their tails as thin streams of vapor emitted by their nuclei, ignited or heated by the Sun. Newton suspected that comets were the origin of the life-supporting component of air. 

As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized that comets are composed of some volatile substance, whose vaporization gives rise to their brilliant displays near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon.

In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This "dirty snowball" model soon became accepted and appeared to be supported by the observations of an armada of spacecraft (including the European Space Agency's Giotto probe and the Soviet Union's Vega 1 and Vega 2) that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material.

On 22 January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to "sprout jets and plumes". According to one of the scientists, "The lines are becoming more and more blurred between comets and asteroids." On 11 August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array (ALMA) for the first time, that detailed the distribution of HCN, HNC, H
2CO, and dust inside the comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON).

Spacecraft missions

• The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion.
• Deep Impact. Debate continues about how much ice is in a comet. In 2001, the Deep Space 1 spacecraft obtained high-resolution images of the surface of Comet Borrelly. It was found that the surface of comet Borrelly is hot and dry, with a temperature of between 26 to 71 °C (79 to 160 °F), and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporised water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on 4 November 2010.
• Stardust. Data from the Stardust mission show that materials retrieved from the tail of Wild 2 were crystalline and could only have been "born in fire," at extremely high temperatures of over 1,000 °C (1,830 °F). Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets also contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the "comet dust resembles asteroid materials". These new results have forced scientists to rethink the nature of comets and their distinction from asteroids.
• Rosetta. The Rosetta probe orbited Comet Churyumov–Gerasimenko. On 12 November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history.

Great comets

Woodcut of the Great Comet of 1577

Approximately once a decade, a comet becomes bright enough to be noticed by a casual observer, leading such comets to be designated as great comets. Predicting whether a comet will become a great comet is notoriously difficult, as many factors may cause a comet's brightness to depart drastically from predictions. Broadly speaking, if a comet has a large and active nucleus, will pass close to the Sun, and is not obscured by the Sun as seen from the Earth when at its brightest, it has a chance of becoming a great comet. However, Comet Kohoutek in 1973 fulfilled all the criteria and was expected to become spectacular but failed to do so. Comet West, which appeared three years later, had much lower expectations but became an extremely impressive comet.

The late 20th century saw a lengthy gap without the appearance of any great comets, followed by the arrival of two in quick succession—Comet Hyakutake in 1996, followed by Hale–Bopp, which reached maximum brightness in 1997 having been discovered two years earlier. The first great comet of the 21st century was C/2006 P1 (McNaught), which became visible to naked eye observers in January 2007. It was the brightest in over 40 years.

Sungrazing comets

A sungrazing comet is a comet that passes extremely close to the Sun at perihelion, generally within a few million kilometres. Although small sungrazers can be completely evaporated during such a close approach to the Sun, larger sungrazers can survive many perihelion passages. However, the strong tidal forces they experience often lead to their fragmentation.

About 90% of the sungrazers observed with SOHO are members of the Kreutz group, which all originate from one giant comet that broke up into many smaller comets during its first passage through the inner Solar System. The remainder contains some sporadic sungrazers, but four other related groups of comets have been identified among them: the Kracht, Kracht 2a, Marsden, and Meyer groups. The Marsden and Kracht groups both appear to be related to Comet 96P/Machholz, which is also the parent of two meteor streams, the Quadrantids and the Arietids.

Unusual comets

Euler diagram showing the types of bodies in the Solar System.

Of the thousands of known comets, some exhibit unusual properties. Comet Encke (2P/Encke) orbits from outside the asteroid belt to just inside the orbit of the planet Mercury whereas the Comet 29P/Schwassmann–Wachmann currently travels in a nearly circular orbit entirely between the orbits of Jupiter and Saturn. 2060 Chiron, whose unstable orbit is between Saturn and Uranus, was originally classified as an asteroid until a faint coma was noticed. Similarly, Comet Shoemaker–Levy 2 was originally designated asteroid 1990 UL₃.

Centaurs

Centaurs typically behave with characteristics of both asteroids and comets. Centaurs can be classified as comets such as 60558 Echeclus, and 166P/NEAT. 166P/NEAT was discovered while it exhibited a coma, and so is classified as a comet despite its orbit, and 60558 Echeclus was discovered without a coma but later became active, and was then classified as both a comet and an asteroid (174P/Echeclus). One plan for Cassini involved sending it to a centaur, but NASA decided to destroy it instead.

Observation

A comet may be discovered photographically using a wide-field telescope or visually with binoculars. However, even without access to optical equipment, it is still possible for the amateur astronomer to discover a sungrazing comet online by downloading images accumulated by some satellite observatories such as SOHO. SOHO's 2000th comet was discovered by Polish amateur astronomer Michał Kusiak on 26 December 2010 and both discoverers of Hale-Bopp used amateur equipment (although Hale was not an amateur).

Lost

A number of periodic comets discovered in earlier decades or previous centuries are now lost comets. Their orbits were never known well enough to predict future appearances or the comets have disintegrated. However, occasionally a "new" comet is discovered, and calculation of its orbit shows it to be an old "lost" comet. An example is Comet 11P/Tempel–Swift–LINEAR, discovered in 1869 but unobservable after 1908 because of perturbations by Jupiter. It was not found again until accidentally rediscovered by LINEAR in 2001. There are at least 18 comets that fit this category.

In popular culture

The depiction of comets in popular culture is firmly rooted in the long Western tradition of seeing comets as harbingers of doom and as omens of world-altering change. Halley's Comet alone has caused a slew of sensationalist publications of all sorts at each of its reappearances. It was especially noted that the birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain (who correctly speculated that he'd "go out with the comet" in 1910) and Eudora Welty, to whose life Mary Chapin Carpenter dedicated the song "Halley Came to Jackson".

In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, the Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale–Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.

In science fiction, the impact of comets has been depicted as a threat overcome by technology and heroism (as in the 1998 films Deep Impact and Armageddon), or as a trigger of global apocalypse (Lucifer's Hammer, 1979) or zombies (Night of the Comet, 1984). In Jules Verne's Off on a Comet a group of people are stranded on a comet orbiting the Sun, while a large manned space expedition visits Halley's Comet in Sir Arthur C. Clarke's novel 2061: Odyssey Three.