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Any successful model of GRB emission must explain the physical process for generating gamma-ray emission that matches the observed diversity of light curves, spectra, and other characteristics.

Particularly challenging is the need to explain the very high efficiencies that are inferred from some explosions: some gamma-ray bursts may convert as much as half (or more) of the explosion energy into gamma-rays.

Early observations of the bright optical counterparts to GRB 990123 and to GRB 080319B, whose optical light curves were extrapolations of the gamma-ray light spectra, have suggested that inverse Compton may be the dominant process in some events.

In this model, pre-existing low-energy photons are scattered by relativistic electrons within the explosion, augmenting their energy by a large factor and transforming them into gamma-rays.

The nature of the longer-wavelength afterglow emission (ranging from X-ray through radio) that follows gamma-ray bursts is better understood.

Any energy released by the explosion not radiated away in the burst itself takes the form of matter or energy moving outward at nearly the speed of light.

As this matter collides with the surrounding interstellar gas, it creates a relativistic shock wave that then propagates forward into interstellar space.

A second shock wave, the reverse shock, may propagate back into the ejected matter.

Extremely energetic electrons within the shock wave are accelerated by strong local magnetic fields and radiate as synchrotron emission across most of the electromagnetic spectrum.

This model has generally been successful in modeling the behavior of many observed afterglows at late times (generally, hours to days after the explosion), although there are difficulties explaining all features of the afterglow very shortly after the gamma-ray burst has occurred.

Considering the universe as a whole, the safest environments for life similar to that on Earth are the lowest density regions in the outskirts of large galaxies.

Our knowledge of galaxy types and their distribution suggests that life as we know it can only exist in about 10% of all galaxies.

Furthermore, galaxies with a redshift, z, higher than 0.5 are unsuitable for life as we know it, because of their higher rate of GRBs and their stellar compactness.

All GRBs observed to date have occurred well outside the Milky Way galaxy and have been harmless to Earth.

However, if a GRB were to occur within the Milky Way within 5,000 to 8,000 light-years and its emission were beamed straight towards Earth, the effects could be harmful and potentially devastating for its ecosystems.

Currently, orbiting satellites detect on average approximately one GRB per day.

The closest observed GRB as of March 2014 was GRB 980425, located away (z=0.0085) in an SBc-type dwarf galaxy.

GRB 980425 was far less energetic than the average GRB and was associated with the Type Ib supernova SN 1998bw.

Estimating the exact rate at which GRBs occur is difficult; for a galaxy of approximately the same size as the Milky Way, estimates of the expected rate (for long-duration GRBs) can range from one burst every 10,000 years, to one burst every 1,000,000 years.

Only a small percentage of these would be beamed towards Earth.

Estimates of rate of occurrence of short-duration GRBs are even more uncertain because of the unknown degree of collimation, but are probably comparable.

Since GRBs are thought to involve beamed emission along two jets in opposing directions, only planets in the path of these jets would be subjected to the high energy gamma radiation.

The immediate effect on life on Earth from a GRB within a few kiloparsecs would only be a short increase in ultraviolet radiation at ground level, lasting from less than a second to tens of seconds.

This ultraviolet radiation could potentially reach dangerous levels depending on the exact nature and distance of the burst, but it seems unlikely to be able to cause a global catastrophe for life on Earth.

The long-term effects from a nearby burst are more dangerous.

Gamma rays cause chemical reactions in the atmosphere involving oxygen and nitrogen molecules, creating first nitrogen oxide then nitrogen dioxide gas.

The nitrogen oxides cause dangerous effects on three levels.

First, they deplete ozone, with models showing a possible global reduction of 25–35%, with as much as 75% in certain locations, an effect that would last for years.

This reduction is enough to cause a dangerously elevated UV index at the surface.

Secondly, the nitrogen oxides cause photochemical smog, which darkens the sky and blocks out parts of the sunlight spectrum.

This would affect photosynthesis, but models show only about a 1% reduction of the total sunlight spectrum, lasting a few years.

However, the smog could potentially cause a cooling effect on Earth's climate, producing a "cosmic winter" (similar to an impact winter, but without an impact), but only if it occurs simultaneously with a global climate instability.

Thirdly, the elevated nitrogen dioxide levels in the atmosphere would wash out and produce acid rain.

Nitric acid is toxic to a variety of organisms, including amphibian life, but models predict that it would not reach levels that would cause a serious global effect.

The nitrates might in fact be of benefit to some plants.

All in all, a GRB within a few kiloparsecs, with its energy directed towards Earth, will mostly damage life by raising the UV levels during the burst itself and for a few years thereafter.

Models show that the destructive effects of this increase can cause up to 16 times the normal levels of DNA damage.

It has proved difficult to assess a reliable evaluation of the consequences of this on the terrestrial ecosystem, because of the uncertainty in biological field and laboratory data.

The major Ordovician–Silurian extinction events 450 million years ago may have been caused by a GRB.

The late Ordovician species of trilobites that spent portions of their lives in the plankton layer near the ocean surface were much harder hit than deep-water dwellers, which tended to remain within quite restricted areas.

This is in contrast to the usual pattern of extinction events, wherein species with more widely spread populations typically fare better.

A possible explanation is that trilobites remaining in deep water would be more shielded from the increased UV radiation associated with a GRB.

Also supportive of this hypothesis is the fact that during the late Ordovician, burrowing bivalve species were less likely to go extinct than bivalves that lived on the surface.

A case has been made that the 774–775 carbon-14 spike was the result of a short GRB, though a very strong solar flare is another possibility.

In light of evolving understanding of gamma-ray bursts and their progenitors, the scientific literature records a growing number of local, past, and future GRB candidates.

Long duration GRBs are related to superluminous supernovae, or hypernovae, and most luminous blue variables (LBVs) and rapidly spinning Wolf–Rayet stars are thought to end their life cycles in core-collapse supernovae with an associated long-duration GRB.

The London Marathon is an annual marathon held in London, United Kingdom.

Founded by athletes Chris Brasher and John Disley in 1981, it was typically held in April but has now moved to October.

The largely flat course is set around the River Thames, starting in Blackheath and finishing at The Mall.

Hugh Brasher (son of Chris) is the current Race Director and Nick Bitel its Chief Executive.

The race has several components: it has a mass race for the public, professional races for men and women long-distance runners, elite level wheelchair races for men and women, plus a 3-mile mini marathon event for under-17 athletes.

The mass race is the largest marathon event in the United Kingdom and its third largest running event (after the Great North Run and Great Manchester Run).

There is a significant charity running aspect to the marathon, with participants helping to raise over £1 billion since its founding, including £66.4 million at the 2019 London Marathon which was the highest amount for a single-day fund-raising event.

Since 2006, the elite race has been part of the World Marathon Majors, which includes six of the world's top level marathon races.

The London Marathon has seen the marathon world record broken on seven occasions: Khalid Khannouchi broke the men's record in 2002, while women's records have been broken by Grete Waitz (1983), Ingrid Kristiansen (1985), Paula Radcliffe (2002, 2003, 2005) and Mary Jepkosgei Keitany (2017).

The current elite course records are held by Eliud Kipchoge (2:02:37 in 2019) and Paula Radcliffe (2:15:25 in 2003).

The current wheelchair course records are held by Marcel Hug (1:26:27 in 2021) and Manuela Schär (1:39:52 in 2021).

The 2020 London Marathon was postponed and only allowed elite participants due to the COVID-19 outbreak.

The Polytechnic Marathon (also known as the Poly) was first held in 1909.

The current London Marathon was founded in 1981 by Olympic champion and journalist Chris Brasher and athlete John Disley.

Last Sunday, in one of the most trouble-stricken cities in the world, 11,532 men and women from 40 countries in the world, assisted by over a million black, white and yellow people, laughed, cheered and suffered during the greatest folk festival the world has seen.

The first London Marathon was held on 29 March 1981, more than 20,000 applied to run.

The Marathon's popularity has steadily grown since then.

As at 2009, 746,635 people have completed the race since its inception.

In 2010, 36,549 people crossed the line, the biggest field since the race began.

The first wheelchair marathon race was held in 1983 and the event was credited with reducing the stigma surrounding disabled athletes.

In 2013 the IPC Athletics Marathon World Cup was held within the London Marathon featuring athletes of both genders in the T42–T46 and T11–T13 categories.

In August 2013 it was announced that the event would be staged in London until 2017 and feature athletes in the T11-T12, T13, T42-T44, T43, T45-46, T51-52 and the T53-54 class.

For many years the London and Polytechnic Marathons competed with each other until, in 1996, the latter folded in due to the popularity of the former.

Following the Boston Marathon bombing, organisers of the 2013 London Marathon undertook a review of their security arrangements, despite no specific threats against the event.

Previously David Bedford and Bitel had overseen a period of great change for the race, including amendments to the course in 2005 which saw the cobbled section by the Tower of London replaced with a flat stretch along the Highway.

Dan Tunstall Pedoe was the medical director of the London Marathon for 25 years between the first one in 1981 until 2005.

Also assisting were more than 1,500 volunteers of St. John Ambulance, who organise over 50 first aid posts along the route, and three field hospitals at the finish.

St John Ambulance also provide a number of healthcare professions for the event, including nurses, paramedics, ambulances with crews.

The BBC covers the event, devoting rolling coverage for most of the morning.

The theme music associated with this coverage, and with the event itself, is called "Main Titles to The Trap", composed by Ron Goodwin for the film The Trap.

There are three separate groups of starters: Elite Women, Wheelchair (Men and Women), and Elite Men followed by Mass Race.

Since 2010, the race has been sponsored by Virgin Money.

Although the race publicity (athlete advice, timing charts and so on) is mile-oriented, the individual timing splits that are available to competitors after the event are kilometre-oriented.

The course begins at three separate points: the 'red start' in southern Greenwich Park on Charlton Way, the 'green start' in St John's Park, and the 'blue start' on Shooter's Hill Road.

From these points around Blackheath at above sea level, south of the River Thames, the route heads east through Charlton.

The three courses converge after in Woolwich, close to the Royal Artillery Barracks.

As the runners reach the , they pass by the Old Royal Naval College and head towards Cutty Sark drydocked in Greenwich.

Heading next into Deptford and Surrey Quays/Rotherhithe in the Docklands, and out towards Bermondsey, competitors race along Jamaica Road before reaching the half-way point as they cross Tower Bridge.

Running east again along The Highway through Wapping, competitors head up towards Limehouse and into Mudchute in the Isle of Dogs via Westferry Road, before heading into Canary Wharf.

As the route leads away from Canary Wharf into Poplar, competitors run west down Poplar High Street back towards Limehouse and on through Commercial Road.

They then move back onto The Highway, onto Lower and Upper Thames Streets.

Heading into the final leg of the race, competitors pass The Tower of London on Tower Hill.

In the penultimate mile along The Embankment, the London Eye comes into view, before the athletes turn right into Birdcage Walk to complete the final , catching the sights of Big Ben and Buckingham Palace, and finishing in The Mall alongside St. James's Palace.

This final section of the route formed part of the 2012 Olympic Marathon Course.

In 1982, the finishing post was moved from Constitution Hill to Westminster Bridge due to construction works.

It remained there for twelve years before moving to its present location at The Mall.

In 2005, the route around the Isle of Dogs between was switched from a clockwise to an anti-clockwise direction, and at the route was diverted to avoid St Katherine Docks and the cobblestoned area near the Tower of London.

In 2008, a suspected gas leak at a pub in Wapping diverted the course, but in 2009 the race followed the same path as in 2007.

Since 2012 mile 21 has become a significant cheer zone with Run Dem Crew transforming the stretch of Commercial Road outside the former Limehouse Town Hall into a street party with music and confetti cannons.

Due to the COVID-19 pandemic, the 2020 London Marathon was run on a non-traditional course, consisting of 19.6 laps of length around St James's Park, taking in The Mall, Horse Guards Parade, Birdcage Walk and Buckingham Palace.

The inaugural marathon had 7,741 entrants, 6,255 of whom completed the race.

The first Men's Elite Race in 1981 was tied between American Dick Beardsley and Norwegian Inge Simonsen, who crossed the finish line holding hands in 2 hours, 11 minutes, 48 seconds.

Organized by the British Sports Association for the Disabled (BASD), 19 people competed and 17 finished.

Gordon Perry of the United Kingdom won the Men's Wheelchair Race, coming in at 3:20:07, and Denise Smith, also of the UK, won the Women's Wheelchair Race in 4:29:03.

World records for marathon running have been set several times.

Khalid Khannouchi, representing the United States, set the men's world record in 2:05:38 in 2002.

Previous women's world records were set in 1983 and 1985 by Grete Waitz and Ingrid Kristiansen respectively, both of Norway.

The current men's course record is 2:02:37 set by Kenyan Eliud Kipchoge in 2019.

In 2002, Lloyd Scott completed the marathon wearing a deep sea diving suit that weighed a total of , with each shoe weighing ; he also set a record for the slowest London Marathon time.

On 19 April 2003, former boxer Michael Watson, who had been told he would never be able to walk again after a fight with Chris Eubank, made headlines by finishing the marathon in six days.

In 2006, Sir Steve Redgrave (winner of five consecutive Olympic gold medals) set a new Guinness World Record for money raised through a marathon by collecting £1.8 million in sponsorship.

This broke the record set the previous year by the founder of the Oasis Trust, Steve Chalke, who had collected over £1.25 million.

The £500 that Claire Squires collected before the race increased to over £1 million after she died having collapsed during the 2012 race.

A small number of runners, known as the "Ever Presents", have completed each of the London Marathons since 1981.

Originally hosted out-of-vision by David Coleman, more recently the main presenters on BBC One have been Sue Barker, Jonathan Edwards and Gabby Logan.

The highlight presenters on BBC Two have been Jonathan Edwards (2007–12), Sonali Shah (2013), and Helen Skelton (2014–15).

Subsequent sponsors have been Mars (1984–1988), ADT (1989–1992), NutraSweet (1993–1995), and Flora (1996–2009).

The current sponsors, Virgin Money, have sponsored the marathon since 2010, after signing a five-year £17m sponsorship deal in 2008.

In April 2013, the London Marathon renewed its sponsorship deal with Virgin Money for a further five years and the race changed its name to the Virgin Money London Marathon.

In 2021 it was announced that Indian consultancy company Tata Consultancy Services will take over sponsorship of the London Marathon from 2022.

The Second Council of Constantinople is the fifth of the first seven ecumenical councils recognized by both the Eastern Orthodox Church and the Catholic Church.

It is also recognized by the Old Catholics and others.

Protestant opinions and recognition of it are varied.

Some Protestants, such as Calvinists, recognize the first four councils, whereas Lutherans accept the first six, and most Anglo-Catholics accept all seven.

Participants were overwhelmingly Eastern bishops—only sixteen Western bishops were present, including nine from Illyricum and seven from Africa, but none from Italy—out of the 152 total.

The main work of the council was to confirm the condemnation issued by edict in 551 by the Emperor Justinian against the Three Chapters.

The council also condemned the teaching that Mary could not be rightly called the Mother of God () but only the mother of the man () or the mother of Christ ().

Justinian hoped that this would contribute to a reunion between the Chalcedonians and monophysites in the eastern provinces of the Empire.

The council, however, proceeded without the pope to condemn the Three Chapters.

And during the seventh session of the council, the bishops had Vigilius stricken from the diptychs for his refusal to appear at the council and approve its proceedings, effectively excommunicating him personally but not the rest of the Western Church.

Vigilius was then imprisoned in Constantinople by the emperor and his advisors were exiled.

After six months, in December 553, he agreed, however, to condemn the Three Chapters, claiming that his hesitation was due to being misled by his advisors.

In Northern Italy the ecclesiastical provinces of Milan and Aquileia broke communion with Rome.

Milan accepted the condemnation only toward the end of the sixth century, whereas Aquileia did not do so until about 700.

The rest of the Western Church accepted the decrees of the council, though without great enthusiasm.

Though ranked as one of the ecumenical councils, it never attained in the West the status of either Nicaea or Chalcedon.

In Visigothic Spain (Reccared having converted a short time prior) the churches never accepted the council; when news of the later Third Council of Constantinople was communicated to them by Rome it was received as the fifth ecumenical council, not the sixth.

Isidore of Seville, in his Chronicle and De Viris Illustribus, judged Justinian a tyrant and persecutor of the orthodox and an admirer of heresy, contrasting him with Facundus of Hermiane and Victor of Tunnuna, who was considered a martyr.

Despite the conflict between the council and the pope, and the inability to reconcile Chalcedonians and non-Chalcedonians, the council still made a significant theological contribution.

The canons condemning the Three Chapters were preceded by ten dogmatic canons which defined Chalcedonian Christology with a new precision, bringing out that Christ has two natures, the human and the divine, in one person.

The 'two natures' defined at Chalcedon were now clearly interpreted as two sets of attributes possessed by a single person, Christ God, the Second Person of the Trinity.

Later Byzantine Christology, as found in Maximus the Confessor and John of Damascus, was built upon this basis.

It used to be argued that the extant acts are incomplete, since they make no mention of the debate over Origenism.

However, the solution generally accepted today is that the bishops signed the canons condemning Origenism before the council formally opened.

There is a Syriac account of the council in the Melkite Chronicle of 641.

Also, one of the Acts of the Council at Constantinople, were the Anathemas issued against those who rejected the Perpetual Virginity of Mary.

An aerobot is an aerial robot, usually used in the context of an unmanned space probe or unmanned aerial vehicle.

While work has been done since the 1960s on robot "rovers" to explore the Moon and other worlds in the Solar System, such machines have limitations.

They tend to be expensive and have limited range, and due to the communications time lags over interplanetary distances, they have to be smart enough to navigate without disabling themselves.

For planets with atmospheres of any substance, however, there is an alternative: an autonomous flying robot, or "aerobot".

Most aerobot concepts are based on aerostats, primarily balloons, but occasionally airships.

Flying above obstructions in the winds, a balloon could explore large regions of a planet in detail for relatively low cost.

They can be made light in weight and are potentially relatively inexpensive.

They can cover a great deal of ground, and their view from a height gives them the ability to examine wide swathes of terrain with far more detail than would be available from an orbiting satellite.

For exploratory missions, their relative lack of directional control is not a major obstacle as there is generally no need to direct them to a specific location.

Balloon designs for possible planetary missions have involved a few unusual concepts.

This is a hot-air balloon where the envelope is made from a material that traps heat from sunlight, or from heat radiated from a planetary surface.

Black is the best color for absorbing heat, but other factors are involved and the material may not necessarily be black.

Solar Montgolfieres have several advantages for planetary exploration, as they can be easier to deploy than a light gas balloon, do not necessarily require a tank of light gas for inflation, and are relatively forgiving of small leaks.

They do have the disadvantage that they are only aloft during daylight hours.

This type of balloon consists of an envelope connected to a reservoir, with the reservoir containing a fluid that is easily vaporized.

The balloon can be made to rise by vaporizing the fluid into gas, and can be made to sink by condensing the gas back into fluid.

There are a number of different ways of implementing this scheme, but the physical principle is the same in all cases.

A balloon designed for planetary exploration will carry a small gondola containing an instrument payload.

The gondola will also carry power, control, and communications subsystems.

Due to weight and power supply constraints, the communications subsystem will generally be small and low power, and interplanetary communications will be performed through an orbiting planetary probe acting as a relay.

A solar Montgolfiere will sink at night, and will have a guide rope attached to the bottom of the gondola that will curl up on the ground and anchor the balloon during the darkness hours.

The guide rope will be made of low friction materials to keep it from catching or tangling on ground features.

Alternatively, a balloon may carry a thicker instrumented "snake" in place of the gondola and guiderope, combining the functions of the two.

This is a convenient scheme for making direct surface measurements.

One of the trickier aspects of planetary balloon operations is inserting them into operation.

Typically, the balloon enters the planetary atmosphere in an "aeroshell", a heat shield in the shape of a flattened cone.

Once operational, the aerobot will be largely on its own and will have to conduct its mission autonomously, accepting only general commands over its long link to Earth.

The aerobot will have to navigate in three dimensions, acquire and store science data, perform flight control by varying its altitude, and possibly make landings at specific sites to provide close-up investigation.

A small balloon, similar in appearance to terrestrial weather balloons, was carried on each of the two Soviet Vega Venus probes, launched in 1984.

The first balloon was inserted into the atmosphere of Venus on 11 June 1985, followed by the second balloon on 15 June 1985.

The first balloon failed after only 56 minutes, but the second operated for a little under two Earth days until its batteries ran down.

The Venus Vega balloons were the idea of Jacques Blamont, chief scientist for CNES and the father of planetary balloon exploration.

He energetically promoted the concept and enlisted international support for the small project.

The scientific results of the Venus VEGA probes were modest.

The atmospheric pressure on Mars is about 150 times less than that of Earth.

In such a thin atmosphere, a balloon with a volume of 5,000 to 10,000 cubic meters (178,500 to 357,000 cubic feet) could carry a payload of 20 kilograms (44 pounds), while a balloon with a volume of 100,000 cubic meters (3,600,000 cubic feet) could carry 200 kilograms (440 pounds).

The French had already conducted extensive experiments with solar Montgolfieres, performing over 30 flights from the late 1970s into the early 1990s.

The Montgolfieres flew at an altitude of 35 kilometers, where the atmosphere was as thin and cold as it would be on Mars, and one spent 69 days aloft, circling the Earth twice.

Early concepts for the Mars balloon featured a "dual balloon" system, with a sealed hydrogen or helium-filled balloon tethered to a solar Montgolfiere.

The light-gas balloon was designed to keep the Montgolfiere off the ground at night.

During the day, the Sun would heat up the Montgolfiere, causing the balloon assembly to rise.

Eventually, the group decided on a cylindrical sealed helium balloon made of aluminized PET film, and with a volume of 5,500 cubic meters (196,000 cubic feet).

The balloon would rise when heated during the day and sink as it cooled at night.

Total mass of the balloon assembly was 65 kilograms (143 pounds), with a 15 kilogram (33 pound) gondola and a 13.5 kilogram (30 pound) instrumented guiderope.

Unfortunately, although considerable development work was performed on the balloon and its subsystems, Russian financial difficulties pushed the Mars probe out from 1992, then to 1994, and then to 1996.

The Mars balloon was dropped from the project due to cost.

The first such experiments focused on a series of reversible-fluid balloons, under the project name ALICE, for "Altitude Control Experiment".

The first such balloon, ALICE 1, flew in 1993, with other flights through ALICE 8 in 1997.

Related work included the characterization of materials for a Venus balloon envelope, and two balloon flights in 1996 to test instrument payloads under the name BARBE, for "Balloon Assisted Radiation Budget Equipment".

By 1996, JPL was working on a full-fledged aerobot experiment named PAT, for "Planetary Aerobot Testbed", which was intended to demonstrate a complete planetary aerobot through flights into Earth's atmosphere.

PAT concepts envisioned a reversible-fluid balloon with a 10-kilogram payload that would include navigation and camera systems, and eventually would operate under autonomous control.

The project turned out to be too ambitious, and was cancelled in 1997.

JPL continued to work on a more focused, low-cost experiments to lead to a Mars aerobot, under the name MABVAP, for "Mars Aerobot Validation Program".

MABVAP experiments included drops of balloon systems from hot-air balloons and helicopters to validate the tricky deployment phase of a planetary aerobot mission, and development of envelopes for superpressure balloons with materials and structures suited to a long-duration Mars mission.

JPL also provided a set of atmospheric and navigation sensors for the Solo Spirit round-the-world manned balloon flights, both to support the balloon missions and to validate technologies for planetary aerobots.

While these tests and experiments were going on, JPL performed a number of speculative studies for planetary aerobot missions to Mars, Venus, Saturn's moon Titan, and the outer planets.

As its name implies, MABTEX was primarily intended to be an operational technology experiment as a precursor to a more ambitious efforts.

MABTEX was envisioned as a small superpressure balloon, carried to Mars on a "microprobe" weighing no more than .