Tuesday, 24 November 2020

Asteroid 2020 VG4 passes the Earth.

Asteroid 2020 VG4 passed by the Earth at a distance of about 794 400 km (2.07 times the average distance between the Earth and the Moon, or 0.53% of the distance between the Earth and the Sun), slightly after 5.10 am GMT on Tuesday 17 November 2020. There was no danger of the asteroid hitting us, though were it to do so it would not have presented a significant threat. 2020 VG4 has an estimated equivalent diameter of 4-12 m (i.e. it is estimated that a spherical object with the same volume would be 4-12 m in diameter), and an object of this size would be expected to explode in an airburst (an explosion caused by superheating from friction with the Earth's atmosphere, which is greater than that caused by simply falling, due to the orbital momentum of the asteroid) between 43 and 30 km above the ground, with only fragmentary material reaching the Earth's  surface.

 
The closest approach of 2020 VG4 to the Earth on 17 November 2020. JPL Small Body Database.

2020 VG4 was discovered on 12 November 2020 (five days before its closest approach to the Earth) by the University of Arizona's Mt. Lemmon Survey at the Steward Observatory on Mount Lemmon in the Catalina Mountains north of Tucson. The designation 2020 VG4 implies that the asteroid was the 107th object (asteroid T3 - in numbering asteroids the letters A-Z excluding I, are assigned numbers from 1 to 25, with a number added to the end each time the alphabet is ended, so that A = 1, A1 = 26, A2 = 51, etc., which means that G4 = (25 x 4) + 7 = 107) discovered in the first half of November 2020 (period 2020 V - the year being split into 24 half-months represented by the letters A-Y, with I being excluded).

 
The orbit and current position of 2020 VG4. The Sky Live 3D Solar System Simulator.

2020 VG4 has a 771 day (2.11 year) orbital period, with an elliptical orbit tilted at an angle of 6.83° to the plain of the Solar System which takes in to 0.95 AU from the Sun (95% of the distance at which the Earth orbits the Sun) and out to 2.34 AU (234% of the distance at which the Earth orbits the Sun, and more than the distance at which the planet Mars orbits the Sun). It is therefore classed as an Apollo Group Asteroid (an asteroid that is on average further from the Sun than the Earth, but which does get closer). This is thought to have been the first close encounter between 2020 VG4 and the Earth, with another such encounter predicted in February 2022, after which no further similar events are anticipated.

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Warnings issued to beachgoers after Portuguese Man 'o War wash up on coast of Wales.

Visitors to beaches in South Wales have been warned to keep Dogs on leads and avoid walking barefoot after a number of Portuguese Man 'o War, Physalia physalis, have been found on beaches in Pembrokeshire and Carmarthenshire this week. The planktonic Animals die rapidly after being exposed on beaches, but remain capable of delivering a powerful and unpleasant sting for some time afterwards, and in extreme circumstances these can prove fatal to both Humans and Dogs. Detatched tentacles, which are harder to spot than their brightly coloured floats, also present a risk. The venemous Cnidarians have been seen on beaches at Cefn Sidan, Llansteffan, Broadhaven, Manorbier, Newton, Freshwater West, and Newgale. They have also been reported at Tywyn, Barmouth and Harlech on the coast of Gwynedd in North Wales, though in lower numbers.

 
A Portuguese Man 'o War, Physalia physalis, on Llansteffan Beach in Carmarthenshire, South Wales, on 19 November 2020.

Portuguese Man o' War are colonial Siphonophores only distantly related to true Jellyfish, Scyphozoa, though commonly referred to as such. Their bodies are made up of thousands of individual zooids, each with their own sting, tentacles and digestive system. New zooids are formed by budding from other members of the colony, but remain attached to these to form a single colony. Each year a generation of specialist sexual zooids (gonozoids) is produced which produce eggs and sperm, with fertilised eggs going on to form new colonies. These animals are anchored to the sea surface by a highly modified zooid which forms an air sack, filled with a mixture of carbon monoxide defused from the zooid and nitrogen, oxygen and argon from the atmosphere, which are brought into the sack through osmosis. Portuguese Man o' War produce an extremely strong venom, for both capturing food and defending the colony, and which is capable of causing extremely painful stings, and sometimes death, in Humans, for which reason people are advised to be extremely cautious on beaches where these animals wash up, not just of entire animals but also detached tentacles, which are less visible but still capable of stinging.

 
A Portuguese Man o' War in the water. NOAA/Wikipedia.

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Monday, 23 November 2020

Hurricane Eta exposes nineteenth century shipwreck on the coast of Florida.

Hurricane Eta swept across central Florida on 12 November 2020, causing eleven fatalities, in addition to the 178 it had already claimed in Central America, in addition to causing widespread flooding and causing about a billion dollars' worth of damage to property. After the storm had passed it was discovered that high tides brought on by the hurricane's storm surge had scoured the beach at the Fort Matanzas National Monument in St John's County on the east coast of Florida, exposing the remains of a ship that had been burried by the sand. 

 
Exposed timbers on the foreshore at Fort Matanzas National Monument, believed to have come from an nineteenth century shipwreck. CNN.

The site is now being investigated archaeologists from the St. Augustine Lighthouse Archaeological Maritime Program and students from Flagler College, who have determined that the remains probably belong to the Caroline Eddy, an American merchant ship which sank in the area on 29 August 1880, after becoming caught in a storm.

 
Chuck Meide of the St. Augustine Lighthouse Archaeological Maritime Program examaning timbers believed to have come from the Caroline Eddy, a merchant ship which sank off the coast of Florida in August 1880. St Augustine.

Tropical storms, known as hurricanes in the Caribbean, are caused by solar energy heating the air above the oceans, which causes the air to rise leading to an inrush of air. If this happens over a large enough area the inrushing air will start to circulate, as the rotation of the Earth causes the winds closer to the equator to move eastwards compared to those further away (the Coriolis Effect). This leads to tropical storms rotating clockwise in the southern hemisphere and anticlockwise in the northern hemisphere. These storms tend to grow in strength as they move across the ocean and lose it as they pass over land (this is not completely true: many tropical storms peter out without reaching land due to wider atmospheric patterns), since the land tends to absorb solar energy while the sea reflects it.

 
The formation of a tropical cyclone. Natural Disaster Management.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms. 

 
The formation and impact of a storm surge. eSchoolToday.

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Magnitude 6.1 Earthquake off the coast of Talca Province, central Chile.

The United States Geological Survey recorded a Magnitude 6.1 Earthquake at a depth of 19.7 km, off the coast of central Chile, roughly 99 km to the northwest of the city of ConstituciĆ³n in Talca Province, slightly before 10.55 pm local time on Saturday 21 November 2020 (slightly before 0.55 am on Sunday 22 November GMT). There are no reports of any injuries associated with this event, but peoplehave reported feeling it over a wide area.

 
The location of the 21 November 2020 Talca Province Earthquake. USGS.

Chile is located on the west coast of South America, which is also the convergent margin between the Nazca and South American Plates. The Nazca Plate is being subducted beneath the South American Plate and is sinking beneath the South American Plate. This is not a smooth process, the rocks of the two plates continuously stick together then, as the pressure builds up, break apart again, causing Earthquakes. As the Nazca Plate sinks deeper it is partially melted by the heat of the Earth's interior. Some of the melted material then rises up through the overlying South American Plate as magma, fuelling the volcanoes of the Chilean Andes.

 
The subduction of the Nazca Plate beneath the South American Plate, and how it causes Earthquakes and volcanoes. Pacific Earthquake Engineering Research Center.
 
Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organisation Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.
 
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