Category Science

          Amazingly, thousands of rocks from space hit the surface of Earth each day. Every year our planet puts on nearly 10,000 tonnes in weight due to meteoroids entering the atmosphere. Many of these are minuscule grains of dust, but some can be many metres in length. The world’s largest known meteorite was discovered in Namibia, Africa, in 1920. It weighs an incredible 55,000 kg (120, 000Ibs).

          To date, there have been nearly 1,100 recovered falls (meteorites seen to fall) and nearly 40,000 finds (found, but not seen to fall). It is estimated that probably 500 meteorites reach the surface of the Earth each year, but less than 10 are recovered. This is because most fall into the ocean, land in remote areas of the Earth, land in places that are not easily accessible, or are just not seen to fall (fall during the day). From a model animation, it appears that lots of small asteroids/large meteoroids pass close to the Earth each day. Most of these are not detected, but recently, three 5–10 meter “asteroids” have been discovered and have passed well within the orbit of the Moon. Also recently, an asteroid about 500 meters in diameter passed about 2 million km from the Earth (five times the distance to the Moon). It is estimated that each day one or two 5–10 meter objects pass within the Moon’s orbit and that there are probably 30 million near-Earth objects! Most of these are too small to ever cause any damage. Five to ten meters is probably the smallest object that would likely survive passage through the Earth’s atmosphere.

          While large impacts are fairly rare, thousands of tiny pieces of spaces of space rock, called meteorites, hit the ground each year. However, the majority of these events are unpredictable and go unnoticed, as they land in vast swathes of uninhabited forest or in the open waters of the ocean, Bill Cooke and Althea Moorhead of NASA’s Meteoroid Environments Office told Space.com. 

          In order to understand meteorite impacts on Earth, it is important to know where the chunks of rock come from. Meteoroids are rocky remnants of a comet or asteroid that travel in outer space, but when these objects enter Earth’s atmosphere, they are considered meteors. Most (between 90 and 95 percent) of these meteors completely burn up in the atmosphere, resulting in a bright streak that can be seen across the night sky, Moorhead said. However, when meteors survive their high-speed plunge toward Earth and drop to the ground, they are called meteorites. 

          The Perseid meteor shower — one of the most popular meteor showers of the year — is expected to put on a particularly breathtaking show Aug. 11 and 12, when the Earth passes through the trail of debris created by Comet Swift-Tuttle. However, viewers should not expect to find any meteorites lying on the ground after this spectacular meteor shower. “Perseids come from Comet Swift-Tuttle and are very fragile, being an ice-dust mix,” Cooke said. “They are not strong enough to survive passage through the atmosphere at 132,000 mph (212,433 km/h) and so never produce meteorites — they are totally vaporized by the time they make it to 50 miles (80 km) altitude.” 

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          Several more probes have been designed and built to visit comets in the near future.

          Stardust, the 4th Discovery mission launched in February 1999, will collect coma samples from the recently deflected comet 81P/Wild 2 on 2 January 2004 and return them to Earth on 15 January 2006 for detailed laboratory analyses. Stardust will be the first mission to bring samples back to Earth from a known comet and also the first to bring back contemporary interstellar particles recently discovered. These samples should provide important insights into the nature and amount of dust released by comets, the roles of comets in planetary systems, clues to the importance of comets in producing dust in our zodiacal cloud as well as circumstellar dust around other stars, and the links between collected meteoritic samples with a known commentary body. Samples are collected in newly invented continuous gradient density silica aerogel. Stardust is facilitated by a magnificent trajectory designed to accomplish a complex and ambitious flyby sample return mission within the Discovery program restrictions. The remaining science payload, which provides important context for the captured samples, includes a time?of?flight spectrometer measuring the chemical and isotopic composition of dust grains; a polyvinylidene fluoride dust flux monitor determining dust flux profiles; a CCD camera for imaging Wild 2 coma and its nucleus; a shared X band transponder providing two?way Doppler shifts to estimate limits to Wild 2 mass and integrated dust fluency; and tracking of the spacecraft’s attitude sensing for the detection of large particle impacts. The graphite composite spacecraft brings the collected sample back to Earth by a direct reentry in a capsule.

          Stardust, the fourth NASA Discovery mission, launched on 7 February 1999, now circles the Sun in an orbit that will cause a close encounter on 2 January 2004 with the comet 81P/Wild 2. Stardust will collect coma dust at 150 km from Wild 2’s nucleus and return it to Earth for detailed laboratory analysis on 15 January 2006. Figure 1 shows an artist’s rendition of the Stardust spacecraft encountering the comet Wild 2 with the sample collector fully deployed. The Halley Intercept Mission (HIM) proposed in 1981 for the last comet Halley apparition inspired the near 2?decade quest for this comet coma sample return mission, Stardust.

          In addition, along the way to Wild 2, the backside of the Wild 2 sample collector will be used to capture interstellar particles (ISP) as bonus science. Besides the primary sample instrument, Stardust also makes in situ investigations to provide important context to the return samples: a time?of?flight spectrometer, a dust flux monitor, an optical navigation camera, an X band transponder for determining integrated dust flux and an estimate of the mass of Wild 2, and monitoring of spacecraft attitude control disturbances for large particle impacts.

          The return of lunar samples by the Apollo program provided the first opportunity to perform detailed laboratory studies of ancient solid materials from a known astronomical body. The highly detailed study of these samples revolutionized our understanding of the Moon and provided fundamental insights into the remarkable and violent processes that occur early in the history of moons and terrestrial planets. This type of space paleontology is not possible with astronomical and remote sensing. Despite these advantages, however, the last US sample return was made by Apollo 17 over 30 years ago! Now, 3 decades later, Stardust is leading a new era of sample return missions, including missions to return samples of solar wind [Burnett et al .,2003], asteroid [Fujiwara et al., 1999], and Mars [Garvin, 2002].

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          If a comet collided with Earth, the results could be disastrous — possibly meaning the end of all life on our planet. Comets can often be caught by the strong gravitational pulls of planets. In 1994, the Shoemaker-Levy 9 comet crashed into Jupiter’s atmosphere. It impacted at more than 200,000km/h (124,000mph), creating balls of fire larger than Earth.

          An impact event is a collision between astronomical objects causing measurable effects. Impact events have physical consequences and have been found to regularly occur in planetary systems, though the most frequent involve asteroids, comets or meteoroids and have minimal effect. When large objects impact terrestrial planets such as the Earth, there can be significant physical and biospheric consequences, though atmospheres mitigate many surface impacts through atmospheric entry. Impact craters and structures are dominant landforms on many of the Solar System’s solid objects and present the strongest empirical evidence for their frequency and scale.

          Impact events appear to have played a significant role in the evolution of the Solar System since its formation. Major impact events have significantly shaped Earth’s history, have been implicated in the formation of the Earth-Moon system, the evolutionary history of life, the origin of water on Earth and several mass extinctions. The prehistoric Chicxulub impact, 66 million years ago, is believed to be the cause of the Cretaceous-Paleocene extinction event.

          The Comet Shoemaker-Levy 9 impact provided the first direct observation of an extraterrestrial collision of Solar System objects, when the comet broke apart and collided with Jupiter in July 1994. An extrasolar impact was observed in 2013, when a massive terrestrial planet impact was detected around the star ID8 in the star cluster NGC 2547 by NASA’s Spitzer space telescope and confirmed by ground observations. Impact events have been a plot and background element in science fiction.

          In April 2018, the B612 Foundation reported “It’s 100 per cent certain we’ll be hit [by a devastating asteroid], but we’re not 100 per cent certain when.” Also in 2018, physicist Stephen Hawking, in his final book Brief Answers to the Big Questions, considered an asteroid collision to be the biggest threat to the planet. In June 2018, the US National Science and Technology Council warned that America is unprepared for an asteroid impact event, and has developed and released the “National Near-Earth Object Preparedness Strategy Action Plan” to better prepare. According to expert testimony in the United States Congress in 2013, NASA would require at least five years of preparation before a mission to intercept an asteroid could be launched.

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          The Space probe Giotto was the first to visit a comet up close. In 1986, it flew into Halley’s cornet and photographed the nucleus in incredible detail. It was able to gather data for almost 10 hours before dust and gas hitting the probe put the cameras out of action. From just 600km (373 miles) away, Giotto determined that Halley’s nucleus measures 15km by 8km (9 miles by 5 miles), and is made up of ice and dust.

          Giotto was a European robotic spacecraft mission from the European Space Agency. The spacecraft flew by and studied Halley’s Comet and in doing so became the first spacecraft to make close up observations of a comet. On 13 March 1986, the spacecraft succeeded in approaching Halley’s nucleus at a distance of 596 kilometers. It was named after the Early Italian Renaissance painter Giotto di Bondone. He had observed Halley’s Comet in 1301 and was inspired to depict it as the star of Bethlehem in his painting Adoration of the Magi.

          The spacecraft was derived from the GEOS research satellite built by British Aerospace in Filton, Bristol, and modified with the addition of a dust shield (Whipple shield) as proposed by Fred Whipple which comprised a thin (1 mm) aluminium sheet separated by a space and a thicker Kevlar sheet. The later Stardust spacecraft would use a similar Whipple shield. A mockup of the spacecraft resides at the Bristol Aero Collection hangar, at Filton, UK.

          The Soviet Vega 1 started returning images of Halley on 4 March 1986, and the first ever of its nucleus, and made its flyby on 6 March, followed by Vega 2 making its flyby on 9 March. Vega 1’s closest approach to Halley was 8?889 km.

          Giotto passed Halley successfully on 14 March 1986 at 596 km distance, and surprisingly survived despite being hit by some small particles. One impact sent it spinning off its stabilized spin axis so that its antenna no longer always pointed at the Earth, and its dust shield no longer protected its instruments. After 32 minutes Giotto re-stabilized itself and continued gathering science data. Another impact destroyed the Halley Multicolor Camera, but not before it took photographs of the nucleus at closest approach.

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          Unlike many other minor bodies in the Solar System, comets have been known about for thousands of years. The Chinese recorded Halley’s Comet as far back as 240BC. The famous Bayeux Tapestry, which was made to commemorate the Norman conquest of England in 1066, shows Halley’s Comet.

          The first known observation of Halley’s took place in 239 B.C., according to the European Space Agency. Chinese astronomers recorded its passage in the Shih Chi and Wen Hsien Thung Khao chronicles. Another study (based on models of Halley’s orbit) pushes that first observation back to 466B.C. which would have made it visible by the Ancient Greeks. 

          When Halley’s returned in 164 B.C. and 87 B.C., it probably was noted in Babylonian records now housed at the British Museum in London. “These texts have important bearing on the orbital motion of the comet in the ancient past,” noted a Nature research paper about the tablets.

          Another appearance of the comet in 1301 possibly inspired Italian painter Giotto’s rendering of the Star of Bethlehem in “The Adoration of the Magi,” according to the Britannic encyclopedia. Halley’s most famous appearance occurred shortly before the 1066 invasion of England by William the Conqueror. It is said that William believed the comet heralded his success. In any case, the comet was put on the Bayeux Tapestry — which chronicles the invasion — in William’s honor.

          Astronomers in these times, however, saw each appearance of Halley’s Comet as an isolated event. Comets were often foreseen as a sign of great disaster or change.

          Even when Shakespeare wrote his play “Julius Caesar” around 1600, just 105 years before Edmond Halley calculated that the comet returns over and over again, one famous phrase spoke of comets as heralds: “When beggars die there are no comets seen; the heavens themselves blaze forth the death of princes.”

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          Because of the solar wind, a comet’s tail always points away from the Sun. If a comet is travelling away from the Sun, its tail will be in front of the nucleus.

          The solid nucleus or core of a comet consists mostly of ice and dust coated with dark organic material, according to NASA, with the ice composed mainly of frozen water but perhaps other frozen substances as well, such as ammonia, carbon dioxide, carbon monoxide and methane. The nucleus may have a small rocky core. 

          As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet’s gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun. Comet tails may spray planets, as was the case in 2013 with Comet Siding Spring and Mars.

          At first glance, comets and asteroids may appear very similar. The difference lies in the presence of the coma and tail. Sometimes, a comet may be misidentified as an asteroid before follow-up observations reveal the presence of either or both of these features. The nuclei of most comets are thought to measure 10 miles (16 kilometers) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million km) wide, and some have tails reaching 100 million miles (160 million km) long.

          We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope. Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when Earth passes through the orbit of the Swift-Tuttle comet.

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