「潜在的に危険な小惑星」、新たに発見

【ワシントンＡＦＰ＝時事】地球の公転軌道と軌道が交差する可能性がある小惑星が、新たに発見された。遠い将来、壊滅的な衝突が起こる確率がわずかだがあるという。１０月３１日、学術誌「アストロノミカル・ジャーナル」に研究論文が掲載された。（資料写真）
　小惑星は直径１．５キロ前後で、２０２２ ＡＰ７と名付けられた。太陽の周りを５年かけて回っている。
　論文の筆頭著者である米カーネギー研究所の天文学者スコット・シェパード氏は「２０２２ ＡＰ７の軌道は地球の軌道と交わっており、潜在的に危険な小惑星であると言える」と説明。ただし、「現在もしくは近い将来、地球と衝突することはない」としている。
　軌道周回する天体は、惑星などのさまざまな重力の影響で軌道が徐々にずれていく。そのため将来的に危険になることもあるが、長期的な予想は難しい。
　複数の観測所が参加する研究グループ、米国立光学・赤外天文学研究所は、２０２２ ＡＰ７は「地球に危害を及ぼす恐れがある天体としては、過去８年で最も大きい」としている。
　シェパード氏によると、衝突が起きた場合、大気中に舞い上がった粉じんで地球が冷やされ、過去数百万年の間に起きたことがない規模で動植物が絶滅する恐れがある。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2022/11/02-15:24）

An international team of astronomers on Monday announced the discovery of a large asteroid whose orbit crosses that of Earth, creating a small chance far in the future of a catastrophic collision.
The 1.5 kilometer- (0.9 mile-) wide asteroid, named 2022 AP7, was discovered in area notoriously difficult to spot objects due to the glare from the Sun.
It was found along with two other near-Earth asteroids using a high-tech instrument on the Victor M. Blanco telescope in Chile that was originally developed to study dark matter.
2022 AP7 crosses Earth?s orbit, which makes it a potentially hazardous asteroid, but it currently does not now or anytime in the future have a trajectory that will have it collide with the Earth, said lead author of the findings, astronomer Scott Sheppard of the Carnegie Institution for Science.
The potential threat comes from the fact that like any orbiting object, its trajectory will be slowly modified due to myriad gravitational forces, notably by planets. Forecasts are therefore difficult on the very long term.
The newly-discovered asteroid is the largest object that is potentially hazardous to Earth to be discovered in the last eight years, said NOIRLab, a US-funded research group that operates multiple observatories.
2022 AP7 takes five years to circle the Sun under its current orbit, which at its closest point to Earth remain several million kilometers away.
The risk is therefore very small, but in case of a collision, an asteroid of that size would have a devastating impact on life as we know it, said Sheppard. He explained that dust launched into the air would have a major cooling effect, provoking an extinction event like hasn?t been seen on Earth in millions of years.
His team's results were published in the scientific journal The Astronomical Journal. The two other asteroids pose no risk to Earth, but one is the closest asteroid to the Sun ever found.
Some 30,000 asteroids of all sizes -- including more than 850 larger than a kilometer wide -- have been catalogued in the vicinity of the Earth, earning them the label Near Earth Objects (NEOs). None of them threaten Earth for the next 100 years.
According to Sheppard, there are likely 20 to 50 large NEOs left to find, but most are on orbits that put them in the Sun's glare.
In preparation for a future discovery of a more threatening object, NASA conducted a test mission in late September in which it collided a spacecraft with an asteroid, proving that it was possible to change its trajectory.