ＮＡＳＡ、小惑星の軌道変える宇宙船打ち上げ

【ワシントンＡＦＰ＝時事】米航空宇宙局（ＮＡＳＡ）は２３日午後１０時２１分（日本時間２４日午後３時２１分）、カリフォルニア州のバンデンバーグ空軍基地から、宇宙船を小惑星に衝突させて軌道を変える「ＤＡＲＴ」ミッションの宇宙船を打ち上げた。ＮＡＳＡ ＴＶがライブ配信した。（写真はＤＡＲＴ宇宙船が小惑星ディモーフォスに衝突する直前の想像図）
　宇宙船は米宇宙開発企業スペースＸの「ファルコン９」ロケットに搭載されている。
　ＤＡＲＴは、「Ｄｏｕｂｌｅ　Ａｓｔｅｒｏｉｄ　Ｒｅｄｉｒｅｃｔｉｏｎ　Ｔｅｓｔ（二重小惑星進路変更実験）」の略で、地球を脅かす小惑星が出現した場合、その進路を変えるのに宇宙船を衝突させるのが有効かを検証するのが目的。
　標的となるのは、太陽を周回する小惑星「ディディモス」（直径約７６０メートル）の衛星「ディモーフォス」（直径約１６０メートル）。
　二重小惑星が地球まで１１００万キロの距離に接近する来年秋に、宇宙船を衝突させ、軌道の変更を試みる。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2021/11/25-14:52）

In the 1998 Hollywood blockbuster Armageddon, Bruce Willis and Ben Affleck race to save the Earth from being pulverized by an asteroid.
While the Earth faces no such immediate danger, NASA plans to crash a spacecraft traveling at a speed of 15,000 miles per hour (24,000 kph) into an asteroid next year in a test of planetary defense.
The Double Asteroid Redirection Test (DART) is to determine whether this is an effective way to deflect the course of an asteroid should one threaten the Earth in the future.
NASA provided details of the DART mission, which carries a price tag of $330 million, in a briefing for reporters on Thursday.
Although there isn't a currently known asteroid that's on an impact course with the Earth, we do know that there is a large population of near-Earth asteroids out there, said Lindley Johnson, NASA's Planetary Defense Officer.
The key to planetary defence is finding them well before they are an impact threat, Johnson said. We don't want to be in a situation where an asteroid is headed towards Earth and then have to test this capability.
The DART spacecraft is scheduled to be launched aboard a SpaceX Falcon 9 rocket at 10:20 pm Pacific time on November 23 from Vandenberg Space Force Base in California.
If the launch takes place at or around that time, impact with the asteroid some 6.8 million miles from Earth would occur between September 26 and October 1 of next year.
The target asteroid, Dimorphos, which means two forms in Greek, is about 525 feet in diameter and orbits around a larger asteroid named Didymos, twin in Greek.
Johnson said that while neither asteroid poses a threat to Earth they are ideal candidates for the test because of the ability to observe them with ground-based telescopes.
Images will also be collected by a miniature camera-equipped satellite contributed by the Italian Space Agency that will be ejected by the DART spacecraft 10 days before impact.
- 'A small nudge' -
Nancy Chabot of the Johns Hopkins Applied Physics Laboratory, which built the DART spacecraft, said Dimorphos completes an orbit around Didymos every 11 hours and 55 minutes just like clockwork.
The DART spacecraft, which will weigh 1,210 pounds at the time of impact, will not destroy the asteroid, Chabot said.
It's just going to give it a small nudge, she said. It's going to deflect its path around the larger asteroid.
It's only going to be a change of about one percent in that orbital period, Chabot said, so what was 11 hours and 55 minutes before might be like 11 hours and 45 minutes.
The test is designed to help scientists understand how much momentum is needed to deflect an asteroid in the event one is headed towards Earth one day.
We are targeting to be as nearly head on as possible to cause the biggest deflection, Chabot said.
The amount of deflection will depend to a certain extent on the composition of Dimorphos and scientists are not entirely certain how porous the asteroid is.
Dimorphos is the most common type of asteroid in space and is some 4.5 billion years old, Chabot said.
It's like ordinary chondrite meteorites, she said. It's a fine grain mixture of rock and metal together.
Johnson, NASA's Planetary Defense Officer, said more than 27,000 near-Earth asteroids have been catalogued but none currently pose a danger to the planet.
An asteroid discovered in 1999 known as Bennu that is 1,650 feet wide will pass within half the distance of the Earth to the Moon in the year 2135 but the probability of an impact is considered very slight.