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Astronomers film record-breaking lunar impact 

Feb. 24, 2014
Courtesy of the Royal Astronomical Society
and World Science staff

A me­te­or­ite the weight of a small car crashed in­to the Moon last Sep­tem­ber, ac­cord­ing to Span­ish as­tro­no­mers, who filmed the event.

The im­pact, the big­gest seen to date, pro­duced a bright flash and would have been easy to spot from the Earth, they said. The sci­en­tists pub­lish their de­scrip­tion of the event in the jour­nal Monthly No­tices of the Roy­al As­tro­nom­i­cal So­ci­e­ty.

The Moon lacks an at­mos­phere that, on Earth, would pre­vent small rocks from space from reach­ing the sur­face. The re­sult is very vis­i­ble – crat­ers large and small pock­mark the whole Moon and rec­ord 4.5 bil­lion years of col­li­sions that span the his­to­ry of the So­lar sys­tem.

Al­though there is al­most no chance of a very large ob­ject strik­ing the Moon or plan­ets, col­li­sions with smaller ob­jects are com­mon even to­day. The odds of see­ing one of these by chance are pret­ty poor, so sci­en­tists have set up net­works of tele­scopes that can de­tect them au­to­mat­ic­ally.

Last Sept. 11, as­tron­o­mer José M. Madiedo of the Uni­vers­ity of Huel­va in Spain was op­er­at­ing two tele­scopes in the south of Spain that were search­ing for these im­pactors. At 8:07 p.m. Green­wich Mean Time he wit­nessed an un­usu­ally long and bright flash in Ma­re Nu­bi­um, an an­cient lava-filled ba­sin with a darker ap­pear­ance than its sur­round­ings.

The flash was briefly al­most as bright as the fa­mil­iar Pole Star, mean­ing that an­y­one on Earth lucky enough to be look­ing at the Moon at that mo­ment would have been able to see it, ac­cord­ing to Madiedo. In a vi­deo re­cord­ing he made, an af­ter­glow re­mained vis­i­ble for a fur­ther eight sec­onds.

It’s the longest and bright­est con­firmed im­pact flash ev­er ob­served on the Moon, he added. “At that mo­ment I real­ized that I had seen a very rare and ex­tra­or­di­nary event.”

The Span­ish tele­scopes are part of the Moon Im­pacts De­tec­tion and Anal­y­sis Sys­tem that mon­i­tors the lu­nar sur­face. The proj­ect is be­ing un­der­taken by Madiedo with José L. Or­tiz of the In­sti­tute of As­t­ro­phys­ics of An­da­lu­sia.

The fast-impacting rocks melt and va­por­ize in­stan­ta­ne­ously, and this pro­duces a hot glow. General­ly, these flashes last just a frac­tion of a sec­ond. But the Sept. 11 flash was much more in­tense and long­er than an­y­thing seen be­fore, Madiedo said.

Madiedo and Or­tiz think it was pro­duced by an im­pactor of around 400 kg (900 lbs.) with a width of be­tween 0.6 and 1.4 me­ters or yards. The rock hit Ma­re Nu­bi­um at about 61,000 km (38,000 miles) per hour and cre­at­ed a new crat­er about 40 me­ters or yards wide. The es­ti­mat­ed im­pact en­er­gy was equiv­a­lent to an ex­plo­sion of roughly 15 tons of TNT, at least three times high­er than the larg­est pre­vi­ously seen event ob­served by NASA in March last year.

Ob­serv­ing im­pacts on the Moon gives as­tro­no­mers an in­sight in­to the risk of si­m­i­lar (but larg­er) ob­jects hit­ting the Earth. One of the con­clu­sions of the Span­ish team is that these one-meter sized ob­jects may strike our plan­et about ten times as of­ten as sci­en­tists pre­vi­ously thought. For­tu­nate­ly, the Earth’s at­mos­phere shields us from rocks as small as the one that hit Ma­re Nu­bi­um, but they can lead to spec­tac­u­lar “fire­ball” me­te­ors.


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A meteorite the weight of a small car crashed into the Moon last September, according to Spanish astronomers, who filmed the event. The impact, the biggest seen to date, produced a bright flash and would have been easy to spot from the Earth, they said. The scientists publish their description of the event in the journal Monthly Notices of the Royal Astronomical Society. The Moon lacks an atmosphere that, on Earth, would prevent small rocks from space from reaching the surface. The result is very visible – craters large and small pockmark the whole moon and record 4.5 billion years of collisions that span the history of the Solar system. Although there is almost no chance of a very large object striking the Moon or planets, collisions with smaller objects are very common even today, astronomers add. The odds of seeing one of these by chance are pretty poor, so scientists have set up networks of telescopes that can detect them automatically. Last Sept. 11, astronomer José M. Madiedo of the University of Huelva in Spain was operating two telescopes in the south of Spain that were searching for these impactors. At 8:07 p.m. Greenwich Mean Time he witnessed an unusually long and bright flash in Mare Nubium, an ancient lava-filled basin with a darker appearance than its surroundings. The flash was briefly almost as bright as the familiar Pole Star, meaning that anyone on Earth lucky enough to be looking at the Moon at that moment would have been able to see it, according to Madiedo. In a video recording he made, an afterglow remained visible for a further eight seconds. It’s the longest and brightest confirmed impact flash ever observed on the Moon, he added. “At that moment I realized that I had seen a very rare and extraordinary event.” The Spanish telescopes are part of the Moon Impacts Detection and Analysis System that monitors the lunar surface. The project is being undertaken by Madiedo with José L. Ortiz of the Institute of Astrophysics of Andalusia and continues a pioneering program that detected sporadic lunar impact flashes for the first time. Since these impacts take place at huge speeds, the rocks melt and vaporize instantaneously, and this produces a hot glow. Generally, these flashes last just a fraction of a second. But the flash detected on 11 September was much more intense and longer than anything observed before, Madiedo said. Madiedo and Ortiz think it was produced by an impactor of around 400 kg (900 lbs.) with a width of between 0.6 and 1.4 meters or yards. The rock hit Mare Nubium at about 61,000 km (38,000 miles) per hour and created a new crater about 40 meters or yards wide. The estimated impact energy was equivalent to an explosion of roughly 15 tons of TNT, at least three times higher than the largest previously seen event observed by NASA in March last year. Observing impacts on the Moon gives astronomers an insight into the risk of similar (but larger) objects hitting the Earth. One of the conclusions of the Spanish team is that these one-meter sized objects may strike our planet about ten times as often as scientists previously thought. Fortunately, the Earth’s atmosphere shields us from rocks as small as the one that hit Mare Nubium, but they can lead to spectacular “fireball” meteors.