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Once-in-a-lifetime eclipse by asteroid to treat Europe

July 5, 2010
Courtesy of the Eu­ropean Space Agen­cy
and World Science staff

In a rare event next Thurs­day, some sky­watch­ers in Eu­rope will be able to see a star briefly van­ish as an as­ter­oid pas­ses be­tween it and us. As­tro­no­mers say the event may be the only eclipse of a star by an as­ter­oid this cen­tu­ry vis­ible with the un­aided eye.

Eve­ry­one is fa­mil­iar with a so­lar eclipse, when our Moon passes in front of the Sun and blocks its light for sev­er­al min­utes.

Star chart showing the sky from central Europe, facing south just be­fore mid­night on July 8. The star Delta Oph­iuchi will be about half-way be­tween the hori­zon and the ze­nith or "top" of the sky. (Cre­dits: ESA, created with Guide 8 by Pro­ject Plu­to, http://www.projectpluto.com)


A si­m­i­lar situa­t­ion can hap­pen with as­ter­oids, the Sun-or­b­it­ing, rocky or me­tal­lic ob­jects left over from the forma­t­ion of the So­lar Sys­tem or formed by crashes be­tween oth­er as­ter­oids. 

We know of about 400,000 of these dark bod­ies, which range in size from a few hun­dred kilo­me­tres (miles) to just a few me­tres (yards). Smaller ones are hard to de­tect.

While an as­ter­oid is far too small to co­ver the Sun, one will oc­ca­sion­ally move di­rectly in front of one of the stars in the night sky and block its light from us, caus­ing a stel­lar eclipse or oc­culta­t­ion. Since as­ter­oids move fast, these events typ­ic­ally last just a few sec­onds. 

Nor­mally the oc­culted star is so faint the event can only be seen via tel­e­scope. But the night of 8-9 Ju­ly, a star vis­i­ble to the na­ked eye, Del­ta Ophi­uchi (the fourth-brightest star in the con­stella­t­ion Ophi­uchi), will be oc­culted by as­ter­oid Ro­ma, about 50 km (30 miles) wide.

This means the eclipse will be vis­i­ble only along a path of about that same width, cross­ing cen­tral Eu­rope, Spain and the Ca­nary Is­lands. At about 11:57 p.m. Cen­tral Eu­ropean Sum­mer Time, ob­servers on a line run­ning be­tween Stock­holm - Copen­hagen - Bre­men - Nantes - Bil­bao will see the star dis­ap­pear for about five sec­onds as its light is blocked by the as­ter­oid.

Since most as­ter­oids are too small to be re­solved with ground-based tel­e­scopes, as­ter­oid oc­culta­t­ions are the only di­rect way of meas­ur­ing the size of such an ob­ject. When sev­er­al ob­servers rec­ord such an event, us­ing vi­deocam­er­as with pre­cise tim­ing, the times when they see the oc­culta­t­ion help to meas­ure the shape of the as­ter­oid.

Since we know the speed of the as­ter­oid, the dura­t­ion of the oc­culta­t­ion can be con­vert­ed di­rectly to a length. This al­lows sci­en­tists to re­con­struct the size and shape of the ob­ject.

As­ter­oids com­ing close to Earth are the fo­cus of a new Eu­ropean Space Agen­cy mon­i­tor­ing pro­gram, known as the Near Earth Ob­ject seg­ment of the Space Situa­t­ional Aware­ness Pre­par­a­to­r Pro­gramme.


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Homepage image: ESA's Optical Ground Station in Tenerife, Spain is used for asteroid observations. (Courtesy ESA)





 

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In a rare event next Thursday, some skywatchers in Europe will be able to see an asteroid briefly block out the light from a star as it passes in front. Astronomers say the event may be the only eclipse of a star by an asteroid this century observable with the naked eye. Everyone is familiar with a solar eclipse, when our Moon passes in front of the Sun and blocks its light for several minutes. A similar situation can happen with asteroids, the Sun-orbiting, rocky or metallic objects that are left over from the formation of the Solar System or were formed by collisions between other asteroids. We know of about 400,000 of these dark bodies, which range in size from a few hundred kilometres (miles) to just a few metres (yards). The smaller ones are hard to detect. While an asteroid is far too small to cover the Sun, one will occasionally move directly in front of one of the stars in the night sky and block its light from us, causing a stellar eclipse or occultation. Since asteroids move fast, these events typically last just a few seconds. Normally the occulted star is so faint the event can only be seen via telescope. But the night of 8-9 July, a star visible to the naked eye, Delta Ophiuchi (the fourth-brightest star in the constellation Ophiuchi), will be occulted by asteroid Roma, about 50 km (30 miles) wide. This means the eclipse will be visible only along a path of about that same width, crossing central Europe, Spain and the Canary Islands. At about 11:57 p.m. Central European Summer Time, observers on a line running between Stockholm—Copenhagen—Bremen—Nantes—Bilbao will see the star disappear for about five seconds as its light is blocked by the asteroid. Since most asteroids are too small to be resolved with ground-based telescopes, asteroid occultations are the only direct way of measuring the size of such an object. When several observers record such an event, using video cameras with precise timing, the times when they see the occultation help to measure the shape of the asteroid. Since we know the speed of the asteroid, the duration of the occultation can be converted directly to a length. This allows scientists to reconstruct the size and shape of the object. Asteroids coming close to Earth are the focus of a new European Space Agency monitoring program, known as the Near Earth Object segment of the Space Situational Awareness Preparatory Programme.