Asteroid Griseldis Grows A Tail!

Image of main-belt asteroid (493) Griseldis with temporary tail taken with the Subaru Telescope on Maunakea. Credit: D. Tholen, S. Sheppard, C. Trujillo
Space rock with a tail. Image of the asteroid 493 Griseldis with a temporary tail taken with the 315-inch (8-meter) Subaru Telescope on Mauna Kea in Hawaii. News of the discovery was released during the annual meeting of the Division for Planetary Sciences near Washington D.C. on Nov. 12. Credit: D. Tholen, S. Sheppard, C. Trujillo

Many if not all the meteorites found on Earth came from collisions between asteroids. Asteroids with diameters of around 12 miles (20 km) suffer a strike from another body once every 10 million years on average. If the crackup happens at high speed, the asteroid shatters to pieces with rocks and dust blasted into space to form a new asteroid family.

A fusion-crusted Saharan meteorite (left) caused by surface melting during atmospheric entry and a slice of the Park Forest meteorite that fell in 2003. Its shattered interior may have resulted from the asteroid impact that sent it flying toward Earth. Credit: Bob King
A fusion-crusted Saharan meteorite (left) caused by surface melting during atmospheric entry. At right, a slice of the Park Forest meteorite that fell in the Chicago area in 2003. Its shattered interior may have resulted from the asteroid impact that sent it flying toward Earth. Credit: Bob King

Sometimes material from the collision finds its way to our planet and collides again — this time with the atmosphere and the surface, landing as heat-blackened meteorites. If the asteroid collision is a gentle one, the two bodies can join together as a single asteroid with two prominent lobes.

Griseldis orbits the sun every 5.5 years inside the main asteroid belt between Mars and Jupiter. It looks crowed in this diagram, but there's a lot of space between each object in real life. Three additional families of asteroids are also shown. Credit: Wikipedia
Griseldis orbits the sun every 5.5 years inside the main asteroid belt between Mars and Jupiter. It looks crowed in this diagram, but there’s a lot of space between each object in real life. Three additional families of asteroids are also shown. Credit: Wikipedia

The rarity of asteroid collisions on a human timescale makes the discovery of a possible impact on 493 Griseldis a stroke of good fortune. Griseldis, a city-sized object 29 miles wide (46 km), orbits the sun in the main belt of asteroids between Mars and Jupiter. Observations taken with the 8-meter Subaru Telescope from the summit Mauna Kea on Hawaii’s Big Island on March 17th this year revealed a wisp of a tail extending from Griseldis where no wisp had ever been seen before.

 

The top three panels are three different exposures with Subaru with asteroid (493) Griseldis moving from left to right as you move from the first panel to the third one. The bottom panel shows all three exposures added together, after suppressing the galaxy that interferes with the "tail" in the first exposure; the asteroid is on the right. Credit: D. Tholen, S. Sheppard, C. Trujillo
The top three panels are three different exposures made with the Subaru telescope with asteroid 493 Griseldis moving from left to right. The bottom panel shows all three exposures added together after suppressing the galaxy that interferes with the “tail” in the first exposure; Griseldis is on the right. Credit: D. Tholen, S. Sheppard, C. Trujillo

Unlike comet tails, which are pushed in the direction opposite the sun by the solar wind and the slight pressure of sunlight itself, this tail pointed in a different direction and only lasted a short time. Additional observations taken with the 256-inch (6.5-meter) Magellan telescope in Chile four nights later still detected the extension, though it had begun to fade. Astronomers took additional photos later that month and again in April and May but failed to see the tail. A check back through the archives in 2010 and 2012 also turned up negative.

A photo taken with the Magellan Telescope in Chile four days later shows a reduced "tail." Credit: S. Sheppard.
A photo taken with the Magellan Telescope in Chile four days later shows a reduced “tail.” Credit: S. Sheppard.

It’s unknown if any rocks – potential future meteorites – might have been knocked loose during the collision, but the dust wafting into space from the event will become part of the zodiacal light, that shoe-shaped cloud of glowing dust seen at nightfall in the west in spring and before dawn in the east in fall. Comets and asteroids both contribute a share of their dust to the phenomenon, made visible by reflected sunlight.

The zodiacal light with Venus photographed back on Sept. 15th at the start of dawn. The cone-shaped light is a combination of dust released by comets and asteroid collsions that glows in reflected sunlight. Credit: Bob King
The zodiacal light with Venus photographed back on Sept. 15th at the start of dawn. The cone-shaped light is a combination of dust released by comets and asteroid collsions that glows in reflected sunlight. The more vertical band is the Milky way with Orion at right. Credit: Bob King