ESA Rosetta: Asteroid Lutetia: postcard from the past

Landslides on Lutetia are thought to have been caused by the vibrations created by impacts elsewhere on the asteroid dislodging pulverised rocks.

Credits: ESA 2011 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

ESA's Rosetta spacecraft has revealed asteroid Lutetia to be a primitive body, left over as the planets were forming in our Solar System. Results from Rosetta's fleeting flyby also suggest that this mini-world tried to grow a metal heart.

Rosetta flew past Lutetia on 10 July 2010 at a speed of 54 000 km/hr and a closest distance of 3170 km.

At the time, the 130 km-long asteroid was the largest encountered by a spacecraft. Since then, scientists have been analysing the data taken during the brief encounter.

All previous flybys went past objects, which were fragments of once-larger bodies. However, during the encounter, scientists speculated that Lutetia might be an older, primitive 'mini-world'.

Several images have been combined into a map of the asteroid.

This image represents the total area viewed by the spacecraft during the flyby, which amounted to more than 50% of Lutetia’s surface.

Credits: ESA 2011 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

Now they are much more certain. Images from the OSIRIS camera reveal that parts of Lutetia's surface are around 3.6 billion years old. Other parts are young by astronomical standards, at 50–80 million years old.

Astronomers estimate the age of airless planets, moons, and asteroids by counting craters. Each bowl-shaped depression on the surface is made by an impact.

The older the surface, the more impacts it will have accumulated. Some parts of Lutetia are heavily cratered, implying that it is very old.

On the other hand, the youngest areas of Lutetia are landslides, probably triggered by the vibrations from particularly jarring nearby impacts.

Debris resulting from these many impacts now lies across the surface as a 1 km-thick layer of pulverised rock.

This map of Lutetia is centred on the north pole. 

The number of craters in the asteroid's various regions have been used to date the surface. 

Some parts of the surface are 3.6 billion years old, while others are just 50–80 million years old.

Credits: ESA 2011 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA

There are also boulders strewn across the surface: some are 300–400 m across, or about half the size of Ayers Rock, in Australia. 

Some impacts must have been so large that they broke off whole chunks of Lutetia, gradually sculpting it into the battered wreck we see today. 

"We don't think Lutetia was born looking like this," says Holger Sierks, Max-Planck-Institut für Sonnensystemforschung, Lindau, Germany. "It was probably round when it formed."

Rosetta's VIRTIS spectrometer found that Lutetia's composition is remarkably uniform across all the observed regions.

"It is striking that an object of this size can bear scars of events so different in age across its surface while not showing any sign of surface compositional variation," says Fabrizio Capaccioni, INAF, Rome, Italy.

 

This is just the start of the mystery.

Read further 

ESA Rosetta: Most pristine known asteroid is denser than granite

The Rosetta spacecraft swung by asteroid 21 Lutetia on 10 July 2010 (Image: ESA 2010 MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA)

Asteroids are generally regarded as the solar system's scrap heap, the battered bits that broke off and were left behind when the planets were forming but the lumpy asteroid 21 Lutetia may be a whole, unbroken building block left nearly untouched since the solar system's birth.

"We think planets were built of things like Lutetia," says Ben Weiss of the Massachusetts Institute of Technology in Cambridge. "We're getting a chance to see one of the building blocks of the solar system up close."

The European spacecraft Rosetta zipped past Lutetia at 50,000 kilometres per hour in July 2010, snapping photos of a cratered world about 121 kilometres long.

That makes it the second-largest asteroid ever visited by a spacecraft, next to 560-kilometre-wide Vesta.

Violent mêlée

Most of the asteroids to get visits from spacecraft are rubble piles, chunks of debris that were loosely held together by gravity. But Lutetia is so dense that it appears to have survived the violent mêlée of collisions in the early solar system intact.

"The real new thing is that it's not a rubble pile, it's a solid block of rock," said Holger Sierks of the Max-Planck Institute for Solar System Research in Germany, lead author of a new paper reporting the observations. "It's really a remnant from the early days."

Even more surprisingly, the spacecraft's measurements suggested the rock has one of the highest asteroid densities ever measured, at 3.4 tonnes per cubic metre.

That is denser than granite and suggests Lutetia might have heavy metals in its core.

JAXA Hayabusa re-entry image: long exposure

In this long exposure photo released by Japan's Wakayama University Institute for Education on Space, two streaks of the Hayabusa probe, the first spacecraft to complete a round-trip journey to an asteroid, and its capsule streak across the sky near Glendambo in southern Australia as they re-enter the Earth's atmosphere

Picture: AP

NASA Viseo of Hayabusa re-entry at 10,000 KM/hour

JAXA Akatsuki: Pale Blue Island Earth

Two Japanese spacecraft, one headed to Venus and another limping home from an asteroid, have beamed home snapshots of Earth that reveal our planet in different hues amid a sea of stars.

The latest photos of Earth come from Japan's brand new Venus Climate Orbiter Akatsuki and the Hayabusa asteroid probe.

Akatsuki launched May 20 alongside a novel solar sail vehicle and other smaller payloads to begin a six-month trek toward the second planet from the sun.

Hayabusa is returning to Earth from the asteroid Itokawa, which it visited in late 2005 and is due to land in Australia in June.

The Technicolor Earth
The photos of Earth from space by Akatsuki reveal a stunning crescent as the planet appeared to the probe's ultraviolet and infrared cameras.

In ultraviolet, the Earth appears as a dazzling blue sliver, while the same crescent has a vibrant orange hue in infrared. Akatsuki (which means "Dawn" in Japanese) was flying about 155,342 miles (250,000 km) from Earth when it photographed the planet.

Akatsuki also used its long-wave infrared camera to take a snapshot of the entire Earth, though the planet may be unrecognizable to the uninitiated. Earth's trademark blue oceans and white clouds are rendered only in black and white.

Japan's Akatsuki mission is expected to observe Venus in unprecedented detail to study its ever-present clouds and hidden surface. The spacecraft is expected to reach Venus in December and spent two years studying the planet.

The IKAROS solar sail vehicle also launched with the Akatsuki probe and will make a pit stop at Venus before heading off to the far side of the sun. Both spacecraft are doing well, JAXA officials said.