ESO: The anatomy of an asteroid

This is a schematic view of the strange peanut-shaped asteroid Itokawa. 

By making exquisitely precise timing measurements using ESO's New Technology Telescope (NTT), and combining them with a model of the asteroid's surface topography, a team of astronomers has found that different parts of this asteroid have different densities. 

As well as revealing secrets about the asteroid's formation, finding out what lies below the surface of asteroids may also shed light on what happens when bodies collide in the Solar System, and provide clues about how planets form. 

The shape model used for this view is based on the images collected by JAXA's Hayabusa spacecraft. 

Credit: ESO. Acknowledgement: JAXA

ESO's New Technology Telescope (NTT) has been used to find the first evidence that asteroids can have a highly varied internal structure.

By making measurements astronomers have found that different parts of the asteroid Itokawa have different densities.

As well as revealing secrets about the asteroid's formation, finding out what lies below the surface may also shed light on what happens when bodies collide in the Solar System, and provide clues about how planets form.

Stephen Lowry

Using very precise ground-based observations, Stephen Lowry (University of Kent, UK) and colleagues have measured the speed at which the near-Earth asteroid (25143) Itokawa spins and how that spin rate is changing over time.

They have combined these delicate observations with new theoretical work on how asteroids radiate heat.

This small asteroid is an intriguing subject as it has a strange peanut shape, as revealed by the Japanese spacecraft Hayabusa in 2005.

To probe its internal structure, Lowry's team used images gathered from 2001 to 2013, by ESO's New Technology Telescope (NTT) at the La Silla Observatory in Chile among others, to measure its brightness variation as it rotates.

This timing data was then used to deduce the asteroid's spin period very accurately and determine how it is changing over time.

When combined with knowledge of the asteroid's shape this allowed them to explore its interior—revealing the complexity within its core for the first time.

"This is the first time we have ever been able to to determine what it is like inside an asteroid," explains Lowry.

"We can see that Itokawa has a highly varied structure—this finding is a significant step forward in our understanding of rocky bodies in the Solar System."

The spin of an asteroid and other small bodies in space can be affected by sunlight. This phenomenon, known as the Yarkovsky-O'Keefe-Radzievskii-Paddack (YORP) effect, occurs when absorbed light from the Sun is re-emitted from the surface of the object in the form of heat.

When the shape of the asteroid is very irregular the heat is not radiated evenly and this creates a tiny, but continuous, torque on the body and changes its spin rate.

Lowry's team measured that the YORP effect was slowly accelerating the rate at which Itokawa spins. The change in rotation period is tiny—a mere 0.045 seconds per year.

But this was very different from what was expected and can only be explained if the two parts of the asteroid's peanut shape have different densities.

This is the first time that astronomers have found evidence for the highly varied internal structure of asteroids.

Up until now, the properties of asteroid interiors could only be inferred using rough overall density measurements.

This rare glimpse into the diverse innards of Itokawa has led to much speculation regarding its formation.

One possibility is that it formed from the two components of a double asteroid after they bumped together and merged.

Lowry added, "Finding that asteroids don't have homogeneous interiors has far-reaching implications, particularly for models of binary asteroid formation."

"It could also help with work on reducing the danger of asteroid collisions with Earth, or with plans for future trips to these rocky bodies."

This new ability to probe the interior of an asteroid is a significant step forward, and may help to unlock many secrets of these mysterious objects.

More information: This research was presented in a paper "The Internal Structure of Asteroid (25143) Itokawa as Revealed by Detection of YORP Spin-up", by Lowry et al., to appear in the journal Astronomy & Astrophysics.

Landed - Japan Itokawa asteroid mission

Japan's Hayabusa probe returns to Earth on Sunday, releasing a capsule into the atmosphere that scientists hope will contain samples from asteroid Itokawa.

If the 40cm-wide conical disc survives the blistering heat of the descent and can deploy a parachute, it should touch down in Australia at about 1400 GMT.

No-one really knows whether Hayabusa succeeded in grabbing samples from Itokawa's surface during a 2005 visit.

Its capture mechanism did not work as designed, analysis has shown.

However, Japanese space agency (Jaxa) officials remain confident.

They believe a lot of dust would have been kicked up when Hayabusa landed on the space rock, and some of this must have found its way inside the probe.

"At the moment of touch-down, Hayabusa collided with the surface and stayed on the surface for 30 minutes, so we believe some material will have found its way into the sample container," senior project team member Professor Hitoshi Kuninaka told BBC News.

If the Hayabusa capsule is confirmed to contain even just dusty fragments from the asteroid, it would represent a scientific first and a remarkable triumph for the Japanese who have had to battle a succession of technical problems.

Although the grab mechanism failed, dust may still have found its way inside

Hayabusa was launched in 2003 and used ion engines to propel itself out to Itokawa, a 500m-long potato-shaped rock which drifts between the orbit of Earth and beyond Mars.

Not long after the mission started, a solar flare damaged the spacecraft, but it was really at its destination that Hayabusa's woes began to mount.

As well as the capture mechanism malfunction, a mini-probe called Minerva that was to have hopped across the asteroid's surface accidentally drifted off into space.

Even on the journey home, the Hayabusa team had to work around communication drop-outs and propulsion glitches.

But each time an issue came up, the scientists and engineers working on the project managed to find an elegant solution.

Now, after one final course correction during the past week, Hayabusa is bound to hit the Earth's atmosphere above Australia.

HAYABUSA'S HIGH-SPEED RETURN

• (1) Three hours before re-entry, sample capsule is released
• (2) At altitude of 200km, probe and capsule encounter atmosphere
• (3) Capsule travels at 12km/s; heat-shield must work at 3,000C
• (4) Main spacecraft has no protection and burns up
• (5) At 10km, capsule dumps shielding; deploys parachute
• (6) Capsule is tracked to landing via beacon and radar

Just a few hours before the spacecraft begins this plunge, it will push out the sample capsule in front of it.

The main spacecraft will be destroyed during the descent, but the container is built to survive the 3,000-degree heat of re-entry.

"The capsule will come into the atmosphere with a very high velocity - 12km/s; and so in order to endure such a high heat input, we need a special heat-shield," said Professor Kuninaka. "Its surface is made of carbon phenolic (resin)."

Assuming it makes it through this test of fire, the capsule will deploy a parachute at an altitude of about 10km. This should bring the conical pod to a gentle stop in the Australian Outback's Woomera Prohibited Range.

Radar tracking and a beacon in the container itself should guide scientists to the drop location.

The requirement to avoid all earthly contamination means the capsule will not immediately be moved from its landing site. The sterile protocols the mission team will follow will not permit the capsule's evacuation and transfer to Japan for a few days.

The container will be delivered to Jaxa's Sagamihara curation facility for analysis. It could be some months before scientists are able to say with confidence that Hayabusa did indeed capture fragments of Itokawa.

"You hope for grams of sample but you can make do with much less than that," observed Dr Michael Zolensky who worked on Nasa's Stardust comet sample-return mission.

"On Stardust, the entire sample return was on the order of thousands of nano-grams. That was thousands of grains, each of which weighed about one nano-gram; and one of those grains you could spend a year studying," he told BBC News.

Such grains would provide new insight into the early history of the Solar System and the formation of the planets more than 4.5 billion years ago.

Professor Trevor Ireland, from the Australian National University, who will get to work on the samples, said no rocks on Earth could provide this information because they had been recycled many times.

"If we look at anything on Earth it has been put through the wringer; it's been messed up by plate-tectonic processes and geochemical processes. So if we want to look at what our Earth was made of, we have to leave Earth. That's the importance of Hayabusa and going to Itokawa."

ASTEROID 25143 ITOKAWA - A 'PILE OF RUBBLE'

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.