September 22, 2019

Hayabusa 2 cleared for second touchdown on asteroid


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Artist’s concept of the Hayabusa 2 spacecraft touching down on asteroid Ryugu. Credit: JAXA/Akihiro Ikeshita

Ground teams have approved plans for Japan’s Hayabusa 2 sample return mission to briefly land on asteroid Ryugu for the second time July 11, aiming for a targeted touch-and-go to gather material exposed by an explosive impactor released by the robot explorer in April.

Officials from the Japan Aerospace Exploration Agency, or JAXA, announced the decision June 27 after weeks of surveys, practice approaches and deliberations to ensure the Hayabusa 2 spacecraft can safely touch down at the rugged site, which is strewn with boulders and rocks that could pose hazards to the probe.

The objective of the July 11 touch-and-go landing is to collect a second set of samples from the carbon-rich asteroid for return to Earth.

Hayabusa 2 is in the final stretch of a nearly 18-month exploration campaign at asteroid Ryugu before firing its ion thrusters late this year for the return trip to Earth. So far, the mission has accomplished a pinpoint landing and takeoff from the asteroid, deployed three daughter probes to hop around Ryugu’s surface, and carved a new crater on the asteroid after dropping an explosive charge.

One more touch-and-go landing and the release of the mission’s final mobile surface scout are planned in the coming weeks.

Hayabusa 2 gathered a first batch of samples from Ryugu’s surface in February after executing a pinpoint touchdown on a different part of the half-mile-wide (900-meter) asteroid.

The robot explorer’s sampling mechanism works by firing a metal bullet into the asteroid once the probe’s sampler horn, which extends from one side of the spacecraft, contacts the surface. The projectile is designed to blast away rock and dust on the asteroid’s surface, then direct the material through the sampler horn into a collection chamber inside the Hayabusa 2 spacecraft.

While there is no direct way to measure how much sample Hayabusa 2 collected in February, scientists said telemetry data broadcast back to Earth suggested the sampling system worked as designed.

In early April, Hayabusa 2 released the Small Carry-On Impactor, an explosive device that drove a copper mass into the asteroid to create a new crater, uncovering rocks that were buried underneath Ryugu’s surface, perhaps for billions of years.

Scientists will target the July 11 landing a short distance from the fresh crater, where they believe material ejected by the impact fell. The new samples may include pristine subsurface specimens that have escaped radiation and other asteroid weathering affects from sunlight and extreme temperature swings.

Hayabusa 2’s optical navigation camera captured this view of asteroid Ryugu from a distance of 6 kilometers (4 miles) on July 20, 2018. Credit: JAXA

A second successful sampling attempt would make Hayabusa 2 the first mission to collect a subsurface specimen from an asteroid for return Earth.

Assuming the July 11 touch-and-go snags a sample that was exposed by the mission’s explosive impactor, scientists expect the material will contain information from the early formation of the solar system 4.6 billion years ago, such as organic molecules that became the building blocks for life.

“Subsurface materials are particularly valuable for sensitive organics,” scientists wrote in a mission update last month.

Engineers designed Hayabusa 2 to collect samples from up to three locations on the asteroid, but mission managers have ruled out gathering a third sample. The mission only needed one sample to meet minimum success criteria.

During preparations for the second sampling attempt, Hayabusa 2 dropped a target marker onto the asteroid May 30 to help the spacecraft guide itself toward the landing zone. The probe also took high-resolution images of the area to help scientists decide if they should press ahead with another touchdown.

Officials mulled the scientific merit and safety risks of a second landing attempt, and the ground team ultimately elected to go ahead with the touchdown July 11.

Hayabusa 2 had until this month to try for a second touchdown. Ryugu is nearing the point in its orbit closest to the Sun, and rising temperatures on the asteroid will prohibit the spacecraft from landing later this year, officials said.

While Hayabusa 2 explores Ryugu, NASA’s OSIRIS-REx mission is surveying asteroid Bennu before moving in to collect a sample there in 2020 for return to scientists on Earth in 2023.

OSIRIS-REx is designed to bring home at least 60 grams, or 2.1 ounces of samples from Bennu, significantly more than Hayabusa 2. But OSIRIS-REx is only expected to collect a single sample from one location on Bennu’s surface.

Once the second sample collection is complete, Hayabusa 2 is expected to deploy the last of its four daughter probes to hop around the asteroid’s surface.

Hayabusa 2’s return journey to Earth is scheduled begin in November or December, with re-entry of the mission’s sample-carrying descent capsule set for late 2020 over Australia, where recovery teams will pick up the specimens for analysis in laboratories in Japan and the United States.

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Follow Stephen Clark on Twitter: @StephenClark1.


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