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Hayabusa2's Summer Holidays at Ryugu: a "Roller Coaster" Ride to Measure the Gravity

On 27th of July 2018, Hayabusa2 spacecraft successfully arrived at Ryugu after completing the approach phase which consisted of 10, perfectly executed, Trajectory Control Manoeuvres (TCMs). The spacecraft is nominally parked (hovering) at 20 km distance from the center of Ryugu at what Hayabusa2 Engineers call Box-A Operation.

Hayabusa2 Project Team: Arrival

We soon got the first images of Ryugu at 20 km distance and already we realized how uniqueness are the features of this small world. "Ryugu! can you tell us your story?" How did you get so many "rocks"? Why do you have a crater at your equatorial ridge? How come such a huge boulder at the top? What are you made of?

The image was taken with ONC-T camera on 26th of Junw, 2018. Image credit ※: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.

Ryugu is a relatively small Top-Shape asteroid of 0.9 km in diameter, the spin-axis is almost perpendicular to the ecliptic which we did not expect from Earth observations. A Ryugu day last just 7.6 hours and the rotation is retrograde so this small world is upside down if compared with the Earth. The north pole is south and the south pole is north, so confusing isn't it?

Every Hayabusa2 operation starts from a 20 km distance, Home-Position (HP), and return to 20 km at the end of the operation. The reference frame used by engineers is called HP Frame which is essential for the navigation. The HP frame is defined in a way that the spacecraft is always located between the Earth-Ryugu line to ease communications (z-axis). The Sun, which is a vital source of power for the spacecraft, is always located in the x-z positive axis plane. I created a simple video that visualize in blue the HP frame.

The first close images of Ryugu were taken on the 20 of July 2018 at a distance of 6 km. This was the first controlled descent operation and the spacecraft performed the so called Box-C operation. The images are outstanding! Such an interesting environment! Seems hard to land with all that boulders!

Asteroid Ryugu from an altitude of 6 km. Image credit ※: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST.

All the images received so far were taken with the ONC-T (Telescopic) camera that has a field of view of 6 degrees. Hayabusa2 spacecraft is also equipped with another navigation camera called ONC-W1 that has field of view of 60 degrees. Scientists are working on the shape of Ryugu which is also one of the input for my mass concentrations (Mascons) model analysis. A good gravity model helps to understand and predict the dynamics around Ryugu.

Shape model of Ryugu by the University of Aizu. Image credit ※1: University of Aizu, Kobe University (shape model creation), Auburn University (video creation), JAXA.

On the 5th of August 2018, the gravity measurement operation started and it will continue until the 7th. You can watch almost real time the images taken from the spacecraft with the camera, ONC-W1 from the Hayabusa2's Project Website at this link:


During the controlled descent, small manoeuvres are applied in the x-, y- and z-coordinates to decent until a 5 km altitude from Ryugu surface. Around 5 km altitude, a break velocity manoeuvre will be performed at 20:16 JST where the spacecraft will start its free-fall until it reaches 1 km altitude. A sort of "roller coaster" experience for Hayabusa2 spacecraft! A delta-V will be than given at 1 km to return to the 5 km altitude where a second delta-V is scheduled to return from 5 km to Home Position at 20 km. This operation allows scientists to measure the gravity constant (GM) of Ryugu through a reconstruction of the Hayabusa2 spacecraft's trajectory (orbit determination). During the free-fall, attitude manoeuvres and manoeuvres in the x- and y-coordinates will be performed to keep the spacecraft along the z-axis direction. This is an exciting operation as the previous Hayabusa mission to Itokawa couldn't perform such a long free-fall due to a problem with the reaction wheels.

Fasten your seat belt and enjoy the Hayabusa2 "roller coaster" ride!

Thank you for watching!

7th August 2018. Gravity descent operation perfectly executed! Now the spacecraft is traveling back to Home Position. Latest close images at 1 km are just amazing! Looking forward for the official press release.


  • Beginning of September: First Touch Down Operation

  • Second Half of September: Minerva-II Rover Deployment

  • Beginning of October: DLR's Mascot Rover Deployment

For further information regarding Hayabusa2 current schedule, please visit the following page

Hayabusa2 Project Website has been recently renewed! Some of the most interesting articles are:

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