22. August 2019

The near-Earth as­ter­oid Ryugu – a frag­ile cos­mic 'rub­ble pile'

Ryugu at night – a ‘cauliflower rock’ with bright minerals
Ryugu at night – a ‘cauliflow­er rock’ with bright min­er­als
Image 1/6, Credit: MASCOT/DLR/JAXA

Ryugu at night – a ‘cauliflower rock’ with bright minerals

The MAS­Cam team iden­ti­fied two types of rock in the im­ages ac­quired dur­ing MAS­COT’s MAS­COT and land­ing. Some were slight­ly brighter (but al­so on­ly re­flect­ed five per­cent of the in­ci­dent sun­light); these were ‘Type 2’ boul­ders with sharp edges and smooth, frac­tured sur­faces. Oth­ers were dark­er, re­flect­ing on­ly four per­cent of the in­ci­dent so­lar ir­ra­di­a­tion; these were more ir­reg­u­lar­ly shaped ‘Type 1’ boul­ders with cauliflow­er-like, crum­pled sur­faces. The lat­ter were im­aged dur­ing the first night at Ryugu, di­rect­ly in front of the MAS­Cam lens, and they were il­lu­mi­nat­ed with the built-in light-emit­ting diodes. A com­bi­na­tion of blue, green and red light was used to cre­ate this colour im­age. The re­searchers were sur­prised by bright­ly re­flect­ing min­er­als rem­i­nis­cent of cal­ci­um- and alu­mini­um-rich splin­ters of a rare class of me­te­orites, the ‘CI-Chon­drites’, which fell to Earth at Tag­ish Lake in Cana­da in 2000. They are among the old­est and most orig­i­nal wit­ness­es of the for­ma­tion of the So­lar Sys­tem found in me­te­orite col­lec­tions on Earth. On the up­per right, the en­larged sec­tion of the rock spur on the low­er right in the large im­age, which was about 25 cen­time­tres in front of MAS­Cam.
MAS­COT free falling to­wards as­ter­oid Ryugu
Video 2/6, Credit: MASCOT/DLR/JAXA

MASCOT free falling towards asteroid Ryugu

Length: 56.4 MB
On 3 Oc­to­ber 2018, the Mo­bile As­ter­oid Sur­face Scout (MAS­COT) sep­a­rat­ed from the Hayabusa2 moth­er craft at an al­ti­tude of 41 me­tres above the as­ter­oid Ryugu. Dur­ing the six-minute bal­lis­tic de­scent, which was not con­trolled by any propul­sion sys­tem, the DLR cam­era MAS­Cam ac­quired 20 im­ages. As ex­pect­ed, MAS­COT’s flight was tur­bu­lent. For this rea­son the cam­era’s ori­en­ta­tion was un­con­trolled and the im­ages were ac­quired in nu­mer­ous di­rec­tions. Thus, im­ages in the di­rec­tion of the Sun (im­ages num­bers 9, 11, 13, 14 and 15) are al­so in­clud­ed in this se­quence, which are strong­ly over-il­lu­mi­nat­ed and do not show any de­tails of the as­ter­oid’s sur­face.
MASCOT's descent and path across the surface of Ryugu
MAS­COT's de­scent and path across the sur­face of Ryugu
Image 3/6, Credit: AXA/U Tokyo/Kochi U/Rikkyo U/Nagoya U/ Chiba Inst Tech/Meiji U/U Aizu/AIST; DLR (digital model of descent and path)

MASCOT's descent and path across the surface of Ryugu

Af­ter MAS­COT was eject­ed from Hayabusa2’s load­ing bay, the lan­der’s Op­ti­cal Nav­i­ga­tion Cam­era (ONC) sys­tem tracked the lan­der’s de­scent from an al­ti­tude of 41 me­tres. These im­ages made it pos­si­ble to project the tra­jec­to­ry of the de­scent path from the sep­a­ra­tion (MR = MAS­COT Re­lease point) to the first con­tact (CP1 = Con­tact Point 1) on­to the sur­face as well as to re­con­struct the fur­ther course of the MAS­COT mis­sion on Ryugu. This im­age shows a dig­i­tal ter­rain mod­el of the land­ing site, in which the height dif­fer­ences from mi­nus 3.6 me­tres to plus 3.6 me­tres – re­lat­ed to a ref­er­ence lev­el – are colour-cod­ed. The im­age sec­tion is ori­ent­ed to the north; the area is ap­prox­i­mate­ly 300 de­grees east and 30 de­grees south on Ryugu.
Meteorite from Tagish Lake
Me­te­orite from Tag­ish Lake
Image 4/6, Credit: Michael Holly, Creative Services, University of Alberta

Meteorite from Tagish Lake

When eval­u­at­ing the MAS­Cam im­ages, the sci­en­tists iden­ti­fied two dif­fer­ent types of rock –one with sharp edges and smooth frac­tured sur­faces, and a sec­ond with sur­faces rem­i­nis­cent of a cauliflow­er. The im­ages of these some­what crum­pled sur­faces, ir­ra­di­at­ed with light-emit­ting diodes ac­quired dur­ing the night, the re­searchers dis­cov­ered bright min­er­al in­clu­sions in the al­most black rock ma­trix, rem­i­nis­cent of min­er­al in­clu­sions in me­te­orites from Lake Tag­ish (pic­ture). On 18 Jan­uary 2000, af­ter the ex­plo­sion of a large fire­ball over Cana­da, hun­dreds of small me­te­orites fell on­to Earth and nu­mer­ous frag­ments were found on the ice of the frozen lake that gave it its name. The ‘Tag­ish Lake Me­te­orites’ are very rare stone me­te­orites from the class of what are re­ferred to as CI-Chon­drites. The C stands for the chem­i­cal el­e­ment car­bon, and the ‘I’ for the sim­i­lar­i­ty with the Ivu­na me­te­orite from Tan­za­nia. They are among the most prim­i­tive and old­est com­po­nents of the So­lar Sys­tem, rem­nants of the first sol­id bod­ies formed in the pri­mor­dial so­lar neb­u­la. They are be­lieved to have evolved in­to the bod­ies of the So­lar Sys­tem.
MASCOT’s 17 hours on the surface of Ryugu
MAS­COT’s 17 hours on the sur­face of Ryugu
Image 5/6, Credit: DLR (CC BY-NC-ND 3.0)

MASCOT’s 17 hours on the surface of Ryugu

MAS­COT was de­ployed from Hayabusa2 on 3 Oc­to­ber 2018 at 01:57:20 UTC at an al­ti­tude of 41 me­tres above the as­ter­oid Ryugu. Dur­ing the bal­lis­tic de­scent at a drop speed of 0.4 kilo­me­tres per hour, the mod­ule turned sev­er­al times as ex­pect­ed, and touched down on the shad­ed side of a boul­der six min­utes lat­er. MAS­COT fell back­wards and ‘rolled’ an­oth­er 17 me­tres be­fore stop­ping in a de­pres­sion (SP1 = Set­tle­ment Point 1; Ryugu’s grav­i­ta­tion­al pull is on­ly one six­ty thou­sandth that of grav­i­ty on Earth – at 10 kilo­grams, MAS­COT ex­pe­ri­enced a grav­i­ta­tion­al force of 0.17 grams on Ryugu). But a ma­noeu­vre to cor­rect the at­ti­tude at the first mea­sure­ment point (MP1) caused MAS­COT to lie on its ‘back’ and some in­stru­ments to look to­wards space. MAS­Cam cap­tured Jupiter and Sat­urn on the first as­ter­oid night in five im­ages. On the morn­ing of the sec­ond Ryugu day, MAS­COT’s swing arm was ac­ti­vat­ed in or­der to reach the sec­ond mea­sur­ing point (MP2). On the sec­ond as­ter­oid night MAS­Cam ac­quired noc­tur­nal im­ages with il­lu­mi­na­tion of the rocks by four light-emit­ting diodes. On the morn­ing of the third day, MAS­COT per­formed a ‘mi­ni move’ of five cen­time­tres to the side to take (MP3) stereo im­ages. Fi­nal­ly, MAS­COT reached its last mea­sure­ment point at MP4 at 19:04 UTC. The lan­der was op­er­a­tional for 17 hours. Then, con­tact with Hayabusa2 was dis­con­tin­ued.
MASCam in the Mobile Asteroid Surface Scout (MASCOT)
MAS­Cam in the Mo­bile As­ter­oid Sur­face Scout (MAS­COT)
Image 6/6, Credit: DLR (CC BY-NC-ND 3.0)

MASCam in the Mobile Asteroid Surface Scout (MASCOT)

The MAS­Cam (MAS­COT Cam­era) imag­ing sys­tem de­vel­oped by the Ger­man Aerospace Cen­ter (DLR) was de­signed to take pic­tures while de­scend­ing on­to the as­ter­oid Ryugu af­ter sep­a­rat­ing from Hayabusa2.
  • The asteroid is similar to carbonaceous, 4.5 billion year old meteorites found in collections on Earth.
  • Ryugu has numerous cavities.
  • Focal points: Space, exploration

In the summer of 2018, the asteroid Ryugu, which measures only approximately 850 metres across, was visited by the Japanese Hayabusa2 spacecraft. On board was the 10-kilogram German-French Mobile Asteroid Surface Scout (MASCOT) – a lander no bigger than a microwave oven and equipped with four instruments. On 3 October 2018 MASCOT, operated by the control centre at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) in Cologne, separated from its mother craft 41 metres above the asteroid. It touched down on the surface for the first time six minutes after deployment, before coming to a halt 11 minutes later, like a dice on a board game moving in slow motion. Over the course of 17 hours, MASCOT carried out experiments in various places amid the large boulders. Evaluation of the image data from DLR's MASCOT camera (MASCam) showing the descent and Ryugu’s surface has now revealed a detailed view of a fragile 'rubble pile' made up of two different, almost black, types of rock with little internal cohesion. The scientific team, led by planetary researcher Ralf Jaumann from the DLR Institute of Planetary Research in Berlin-Adlershof, have now reported on this in the current issue of Science.

"If Ryugu or another similar asteroid were ever to come dangerously close to Earth and an attempt had to be made to divert it, this would need to be done with great care. In the event that it was impacted with great force, the entire asteroid, weighing approximately half-a-billion tonnes, would break up into numerous fragments. Then, many individual parts weighing several tonnes would impact Earth," says Jaumann, who is supervising the MASCam experiment, interpreting the observations. The asteroid is very similar to carbonaceous meteorites found on Earth, which date back 4.5 billion years. With an average density of just 1.2 grams per cubic centimetre, Ryugu is only a little 'heavier' than water ice. But as the asteroid is made up of numerous pieces of rock of different sizes, this means that much of its volume must be traversed by cavities, which probably makes this diamond-shaped body extremely fragile. This is also indicated by the measurements conducted by the DLR MASCOT Radiometer (MARA) experiment, which were published recently.

Scientists writing space history

"The evaluation of the MASCOT experiments is yielding highly interesting results. To me, it is fascinating to see what this small, high-tech box has achieved on Ryugu, an asteroid 300 million kilometres from Earth," enthuses Hansjörg Dittus, DLR Executive Board Member for Space Research and Technology. "With MASCOT, we have written a small chapter of space history with our Japanese and French colleagues."

MASCOT 'hopped' across the surface using an internal swing arm. "Upon landing and arrival at the initial location, MASCOT had to perform a manoeuvre to correctly align the scientific instruments with the asteroid surface," explains MASCOT Project Manager Tra-Mi Ho of the DLR Institute of Space Systems in Bremen. "This was followed by three more changes in position, with additional measurements."

Two different types of rock, but no dust

The boulders that can be seen in the images acquired by the camera during MASCOT’s descent and on the surface are mostly dark and measure between 10 centimetres and one metre across. Although most of them are angular, some are smooth. Boulders with level, fractured surfaces and sharp edges are slightly lighter in colour than those with more irregular, cauliflower-like and partially crumbly surfaces. On average, Ryugu reflects only 4.5 percent of the incident sunlight, comparable with charcoal, making it among the darkest objects in the Solar System. MASCam was able to acquire images throughout the day and even at night. The camera system was equipped with light-emitting diodes for this purpose, which illuminated the immediate surroundings in different, clearly defined colour wavelengths in visible light and near-infrared, in order to record the reflective behaviour of their environment in different spectral channels.

The two types of rock observed are distributed approximately equally over Ryugu's surface. This suggests two possible origins: "Firstly," explains Jaumann, “Ryugu could have been formed following the collision of two bodies made of different materials. As a result, it would have broken up, before the fragments came together under the influence of gravity to form a new body made up of the two different types of rock. Alternatively, Ryugu could be the remnant of a single body whose inner zones had different temperature and pressure conditions, thus resulting in the formation of two types of rock.”

Ralf Jaumann and his team were particularly surprised by the lack of dust: "Ryugu's entire surface is littered with boulders, but we have not discovered dust anywhere. It should be present, due to the bombardment of the asteroid by micrometeorites over billions of years, and their weathering effect. However, as the asteroid has very low gravity – only one-sixtieth of that experienced on Earth’s surface – the dust has either disappeared into cavities on the asteroid or has escaped into space. This gives an indication of the complex geophysical processes occurring on the surface of this small asteroid.”

Boulders reminiscent of materials from the primordial solar nebula

Until now, the MASCOT scientists believed that Ryugu was similar to two meteorites that fell to Earth in 1969 in Allende, Mexico, and Murchison, Australia. However, those meteorites barely contain bright particles, probably due to the weathering effect of water in the crystal grid of these minerals. The bright inclusions that have now been observed have led the scientists to conclude that Ryugu's cauliflower-like rocks bear greater similarities to meteorites from Tagish Lake. On 18 January 2000, hundreds of small meteorites rained down on Earth following the explosion of a large fireball over Canada, and numerous fragments were found on the ice of the frozen lake.

These are very rare stony meteorites from what is referred to as the CI chondrite class. The C stands for the chemical element carbon, and the I for the similarity with the Ivuna meteorite found in Tanzania. They are among the oldest and most primitive components of the Solar System, remnants of the first solid bodies to be formed in the primordial solar nebula.

Ryugu is a 'Near-Earth Object' (NEO) – that is, an asteroid or comet that comes close to or intersects Earth’s orbit. In some cases, these might be on a collision course with Earth. Ryugu's orbit around the Sun is almost coplanar to that of Earth and approaches it at an angle of 5.9 degrees to within a distance of approximately 100,000 kilometres. Ryugu will never come within the immediate vicinity of Earth, but knowing the properties of bodies like Ryugu is of great importance when it comes to assessing how such Near Earth Objects (NEOs) could be dealt with in the future.

Preparing for the return to Earth

While the MASCOT sub-mission was being completed, Hayabusa2 carried out numerous manoeuvres, mapped the asteroid at high resolution and collected samples from various parts of the surface with a sampler horn. These were then sealed in a transport container that will embark on its return journey in December 2019. The samples will then descend through the atmosphere and land on Earth in 2020.

About the Hayabusa2 mission and MASCOT

Hayabusa2 is a Japanese space agency (Japan Aerospace Exploration Agency; JAXA) mission to the near-Earth asteroid Ryugu. The German-French lander MASCOT on board Hayabusa2 was developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and built in close cooperation with the French space agency CNES (Centre National d'Études Spatiales). DLR, the Institute d'Astrophysique Spatiale and the Technical University of Braunschweig have contributed the scientific experiments on board MASCOT. The MASCOT lander and its experiments were operated and controlled by DLR with support from CNES and in constant interaction with the Hayabusa2 team at JAXA.

The DLR Institute of Space Systems in Bremen was responsible for developing and testing the lander together with CNES. The DLR Institute of Composite Structures and Adaptive Systems in Braunschweig was responsible for the stable structure of the lander. The DLR Robotics and Mechatronics Center in Oberpfaffenhofen developed the swing arm that allowed MASCOT to ‘hop’ on the asteroid. The DLR Institute of Planetary Research in Berlin contributed the MASCAM camera and the MARA radiometer. The asteroid lander was monitored and operated from the MASCOT Control Center in the Microgravity User Support Center (MUSC) at the DLR site in Cologne.

  • Falk Dambowsky
    Ger­man Aerospace Cen­ter (DLR)

    Com­mu­ni­ca­tions and Me­dia Re­la­tions
    Telephone: +49 2203 601-3959
    Linder Höhe
    51147 Cologne
  • Prof.Dr. Ralf Jaumann
    Freie Uni­ver­sität Berlin
    In­sti­tute of Ge­o­log­i­cal Sci­ences
    Plan­e­tary Sci­ences and Re­mote Sens­ing
    Telephone: +49-172-2355864
    Malteserstr. 74-100
    12249 Berlin
  • Tra-Mi Ho
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Space Sys­tems
    Telephone: +49 421 24420-1171
    Robert-Hooke-Straße 7
    28359 Bremen
  • Ulrich Köhler
    Ger­man Aerospace Cen­ter (DLR)
    In­sti­tute of Plan­e­tary Re­search
    Rutherfordstraße 2
    12489 Berlin

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