Hayabusa 2 (Хаябуса-2), Procyon – H-IIA F26 – Танэгасима – 03.12.2014 04:22:04 UTC

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tnt22

Цитировать HAYABUSA2@JAXA‏ @haya2e_jaxa 3 ч. назад

These are thermal images captured with the TIR. We see the dayside of Ryugu, but notably the temperature of the boulders & surrounding soil is similar, regardless of location. This is the focus of a @nature paper: https://www.nature.com/articles/s41586-020-2102-6 ... ( JAXA, Rikkyo University & collaborators)

https://video.twimg.com/ext_tw_video/1239823601739522050/pu/vid/1280x720/f1cJ6Fyv7Lvaf_WM.mp4 (0:36)


3 ч. назад

Thermal inertia (ability to heat & cool) for the surface of Ryugu is estimated from variations in temperature. This calculation implies that Ryugu material has a low thermal inertia (easy to heat & cool) and suggests the material of Ryugu is full of gaps and very porous.


3 ч. назад

Although there is little difference in temperature during the daytime on Ryugu, the temperature drops sharply at the right edge of Ryugu as we move from evening to night. This implies Ryugu may be an object made from fluffy dust, on the way to becoming a denser celestial body.

tnt22

https://nauka.tass.ru/nauka/8001411
Цитировать17 МАР, 16:58
Причиной образования астероида Рюгу назвали "космическое ДТП"
Планетологи считают, что этот астероид образовался в результате столкновения обломков планетезималей – своеобразных "зародышей" планет. Именно этим ученые объясняют пористую структуру Рюгу


Поверхность астероида Рюгу
© AXA via AP

ТАСС, 17 марта. Планетологи выяснили, что весь астероид Рюгу, включая гигантские булыжники на его поверхности, состоит из очень пористой материи, которая появилась в результате разрушения зародышей планет и столкновений их обломков. Результаты этого исследования опубликовал научный журнал Nature.

"Хрупкие и очень пористые астероиды, подобные Рюгу, скорее всего, являются "связующим звеном", которое объясняет то, как скопления космической пыли превращаются в планеты и другие небесные тела. Это открытие закрывает самый большой пробел в нашем понимании того, как формируются планеты, так как этот материал невозможно найти в метеоритах, падающих на Землю", – объяснил суть исследования один из его авторов, планетолог Маттиас Гротт.

Астероид Рюгу исследовал зонд "Хаябуса-2", который запустили в космос в начале декабря 2014 года. Планетологи надеются, что аппарат впервые доставит на Землю "чистые" образцы первичной материи Солнечной системы, которые он собрал в середине июля прошлого года.

"Хаябуса-2" доставил к астероиду четыре спускаемых аппарата – три японских ровера, аналоги которых японские специалисты пытались высадить на поверхность астероида Итокава в ходе предыдущей миссии "Хаябуса-1", а также европейский аппарат MASCOT. Роботы Rover-1A и Rover-1B были успешно сброшены на поверхность Рюгу в конце сентября 2018 года, через месяц за ним последовал MASCOT, а последний аппарат, MINERVA-II2, был отправлен на Рюгу в октябре 2019 года.

Все аппараты полностью выполнили запланированные задачи и собрали множество сведений о том, как устроен этот астероид. В частности, MASCOT обнаружил необычно малое количество пыли на поверхности Рюгу и огромное число кубообразных булыжников, другие роверы записали и передали на Землю первое видео с поверхности малого небесного тела. Вдобавок замеры роверов указали на необычно высокую пористость поверхности астероида, что стало большой неожиданностью для ученых.

Первенцы Солнечной системы

Гротт и его коллеги проанализировали данные о свойствах недр астероида, которые собрали инструменты спускаемых аппаратов и инфракрасная камера TIR, которая находится на борту "Хаябусы-2". Наблюдая за тем, как быстро астероид теряет тепло, ученые оценили плотность и структуру всей материи на его поверхности.

Оказалось, что весь Рюгу сложен из такой же легкой, непрочной и пористой материи, следы которой в отдельных точках его поверхности зафиксировал ровер MASCOT и другие спускаемые аппараты. Это же касалось и всех крупных булыжников, а также других структур, которые планетологи раньше считали более прочными.

"Быстрый нагрев поверхности Рюгу после восхода Солнца, во время которого температура камней повышалась с –43 до +27 °C, говорит о том, что астероид состоит из очень пористой и неплотной материи. Лишь один процент булыжников на его поверхности вел себя как метеориты, упавшие на Землю", – объяснил Гротт.

Такую структуру у Рюгу, по словам ученого, можно объяснить только в том случае, если этот астероид возник в первые мгновения жизни Солнечной системы. Его прародителями, как предполагают астрономы, были планетизимали – своеобразные зародыши планет.

Планетезимали появлялись в результате мягких столкновений между зернами первичной материи, из которой родились Земля и все другие миры. Благодаря этому в их недрах было очень много пустот, а сами подобные небесные тела были очень непрочными, разрушаясь даже при относительно слабых столкновениях друг с другом.

Одним из результатов подобных "космических ДТП", как предполагают Гротт и его коллеги, стал и Рюгу. Ученые считают, что он сформировался из большого числа обломков протопланетных тел.

Так это или нет, ученые смогут выяснить в конце 2020 года, когда "Хаябуса-2" вернется на Землю и сбросит на нее капсулы с образцами материи этого астероида.

tnt22

#1105
http://www.hayabusa2.jaxa.jp/en/topics/20200318_nature/
ЦитироватьThermal images from the TIR suggest Ryugu boulders may be "fluffy"
by Elizabeth Tasker (JAXA)

The first photographs from Hayabusa2 of the surface of asteroid Ryugu revealed a treacherous landscape, with large boulders carpeting the asteroid to form a rugged topology. Yet when the spacecraft turned on its thermal infrared imager (TIR), it saw a surprisingly homogenous surface in the thermographic images.

How effectively a planetary surface conducts and stores heat is measured by a property known as the thermal inertia. A surface with a high thermal inertia will gain and lose heat slowly, resulting in smaller temperature differences between night and day and summer and winter.

Thermal inertia depends on composition and also density. Boulders like those scattered over Ryugu are expected to be dense and should therefore have a high thermal inertia, changing temperature more slowly than the surrounding material. On the dayside of Ryugu, this would make the boulders stand out on the TIR image as cold spots. But this was not what was seen.

A paper published in the journal, Nature, this week (March 16, 2020) led by Tatsuaki Okada (ISAS & University of Tokyo) revealed the first global thermal image of an asteroid during a complete rotation, in addition to high resolution images taken close to the surface. This data captured by the TIR on Hayabusa2 showed little temperature difference between the majority of boulders and the surrounding material on the surface of Ryugu, pointing to a similar thermal inertia value of about (300 tiu).


Left: "Ryugoid", the expected thermal distribution of the asteroid before arrival. The assumed thermal inertia was 1600 tiu for the boulders and 300 tiu for the regolith. Right: Observed Ryugu, from the global TIR observations at an altitude of about 5km (~4.5 m / pixel). The derived thermal inertia of both boulders and surroundings is about 300 tiu.

The implication is that Ryugu is covered with low-density, porous boulders that are surrounded by similarly porous fragments greater than 10 cm in size. Since the initial expectation was the asteroid should have dense boulders embedded in a layer of fine regolith, this is a startling discovery. The reverberations have important implications for how planets are formed.

Ryugu is an example of a C-type or Carbonaceous asteroid. C-types are primitive asteroids consisting of material that has experienced little change since the formation of the Solar System 4.6 billion years ago. This makes them important time capsules for the early days of planet formation, revealing the material that built the Solar System.

It is suspected that the carbonaceous chondrite meteorites found on Earth are fragments of C-type asteroids. The accessibility of meteorites makes these much studied objects for understanding the Earth's earliest days. But these meteorites are significantly less porous that the majority of rocks on Ryugu.

The close-up TIR images captured during the Hayabusa2 descent operations reveal a few colder boulders with thermal inertia values corresponding to 600 - 1,000 tiu; a value more typical of the carbonaceous chondrites. But these dense additions are in the minority on the asteroid surface.


Cold spots discovered during close-up thermal images. These images were captured during the TD1-R1-A operation on the 15 October, 2018 from an altitude of 78.8m. The temperature profile along the yellow line is shown in the adjacent panel. The cold spots are colder by more than 20K, indicated a dense and consolidated boulder with high thermal inertia. Only a few cold spots (boulders) are found on the surface of Ryugu: other boulders have the same temperature as their surroundings. Scale bar is 5m in panel a and 1m in panel c.

One possibility is that C-type asteroids may have a far less consolidated nature than expected, consisting of a loose conglomerate of fluffy dust and pebbles. Ryugu would then be a fragment from such a porous parent, with a handful of denser boulders that were formed in the deepest part of the parent asteroid, or even had a separate origin and were delivered to the asteroid during meteoritic impacts.

These fluffy asteroids would struggle to survive the high temperatures and shockwaves experienced during a descent through the atmosphere of Earth, resulting in meteorites that consist of only the dense minor components of the asteroid.


Formation scenario for Ryugu from a porous parent body. (1) Formation begins with the fluffy dust in the Solar nebula. (2) Porous planetesimals were formedr through accretion of dust or pebbles. (3) The parent body of Ryugu may have remained porous owing to a low degree of consolidation. A clear boundary at the inner core is illustrated here, but a gradual increase of consolidation by depth might also have occurred. (4) Impact fragmentation of the parent body, with a few large fragments forming the boulders on Ryugu. (5) Fragments re-accrete to form Ryugu, with porous boulders and sediment on the surface and a small number of dense boulders originating from the parent's inner core. (6) Re-shaping during a rapid rotation phase created a double-top-shape.

If this fluffy composition was typical of the material that built the planets, then the growth process that took the Solar System from dust to planets might be very different from models that assume a more rigid building material. For example, the result of impacts between porous planetesimals may prove to result more often in coagulation than fragmentation, allowing for a swifter growth and altering the time-scale for planet formation.

It is also possible that Ryugu's porosity may be due to a composition of completely different materials to the carbonaceous chondrite meteorites. For example, the asteroid could consist of carbon-rich material similar to that discovered on comet-67P/Churymov-Gerasimenko. This will ultimately be resolved when Hayabusa2 returns to Earth at the end of 2020 with samples of both surface and subsurface material.


Nature (2020)
Paper Title:Highly porous nature of a primitive asteroid revealed by thermal imaging
Authors:Okada, T., Fukuhara, T., Tanaka, S. et al.   Author Names
DOI:10.1038/s41586-020-2102-6

ref: JAXA Press Release(In Japanese)
       Material for the press briefing

Hayabusa2 Project 
2020.03.18

tnt22

Цитировать HAYABUSA2 @JAXA✔@haya2e_jaxa 4:46 AM - Mar 20, 2020

Results from the impact experiment (SCI) that generated the artificial crater on asteroid Ryugu was published in Science Magazine today JST (Match 20). This research was led by Professor Masahiko Arakawa (Kobe University). https://science.sciencemag.org/content/early/2020/03/18/science.aaz1701 ...

ЦитироватьAn artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime
The Hayabusa2 spacecraft investigated the small asteroid Ryugu, which has a rubble pile structure. We describe an impact experiment on Ryugu using Hayabusa2's Small Carry-on Impactor (SCI). The...
science.sciencemag.org

PDF-версия - science.aaz1701.full.pdf - 1.5 MB, 10 стр

tnt22

Цитировать HAYABUSA2 @JAXA✔@haya2e_jaxa 4:49 AM - Mar 20, 2020

This sequence of images shows the ejecta thrown out of the artificial crater. Images were taken 185s before the impact and 3s, 5s, 36s, 100s, 192s, 396s and 489s after the SCI collision. The right image is an enlargement of the left.
( JAXA, Kobe University and collaborators.)

https://video.twimg.com/ext_tw_video/1240817733052166144/pu/vid/1280x640/gdRL37O79t6x3ciJ.mp4 (0:03)


4:53 AM - Mar 20, 2020

This is a before & after comparison image of the area where the impactor (SCI) hit the asteroid surface. These changes were part of the studies in the above journal paper. You can clearly see the change in surface topography.
( JAXA, Kobe University and collaborators).

https://video.twimg.com/ext_tw_video/1240818733720821762/pu/vid/600x600/GUsKKq0lnkrvI4Fl.mp4


5:00 AM - Mar 20, 2020

Within the Hayabusa2 Project, the artificial crater is known as the "SCI Crater" or the "Omusubi-Kororin Crater" (from the folktale of the "rolling rice ball" due to the shape of boulders in the vicinity that may roll down into the crater).


5:04 AM - Mar 20, 2020

Digital Elevation Map (DEM) used to measure the shape of the artificial crater. Color indicates terrain height. The diameter of the crater is about 14.5m and marked by the red line. The diameter of the surrounding rim is 17.6m and shown in black. ©Arakawa et al., Science 2020.




5:06 AM - Mar 20, 2020

The green dotted line (previous image) is a pit with diameter about 3m. The depth of the artificial crater (to the bottom of the pit) is estimated to be about 1.7m, and the depth from the rim top to the pit bottom is estimated at about 2.7m.


5:17 AM - Mar 20, 2020

The paper by Prof. Masahiko Arakawa (Kobe U.) in Science found that the artificial crater formed on Ryugu is 7x larger than it would be on Earth. Ryugu's surface age turns out to be quite young, of order 10 million years. This crater formation on an asteroid has been informative!

tnt22

https://nauka.tass.ru/nauka/8031713
Цитировать20 МАР, 02:58
"Бомбардировка" астероида Рюгу помогла ученым вычислить возраст его поверхности
Возраст материи астероида не превышает 158 млн лет

ТАСС, 20 марта. Данные, собранные зондом "Хаябуса-2" во время формирования рукотворного кратера на поверхности астероида Рюгу, помогли астрономам раскрыть структуру материи его поверхности и вычислить ее возраст, не превышающий 158 миллионов лет. Выводы и результаты наблюдений за взрывом на этом небесном теле были опубликованы в журнале Science.

"Снимки, полученные камерой DCAM3, показывают, что формирование кратера зависело исключительно от силы притяжения, а не того, насколько крепкой была материя Рюгу в этой точке. Это говорит о том, что поверхность астероида относительно молода и сформирована из рыхлого пескообразного материала", - пишут планетологи.

Зонд "Хаябуса-2" был отправлен в космос в начале декабря 2014 года для изучения, забора и возврата проб с астероида Рюгу (в переводе с японского - "Дворец дракона"). Планетологи надеются, что он впервые доставит на Землю "чистые" образцы первичной материи Солнечной системы, собранные аппаратом в середине июля прошлого года.

Сбор этих образцов зонд осуществил в два этапа. Сначала он сблизился с поверхностью астероида в феврале 2019 года в точке "Таматебако" ("Волшебная шкатулка") и выстрелил в нее пятиграммовой танталовой пулей. Через два месяца пилоты и научная команда миссии успешно реализовали вторую фазу, сбросив на Рюгу взрывпакет, чья масса составляла примерно 15 кг.

Он содержал в себе примерно 4,5 кг октогена и был устроен таким образом, что вся сила взрыва была направлена вниз, в сторону поверхности астероида. Успешно осуществив эту операцию, "Хаябуса-2" в июле прошлого года совершила еще одно сближение с поверхностью астероида в точке взрыва, забрав чистые образцы первичной материи Солнечной системы.

Многослойный астероид

Незадолго до этого, как пишет научный руководитель миссии Сэйитиро Ватанабэ и его коллеги, зонд передал на Землю научные данные, собранные инструментами "Хаябусы-2", а также кадры, полученные автономной камерой DCAM3, оставленной над точкой сброса взрывпакета.

Эта информация была важна для ученых, она показывает, как устроены верхние слои поверхности Рюгу, что крайне важно понимать для вычисления возраста астероида и оценки того, как быстро исчезают кратеры с его поверхности.

Для этого ученые сравнили фотографии, полученные DCAM3 и бортовыми камерами "Хаябусы-2" с разных ракурсов, и составили трехмерную карту рукотворного кратера. После этого они изучили, как далеко были выброшены обломки астероида после взрыва и оценили плотность его грунта, а также другие механические и физические параметры.

Эти замеры показали, что подрыв устройства привел к рождению достаточно крупного кратера диаметром примерно в 15 метров и глубиной в два метра, внутри которого возникла еще одна менее крупная выемка. Ее структура, а также характер разбросанных фрагментов грунта указывают на то, что поверхность Рюгу покрывает тонкий слой из очень рыхлой и непрочной пескообразной материи, скрывающей под собой более плотный грунт.

Подобная структура верхнего слоя недр астероида, как отмечают Ватанабэ и его коллеги, говорит о том, что поверхность Рюгу достаточно молода и постоянно обновляется. Следы небольших кратеров, чья глубина составляет несколько метров, должны исчезать с нее за мгновения по космическим меркам - около 10 тыс. лет.

В целом возраст его поверхности оказался достаточно молодым - он должен быть больше, чем 10  млн лет, но при этом не превышать отметки в 158 млн лет. Это следует учитывать при анализе образцов первичной материи Солнечной системы, которые будут доставлены на Землю в конце этого года, заключают ученые.

tnt22

Цитировать HAYABUSA2 @JAXA✔@haya2e_jaxa 7:01 AM - Apr 17, 2020

We took an image of the starry sky to check Hayabusa2's Optical Navigation Camera - Telescopic (ONC-T) and were able to capture a stunning shot of the Milky Way! This image was taken on April 6, 2020 about 09:10 JST. ( JAXA, AIST & collaborators.)




7:15 AM - Apr 17, 2020

This is the same image of the Milky Way coloured blue for artistry. Wrapped in stars, Hayabusa2 is now headed for the Earth. Do you know where in the Milky Way is shown in this picture? ( JAXA, AIST & collaborators.)




12:22 PM - Apr 18, 2020

Thank you for all your replies about the location in the Milky Way that was imaged with the ONC-T! The correct answer is shown in this diagram (created with StellaNavigator). The location is near the toes of Ophiuchus, close to the center of our Milky Way galaxy.


tnt22

https://tass.ru/nauka/8422283
Цитировать7 МАЯ, 21:32
Астрономы выяснили, как на астероиде Рюгу возникли красные полосы

ТАСС, 7 мая. Японские планетологи проанализировали снимки, полученные камерами зонда "Хаябуса-2" в момент его посадки на поверхность астероида Рюгу, и нашли намеки на то, что загадочные залежи красных пород на его поверхности возникли в ходе его недавнего сближения с Солнцем. Выводы ученых были опубликованы в журнале Science.

"Присутствие красной материи на астероиде Рюгу можно объяснить тем, что примерно 300 тыс. лет назад его орбита временно поменялась, в результате чего он сблизился с Солнцем. Его поверхность начала сильнее прогреваться, что привело к покраснению залежей органики на его поверхности", - пишут ученые.

Зонд "Хаябуса-2" был отправлен в космос в начале декабря 2014 года для изучения, забора и возврата проб с астероида Рюгу (в переводе с японского - Дворец дракона). Планетологи надеются, что он впервые доставит на Землю "чистые" образцы первичной материи Солнечной системы, собранные аппаратом в середине июля прошлого года.

Сбор этих образцов зонд осуществил в два этапа. Сначала он сблизился с поверхностью астероида в феврале 2019 года в точке "Таматебако" ("Волшебная шкатулка") и выстрелил в нее пятиграммовой танталовой пулей. Через два месяца пилоты и научная команда миссии успешно реализовали вторую фазу, сбросив на Рюгу взрывпакет, чья масса составляла примерно 15 кг.

Он содержал в себе примерно 4,5 кг октогена и был устроен таким образом, что вся сила взрыва была направлена вниз, в сторону поверхности астероида. Успешно осуществив эту операцию, "Хаябуса-2" в июле прошлого года совершила еще одно сближение с поверхностью астероида в точке взрыва, забрав предположительно чистые образцы первичной материи Солнечной системы.

Сине-красный астероид

Как пишет научный руководитель миссии Сэйитиро Ватанабэ и его коллеги, сейчас японские планетологи начали сомневаться в этом, проанализировав снимки, собранные камерами аппарата в тот момент, когда он впервые опустился на поверхность астероида и поймал образцы пород, подброшенные выстрелом танталовой пули.

Дело в том, что сразу после прибытия "Хаябусы-2" к Рюгу ученые заметили, что его поверхность неоднородна по своему составу. Она покрыта чередующимися полосами из пород красного и синего цвета, природа которых оставалась загадкой для астрономов до недавнего времени.

В точке "Таматебако", как отмечают ученые, преобладали породы синего цвета, которые планетологи ассоциировали с первичной материей Солнечной системы, а красная материя ассоциировалась с менее первозданной материей, попавшей в недра астероида в момент гибели его прародителя и формирования Рюгу из его обломков.

Когда ученые начали рассматривать кадры посадки и взлета "Хаябусы-2", они обнаружили, что значительная часть этой красной материи представляла собой пескообразный материал, который легко "сдувался" с поверхности Рюгу двигателями зонда. Достаточно большие ее количества, как предполагают планетологи, попали внутрь межпланетной станции вместе с образцами синей первичной материи.

В свою очередь подобная структура красных регионов астероида, по словам планетологов, говорит о том, что они возникли на поверхности астероида относительно недавно, примерно 300 тыс. лет назад. Как считают ученые, это произошло в результате того, что орбита Рюгу в то время резко изменилась, в результате чего он впервые сбизился с Солнцем и покрылся тонким слоем красного "песка".

Синие полосы на поверхности Рюгу, как отмечают ученые, возникли значительно позже, в результате того, что часть красного песка была выброшена из экваториальных регионов астероида под действием силы вращения, а также в результате формирования новых кратеров в других участках его поверхности. Изучение образцов красной и синей материи, которые "Хаябуса-2" доставит на Землю в конце этого года, поможет ученым проверить эту гипотезу и окончательно раскрыть историю формирования Рюгу.

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en - в бесплатном доступе
Цитироватьhttps://science.sciencemag.org/content/368/6491/654
Sample collection from asteroid (162173) Ryugu by Hayabusa2: Implications for surface evolution
....
Science  08 May 2020:
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jp https://www.jaxa.jp/press/2020/05/20200508-1_j.html

Цитировать https://www3.nhk.or.jp/nhkworld/en/news/20200508_03/
...
Japan's space agency says it has found that the asteroid Ryugu may have orbited between the Sun and Mercury in the distant past. That's different from its current orbit, which passes between Earth and Mars, but not between the Sun and Mercury.

The Japan Aerospace Exploration Agency, or JAXA, says it analyzed images of the asteroid taken by a camera on its probe Hayabusa2.

JAXA says much of the sand and rock on the surface of Ryugu turned red when exposed to heat of more than 600 degrees Celsius. It says such high temperatures are inconceivable even when the asteroid is at its closest point to the Sun in its current orbit.

JAXA says the sand and rock in a recent crater on Ryugu is blue, as it has not been heated to high temperatures. It says this is because the asteroid's orbit moved farther away from the Sun in the relatively recent past.

JAXA says these findings suggest that Ryugu's shortest distance from the Sun about 300,000 to 8 million years ago was roughly one-third to one-fifth the current one.

JAXA adds that Ryugu's orbit at the time may have passed between the Sun and Mercury before shifting to the present trajectory. Mercury is the closest planet to the Sun in our solar system.

JAXA says it remains unknown what caused the change in Ryugu's orbit, but that gravity could be one reason.
...

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Цитировать HAYABUSA2@JAXA @haya2e_jaxa · 16 ч. назад

The second phase of the ion engine operations during the Hayabusa2 Earth-return Operation has begun. The preparation of the ion engines is complete and the operation will start tomorrow morning (May 12 JST). We will do our best while keeping safe from the novel coronavirus!


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Цитировать HAYABUSA2@JAXA @haya2e_jaxa · 42 мин. назад

The Hayabusa2 second-stage ion engine operation has begun. The ion engine was ignited today (May 12) at 7:00 JST (onboard time) and confirmed stable at 7:25 JST (ground time). The condition of the spacecraft is normal. This photo shows the ion engines being tested (©JAXA).




38 мин. назад

Currently, Hayabusa2 is far from the Sun, so only one ion engine is operating. The 2nd phase of the ion engine operation will continue until around September. After this ion engine operation, Hayabusa2 will be in an orbit that can deliver the capsule to Earth.

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http://www.hayabusa2.jaxa.jp/en/topics/20200508_science/

ЦитироватьA sunburned Ryugu: the asteroid surface has been weathered by the Sun!

New information about the surface and orbit of Ryugu has been uncovered from analyzing data obtained during touchdown and the global observations performed by the asteroid explorer, Hayabusa2. These findings have been published in the electronic version of the US scientific journal Science, on May 7, 2020 (May 8, JST: Morota et al, 2002). The paper was led by Associate Professor Tomokatsu Morota from the University of Tokyo and member of the Hayabusa2 science team.

Journal paper title:Sample collection from asteroid 162173 Ryugu by Hayabusa2: implicationsfor surface evolution
Authors:T. Morota, S. Sugita, Y. Cho, et al.    Full list
DOI:10.1126/science.aaz6306
JAXA press releases (Japanese)
Reference materials

■ Paper summary

On February 22, 2019 (JST), Hayabusa2 successfully completed the first touchdown to sample materials from asteroid Ryugu. The extremely high-resolution images acquired during touchdown revealed a large quantity of reddish-dark particles thrown upwards with the rocks. It is thought that these particles had been adhered to the surfaces and inside pores of rocks but were disturbed during the impact of the bullet and thruster jets during touchdown.

These reddish-dark materials are sparsely distributed in a depth ranging from Ryugu's surface to a few meters. The interiors of older craters also appear reddish-dark, while younger craters have a bluer interior than their surroundings. From comparison of the age of these craters, surface reddening of Ryugu occurred during a short period between 300,000 and 8 million years ago. The interiors of craters formed before this period turned reddish-dark along with the asteroid surface, while younger craters were evacuated after the red material had formed and exposed fresh interior rock unaffected by the coating. The best explanation is the reddish-dark material was created when the surface of Ryugu was burned by the Sun.

This suggests that Ryugu was in an orbit closer to the Sun in the past. As both reddish-dark material and the unaltered blue-bright material were present on the surface at the landing site, it is expected that both the altered and unaltered materials have been collected.

■Materials thrown upwards during the 1st touchdown

The observations from Hayabusa2 reveal that the surface of Ryugu was covered with rocks, and the existence of fine particles such as those found on the lunar surface could not be confirmed. Hayabusa2 performed the first touchdown on February 22, 2019 (JST) to collect samples from Ryugu. Figure 1 shows an image captured with the Optical Navigation Camera – Wide angle (ONC-W1) during touchdown. Figure 2 is a movie showing the images taken with both ONC-W1 and the small monitor camera (CAM-H). Figure 3 shows a movie captured only with ONC-W1.


Figure 1: Surface of Ryugu before and after the first touchdown taken with the ONC-W1. Date and time listed is in UTC. 
(©JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST). 

http://www.hayabusa2.jaxa.jp/en/topics/20200508_science/img/HY2_CAMH_ONC_20190221_TD1-L08E1_4fps_512.mp4
Figure 2: Movie of the image sequence taken with CAM-H and ONC-W1 at the time of the first touchdown, at matching times. (Movie) 
(©JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu, AIST).

http://www.hayabusa2.jaxa.jp/en/topics/20200508_science/img/aaz6306_Supplementary%20Movie_mov1_seq1_v2.mp4
Figure 3: Movie of images taken with the ONC-W1 at the time of the first touchdown. (Movie) 
(©Morota et al., 2020) 


Touchdown lifted not only rocks, but a large quantity of dark particles off the surface. Rocks blown by the force of the touchdown turned white, suggesting that the dark particles had adhered to the surface and inside pores of the rocks. The soaring dark particles were deposited over an area of 10 m in diameter, centered on the touchdown point. Figure 4 shows how the reflectance and slope (color) of the reflectance spectrum changed before and after touchdown. As you can see from the figure, the color near the touchdown point changed to reddish-dark after touchdown. It is probable that dark particles floated upwards during the touchdown had this reddish color.


Figure 4: Color change around the touchdown point. The change in reflectance and the slope of the reflectance spectrum before touchdown (A, B : Oct. 2, 2018) and after touchdown (C, D : April 4, 2019) is shown. (A) and (C) show the reflectance and (B) and (D) show the slope of the reflectance spectrum. The center of the dotted circle is the touchdown down point. 
(Partially modified from ©Morota et al., 2020).

■Comparison with remote sensing observations

Global observations show that the surface of Ryugu is slightly blue-bright at the equator and reddish-dark at mid-latitudes. The poles are also particularly blue-bright (Figure 5). Detailed observation of colors inside craters superposed on craters shows that the color distribution inside the relatively older (lower) crater has the same redness as the surroundings. Within the upper (younger) crater, the inside is bluer than the surroundings (Figure 5). This suggests that there was an event in the past that turned Ryugu's surface red. The red craters were formed before this event, while the craters with a bluer interior were formed after the reddening of Ryugu's surface occurred and exposed fresh bluer materials within their base. Both the latitude dependence of the red distribution, and the clear division between red and blue craters, supports the idea that the reddening event on the surface of Ryugu was caused by heating or weathering by the Sun and happened in a short time period.


Figure 5: Map of variations in the spectral slope of the reflectance spectrum on the surface of Ryugu. The black circles in (A) and white circles in (C) denote the craters. The B1 and B2 craters, which have blue interiors, lie over the other craters, indicating that these are young craters. 
(Partially modified from ©Morota et al., 2020.)

■Ryugu's past approach to the Sun

These observations suggest that Ryugu was temporarily on an orbit that approached the Sun in the past. The blue craters were formed after the surface reddening, so based on their number density, we can infer the age of the surface reddening. The resultant date is estimated to be between 300,000 and 8 million years ago. Figure 6 summarizes the evolutionary history of Ryugu, based on these results.


Figure 6: Estimated evolutionary history of Ryugu. (©Morota et al., 2020).

The reddish-dark particles observed during touchdown are believed to have been crushed material that had been altered during a past approach towards the Sun by Ryugu. As both reddish-dark material and also blue-bright material were on the surface at the landing site, it is expected that both altered and unaltered materials were collected.

Reference figure 


This figure shows an approximate image of how the orbit of Ryugu changed in relation to its evolution shown in Fig.6. Although Ryugu was formed in the astroid belt, the perihelion (point closest to the Sun) of its orbit changed to move near the orbit of Mercury. The orbit later evolved again to become the current path of Ryugu around the Sun. (© University of Tokyo, JAXA) 

Hayabusa2 Project
2020.05.08

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Цитата: undefined HAYABUSA2@JAXA @haya2e_jaxa 3 ч. назад

Today (5/25), Hayabusa2 achieved 2000 days of space flight & passed the mid-point for the return trip! The remaining distance is ~400 million km. Ion engines & flight course are good. Operations continue, hoping that Ryugu's treasure will arrive at a peaceful Earth --PM Tsuda.

Изображение

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Цитировать HAYABUSA2@JAXA @haya2e_jaxa 24 июн.

Currently, Hayabusa2 & Ryugu are very close to Mars. This figure shows the trajectory map displayed in the control room. On this scale, the 3 celestial bodies apppear very close. Mars will rise at midnight & Hayabusa2 will be nearby. From near Mars, Hayabusa2 will return to Earth


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#1117
http://www.hayabusa2.jaxa.jp/en/topics/20200626_Icarus/

Цитата: undefinedRyugu's porous boulder covered rough surface, everywhere!

New thermophysical properties of the surface of Ryugu have been uncovered from analyzing data obtained during the global observations performed by the asteroid explorer, Hayabusa2. These findings have been published in the electronic version of the US scientific journal Icarus, on May 16, 2020 (May 17, JST: Shimaki et al., 2020). The paper was led by researcher Yuri Shimaki from ISAS/JAXA and members of the Hayabusa2 science team.

Journal paper title:Thermophysical properties of the surface of asteroid 162173 Ryugu: Infrared observations and thermal inertia mapping
Authors:Y. Shimaki, H. Senshu, N. Sakatani, et al.    Full list
DOI:10.1016/j.icarus.2020.113835

■Paper summary
The thermal infrared imager (TIR) on Hayabusa2 carried out thermography of asteroid Ryugu for one rotation period on August 1, 2018. The research team compared the observed temperatures with temperatures calculated using a thermophysical model that considered the effect of surface roughness (TPM2). As a result, it was found that the thermal inertia of the surface of Ryugu is ubiquitously small, and that porous boulders are distributed all over the surface of Ryugu. Additionally, the surface of Ryugu is as rough as the surface of Hawaii's a'a lava*. The thermal inertia and surface roughness of Ryugu estimated in this study affect the calculation of the orbital evolution of Ryugu.
*Ryugu is uneven over a scale of several square metres, whereas a'a lava is uneven over tens of centimeter squared.

■The effect of roughness on the surface temperatures of an asteroid
The surface temperature of an asteroid is a significant index that indicates surface composition or grain size. The surface temperature of a flat surface is determined by its thermal inertia, if the incident solar energy is constant. On the other hand, a rough surface produces a thermal trap in concave terrain (via re-absorption of secondary radiation). It also changes the apparent brightness temperature depending on the observation direction, caused by the existence of hot-sunlit regions and cold-shadow regions (Figure 1). This effect is called the roughness effect. For this reason, rough terrain is observed as hot from the Sun direction but observed as cold from the night-side direction.


Figure 1: Roughness effect (©JAXA, Chiba Institute of Technology)

■Comparison between the observed data and calculation using the thermal model
The thermography of Ryugu with the TIR showed a small temperature difference over the image (Figure 2). This observation results from a small change in the diurnal brightness temperature of the surface of Ryugu. Figure 3a shows the surface brightness temperature of Ryugu observed by TIR and the temperature predictions from a thermophysical model that does not consider the effect of roughness (TPM1). Notably, this model failed to reproduce the observed temperatures, even if we changed the thermal inertia from 20 to 800 J m–2 s–0.5 K–1 (hereafter tiu). Okada et al., Nature 579, 518–522, 2020 estimated the global thermal inertia of Ryugu as 300 ± 100 tiu from the maximum diurnal temperatures of Ryugu observed by the TIR and those calculated by TPM1. They revealed that highly porous boulders cover the surface of Ryugu.
In this study, the research team carried out a numerical calculation by a thermophysical model that included an effect of surface roughness (TPM2). The team compared the calculated results using TPM2 and the observed data acquired on August 1, 2018, and confirmed that the model reproduced the observed diurnal brightness temperature well (Figure 3b). The roughness effect has been expected by ground and flyby observations; however, time- and spatial-resolutions were limited. The thermographic observation of the asteroid for one rotation period during the hovering operation of Hayabusa2 allows us to acquire highly resolved images with sufficient time intervals.


Figure 2: Thermography of Ryugu on August 1, 2018 (©Shimaki et al., 2020)


Figure 3: Comparison between Ryugu's surface temperature and calculated results from thermophysical models. (a) Observed data by TIR (squares) and calculated results from a thermophysical model without consideration of the roughness effect (TPM1) (solid and dashed lines). (b) Calculated results from a thermophysical model with consideration of the roughness effect (TPM2) (plus sign). Gamma and sigma are thermal inertia and surface roughness, respectively. (© Shimaki et al., 2020)

■ The global thermophysical properties of asteroid Ryugu
From the analysis using TMP2, we estimated thermal inertia and surface roughness (variance of the vertical length relative to the horizontal length of a rough surface) within a TIR pixel that corresponded to 4.5 m (Figures 4 and 5). The global thermal inertia of Ryugu was found to be 225 ± 45 tiu, indicating the ubiquitous distribution of porous boulders over the Ryugu surface. Considering the thermal inertia of dense rock such as basalt (>2000 tiu) and typical carbonaceous chondrites (600-1000 tiu), Ryugu's boulders are expected to be highly porous.
The global surface roughness of Ryugu, covered by numerous boulders (Figure 6), was estimated to be 0.41 ± 0.08, corresponding to the RMS surface slope (average slope of a rough surface) of 47 ± 5°. The RMS surface slope of Ryugu is comparable to that of Hawaii's a'a lava (Figure 7). Note that the RMS slope of Ryugu is for several meter squares, but that of a'a lava for several tens centimeter squares, respectively. These similarities are only in their morphologies, so they do not represent similarities in the compositions and formation processes (Ryugu's boulder is expected to be carbonaceous chondrites rich in organics, but a'a lava is a basaltic igneous rock). However, these results suggest that large-scale roughness can be maintained on the surface of Ryugu because of the tiny gravity and no-weathering environment. Additionally, one can imagine the difficulty of the touchdown operations by picturing a 1/10 scale Hayabusa2 attempting to touchdown on the surface of a'a lava. We see a relatively low surface roughness region around the equatorial ridge (Figure 5), indicating mass movement between the equatorial ridge and mid-latitudes in the past.


Figure 4: Thermal inertia map of Ryugu. Circles indicate locations of major craters. (© Shimaki et al., 2020)


Figure 5: Figure 5: Surface roughness map of Ryugu. A low surface roughness band can be seen in the equatorial ridge. (©Shimaki et al., 2020)


Figure 6: Ryugu's surface taken by MINERVA-II1 Rober-1B. (©JAXA)


Figure 7: Hawaii's a'a lava (©Rina Noguchi). Hayabusa2's touchdown to Ryugu corresponds to 1/10 scale Hayabusa2's touchdown to a'a lava.

■Thermophysical properties of craters and boulders on Ryugu
Besides optical images, the thermal inertia and surface roughness provides us with the physical properties of Ryugu's surface. For example, ejecta deposits around lunar craters were detected as anomalies of thermal inertia. Figure 8 shows the ONC image, thermal inertia, and surface roughness of large craters and large boulders on Ryugu. The thermal inertia values of large craters are within the range of the global average. We see no anomalies in thermal inertia to indicate ejecta deposits or consolidated materials by the crater forming process. Additionally, a boulder (b2) in the Kintaro crater showed a thermal inertia smaller than the global average, indicating a more porous structure, such as breccias containing porous pebbles.


Figure 8: ONC image, thermal inertia, and surface roughness of large craters and boulders. Color scales are as same as Figures 4 and 5. (©Shimaki et al., 2020)

■Implication for the numerical calculation of the orbital evolution of Ryugu
The thermal inertia and surface roughness of Ryugu estimated in this study affect the numerical calculation of the orbital evolution of Ryugu. The hottest region of an asteroid slightly shifts from the sub-solar point due to asteroid rotation and thermal inertia. Thermal radiation from the hottest region pushes the asteroid that results in acceleration or deceleration of the asteroid in the orbital direction, depending on the rotation direction (Yarkovsky effect, Figure 9). Besides orbital motion, the thermal inertia of an asteroid changes the rotation period if the asteroid has an asymmetrical shape relative to the rotation axis (YORP effect). The effect of the Yarkovsky effect on Ryugu is an ongoing topic in our team. It is expected that the precise determination of Ryugu's orbit during the proximity phase will confirm the model prediction.


Figure 9: Yarkovsky effect (©JAXA, Chiba Institute of Technology)

Yuri Shimaki (Hayabusa2 Project)
2020.06.26

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Опубликованы материалы пресс-брифинга 11 июня с.г.

Hayabusa2_Press_20200611_ver8_en2.pdf - 2.9 MB, 27 стр, 22.06.2020 02:33:30 UTC

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