Explanation: An abundance of boulders litters the surface asteroid 101955 Bennu in this dramatic close-up from the OSIRIS-REx spacecraft. Taken on March 28 from a distance of just 3.4 kilometers (2.1 miles) the field of view is about 50 meters across while the light colored boulder at top right is 4.8 meters tall. Likely a loose conglomerate rubble pile asteroid, Bennu itself spans less than 500 meters. That's about the height of the Empire State Building. Mapping the near Earth asteroid since the spacecraft's arrival in December of 2018, the OSIRIS-REx mission plans a TAG (Touch-and-Go) maneuver for July 2020 to sample Bennu's rugged surface, returning the sample to planet Earth in September 2023. Citizen scientists have been invited to help choose the sample collection site.
Awesome! Crystal clear photograph. There is no substitute for actually going there.
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 4:50 am
by shaileshs
I wonder what keeps the stuff in place on such objects ? Most bigger objects will have some core (like molten iron etc) to create gravity but what about these light and floating/wandering/drifting objects ? No core, no molten iron, then where's the gravity coming from ? Why the crust/surface objects don't disintegrate into pieces and scatter off ?
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 5:23 am
by Chris Peterson
shaileshs wrote: ↑Fri May 24, 2019 4:50 am
I wonder what keeps the stuff in place on such objects ? Most bigger objects will have some core (like molten iron etc) to create gravity but what about these light and floating/wandering/drifting objects ? No core, no molten iron, then where's the gravity coming from ? Why the crust/surface objects don't disintegrate into pieces and scatter off ?
Cores don't generate gravity. Gravity is the result of mass, and this body certainly has mass: somewhere from 1010 to 1011 kg. Enough to create a surface gravity of around 10 μg (g-force). That's what holds this rubble pile together, and keeps material on its surface. The four hour rotation period of the body (which may have once been higher) has altered the shape and may have resulted in some material being lost, but currently, it isn't enough to overcome the self-gravity of the object, which remains intact. Why would objects on the surface "disintegrate"?
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 7:13 am
by madtom1999
I guess there is some relationship with the G at the surface of an object to the minimum size of pebble/dust it can hold, along with perihelion too!
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 7:22 am
by Boomer12k
Great shot. Interesting place...
:---[===] *
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 8:53 am
by heehaw
I look from fragment to fragment and wonder, what was its previous history, before it ended up as part of this cosmic rubbish collection?
It's incredible, looks like any other dump on earth! Definitely it is a miracle that we do exist!
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 11:24 am
by orin stepanek
I wonder if it was some planets mountaintop blown into space by a volcanic eruption!
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 11:31 am
by Whiskybreath
LMMdT wrote: ↑Fri May 24, 2019 10:36 am
It's incredible, looks like any other dump on earth! Definitely it is a miracle that we do exist!
It actually looks very like the rubble field left behind after an open-pit mining blast, but one which has not been particularly successful in reducing the rubble to average, manageable sizes. Certainly the result of violent interactions between larger masses.
LMMdT wrote: ↑Fri May 24, 2019 10:36 am
It's incredible, looks like any other dump on earth! Definitely it is a miracle that we do exist!
It actually looks very like the rubble field left behind after an open-pit mining blast, but one which has not been particularly successful in reducing the rubble to average, manageable sizes. Certainly the result of violent interactions between larger masses.
Yes to you both. And speaking of "manageable sizes", I wonder about the size requirements for the sample collection/return part of this mission. Finding the right spot to land and grab up a scoop full of pebbles looks to be extremely daunting in all that mess.
Bruce
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 1:14 pm
by Chris Peterson
heehaw wrote: ↑Fri May 24, 2019 8:53 am
I look from fragment to fragment and wonder, what was its previous history, before it ended up as part of this cosmic rubbish collection?
The first major papers just came out a few weeks ago about Ryugu, visited by Hayabusa2, and a very similar carbonaceous asteroid to Bennu. These papers describe a body created and modified in a highly collisional environment, shaped by rapid rotation which has gradually decreased, with a relatively young surface. I imagine a similar scenario is likely for Bennu.
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 2:38 pm
by gps35n
A normal human could take a boulder that would weigh hundreds of pounds on Earth and throw it into orbit around Bennu. That would make a great video. Of course he would also launch himself up doing a jumping jack.
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Fri May 24, 2019 4:33 pm
by MarkBour
One of many fascinating questions that this image evokes ... If I saw this rubble on Earth, I would assume that the smaller pieces were formed by the steady erosion breakdown of larger pieces. I'm no geologist, so maybe I would be wrong about that, even on Earth. But for Bennu, where and how would this (size) distribution of rubble arise?
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Sat May 25, 2019 12:41 am
by Chunks
MarkBour wrote: ↑Fri May 24, 2019 4:33 pm
One of many fascinating questions that this image evokes ... If I saw this rubble on Earth, I would assume that the smaller pieces were formed by the steady erosion breakdown of larger pieces. I'm no geologist, so maybe I would be wrong about that, even on Earth. But for Bennu, where and how would this (size) distribution of rubble arise?
Planetesimal collisional rubble aggregate - bang the rock together,guys!
https://www.etymonline.com/word/boulder#etymonline_v_15658 wrote:
<<boulder (n.)1610s, "water-worn rounded stone of medium or large size," variant of Middle English bulder ston "stone worn round, cobblestone" (c. 1300), from a Scandinavian source akin to Swedish dialectal bullersten "noisy stone" (large stone in a stream, causing water to roar around it), from bullra "to roar" + sten "stone." Or the first element might be from *buller- "round object," from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell." Specific geological sense "large weather-worn block of stone standing by itself" is from 1813.>>
MarkBour wrote: ↑Fri May 24, 2019 4:33 pm
One of many fascinating questions that this image evokes ... If I saw this rubble on Earth, I would assume that the smaller pieces were formed by the steady erosion breakdown of larger pieces. I'm no geologist, so maybe I would be wrong about that, even on Earth. But for Bennu, where and how would this (size) distribution of rubble arise?
Planetesimal collisional rubble aggregate - bang the rock together,guys!
Are you saying (... planetesimals are generally larger than Bennu). At some point, as larger bodies than Bennu had aggregated, some of their collisions would produce smaller clumps like Bennu that broke off, that were never pulled back in to a larger body? Does this better explain the size distribution of rubble here than a notion that this body formed as its own aggregate, never having been part of a larger body?
MarkBour wrote: ↑Fri May 24, 2019 4:33 pm
One of many fascinating questions that this image evokes ... If I saw this rubble on Earth, I would assume that the smaller pieces were formed by the steady erosion breakdown of larger pieces. I'm no geologist, so maybe I would be wrong about that, even on Earth. But for Bennu, where and how would this (size) distribution of rubble arise?
Planetesimal collisional rubble aggregate - bang the rock together,guys!
Are you saying (... planetesimals are generally larger than Bennu). At some point, as larger bodies than Bennu had aggregated, some of their collisions would produce smaller clumps like Bennu that broke off, that were never pulled back in to a larger body? Does this better explain the size distribution of rubble here than a notion that this body formed as its own aggregate, never having been part of a larger body?
https://en.wikipedia.org/wiki/101955_Bennu#Origin_and_evolution wrote:
<<The carbonaceous material that composes Bennu originally came from the breakup of a much larger parent body—a planetoid or a proto-planet. According to the accretion theory, this material came together 4.5 billion years ago during the formation of the Solar System. Bennu's basic mineralogy and chemical nature would have been established during the first 10 million years of the Solar System's formation, where the carbonaceous material underwent some geologic heating and chemical transformation inside a much larger planetoid or a proto-planet capable of producing the requisite pressure, heat and of course the hydration (if need be)—into more complex minerals. Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family.>>
Click to play embedded YouTube video.
The breakup of a larger Austrian Naval parent body
https://en.wikipedia.org/wiki/142_Polana wrote:
<<Polana (minor planet designation: 142 Polana) is a very dark asteroid from the asteroid belt. It was discovered by Johann Palisa on January 28, 1875, and named after the city of Pola (now Pula, Croatia), home of the Austrian Naval Observatory where he made the discovery. It is a major member of the eponymously named Polana family, which is a subgroup of the Nysa family. The asteroid has an estimated diameter of about 55.3 km and a low albedo of 0.045. It is orbiting at a distance of 2.419 times the separation of the Earth from the Sun, with an orbital period of 3.76 years and an eccentricity of 0.14. In the Tholen classification scheme, Polana is a primitive carbonaceous asteroid of type F, which is a subdivision of more common C-type. The spectrum of this object suggests the presence of magnetite (Fe3O4), which gives it the spectrally blue coloration.>>
https://www.etymonline.com/word/boulder#etymonline_v_15658 wrote:
<<boulder (n.)1610s, "water-worn rounded stone of medium or large size," variant of Middle English bulder ston "stone worn round, cobblestone" (c. 1300), from a Scandinavian source akin to Swedish dialectal bullersten "noisy stone" (large stone in a stream, causing water to roar around it), from bullra "to roar" + sten "stone." Or the first element might be from *buller- "round object," from Proto-Germanic *bul-, from PIE root *bhel- (2) "to blow, swell." Specific geological sense "large weather-worn block of stone standing by itself" is from 1813.>>
Aha! So there are no boulders on Bennu?
(So "boulder" originates from a Swedish word, "bullersten"? Indeed, that sounds perfectly Swedish to me and makes perfect sense, but the word is completely unknown to me.)
Ann
Re: APOD: Boulders on Bennu (2019 May 24)
Posted: Tue May 28, 2019 4:43 pm
by MarkBour
neufer wrote: ↑Sat May 25, 2019 8:06 pm
... Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family.
It often amazes me, the level of sleuthery that astronomers have achieved in such matters.
I think it may be a symptom of having too much computer time on their hands.
neufer wrote: ↑Sat May 25, 2019 8:06 pm
... Bennu probably began in the inner asteroid belt as a fragment from a larger body with a diameter of 100 km. Simulations suggest a 70% chance it came from the Polana family and a 30% chance it derived from the Eulalia family.
It often amazes me, the level of sleuthery that astronomers have achieved in such matters.
I think it may be a symptom of having too much computer time on their hands.
Boulders on Bennu
