APOD: Rotating Asteroid Bennu from OSIRIS-REx (2018 Nov 13)

[APOD Robot]

Rotating Asteroid Bennu from OSIRIS-REx

Explanation: Could this close-by asteroid ever hit the Earth? Eventually yes -- but probably not for a very long time, even though the asteroid is expected to pass inside the orbit of the Moon next century. However, to better understand the nature and orbit of all near-Earth asteroids, NASA sent the robotic Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer (OSIRIS-REx) to investigate this one: the 500-meter across asteroid 101955 Bennu. Launched in 2016, OSIRIS-REx is now approaching Bennu, and is first scheduled to map the minor planet's rough surface. The featured time-lapse video taken earlier this month compacts Bennu's 4.25-hour rotation period into about 7 seconds. Bennu's diamond-like appearance is similar to asteroid Ryugu currently being visited by the Japanese spacecraft Hayabusa2. The exact future orbit of Bennu is a bit uncertain due to close passes near the Earth and the Yarkovsky effect: a slight force created by an object's rotationally-induced, asymmetric infrared glow. If all goes according to plan, ORISIS-Rx will actually touch the asteroid in 2020, collect soil samples, and return them to Earth in 2023 for detailed analyses.

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[Ann]

Bennu is rotating. Wouldn't it have been cool if it had been tumbling?

'Oumuamua is tumbling, which I find really fascinating.

I apologize for the silliness of the Youtube video below, but I just had to post something that showed the difference between tumbling and rotating.

https://www.youtube.com/watch?v=XWml_BoI0Mc

Ann

[Confused]

Bennu's diamond-like appearance

I think it looks more like a Grey tater tot except after looking at Google Images it looks more like a Sweet Potato tater tot. There are many images to choose from but I can't find one that is not commercial in some way.

[neufer]

viewtopic.php?f=8&t=18688&p=287008#p287008

[Astronymus]

viewtopic.php?f=8&t=18688&p=287008#p287008

Thanks for the link. I guessed there is a reason for these similarities. I can imagine we'll find more planetoids like these two in the future.

[MarkBour]

Nice images. I love the boulders littered on the surface. Would this body qualify as a rubble heap? Perhaps easily crumbled? It is so small, I assume we have the capability to alter its trajectory with conventional propulsion ... should the need arise. And, if OSIRIS is going to go bump into it, take a scoop, and then push off of it, we may create the problem ourselves. But, I have confidence that NASA scientists already thought about that before launching this mission. Indeed, part of the mission objectives involve fine measurements of Bennu's orbit and small effects like the Yarkovsky effect. Bennu has a mass of about 60-80 billion kg.

[starsurfer]

So cool to see this!

Also noticed this mistake in the last sentence:

ORISIS-Rx

[neufer]

---

Also noticed this mistake in the last sentence: ORISIS-Rx

<<'℞' is a symbol meaning "recipe". It is sometimes transliterated as "Rx" or just "Rx". This symbol originated in medieval manuscripts as an abbreviation of the late Latin verb recipere, specifically the second person singular imperative form recipe meaning "take", thus: "take thou". Originally abbreviated Rc, the c was simplified and finally written as a straight stroke making it look like an x in combination with the right "leg" of R. Medieval prescriptions invariably began with the command to "take" certain materials and compound them in specified ways. Folk theories about the origin of the symbol '℞' note its similarity to the Eye of Horus, or to the ancient symbol for Zeus or Jupiter, (♃), gods whose protection may have been sought in medical contexts.>>

[BDanielMayfield]

Nice images. I love the boulders littered on the surface. Would this body qualify as a rubble heap? Perhaps easily crumbled? It is so small, I assume we have the capability to alter its trajectory with conventional propulsion ... should the need arise. And, if OSIRIS is going to go bump into it, take a scoop, and then push off of it, we may create the problem ourselves. But, I have confidence that NASA scientists already thought about that before launching this mission. Indeed, part of the mission objectives involve fine measurements of Bennu's orbit and small effects like the Yarkovsky effect. Bennu has a mass of about 60-80 billion kg.

It certainly is a rubble heap, and it's low gravity is barely able to hold it together I would guess.

Better not bump it too hard... Imagine the liability if it was accidentally pushed into an Earth intersecting orbit... The ultimate butterfly effect.

[ta152h0]

Our already great country landed on Eros, within my memory bank.

[Chris Peterson]

Nice images. I love the boulders littered on the surface. Would this body qualify as a rubble heap? Perhaps easily crumbled? It is so small, I assume we have the capability to alter its trajectory with conventional propulsion ... should the need arise. And, if OSIRIS is going to go bump into it, take a scoop, and then push off of it, we may create the problem ourselves. But, I have confidence that NASA scientists already thought about that before launching this mission. Indeed, part of the mission objectives involve fine measurements of Bennu's orbit and small effects like the Yarkovsky effect. Bennu has a mass of about 60-80 billion kg.

It certainly is a rubble heap, and it's low gravity is barely able to hold it together I would guess.

Well, it's a good bet. But I'd be cautious with "certainly". By looking at the way the body influences the probe's orbit and trajectory when it gets close it should be possible to measure its density, and therefore determine if it is porous (a rubble pile) or is a fairly solid stony object. The current density estimate, which indicates a highly porous object, is based on modeling the orbital behavior due to the Yarkovsky effect. This should be pretty reliable, but not as good as a direct measurement obtained by orbiting the body.

Better not bump it too hard... Imagine the liability if it was accidentally pushed into an Earth intersecting orbit... The ultimate butterfly effect.

Of course, the tiniest bump could as easily prevent a collision with Earth. That's the thing about chaos theory. No way to tell.

[neufer]

Better not bump it too hard... Imagine the liability if it was accidentally pushed into an Earth intersecting orbit... The ultimate butterfly effect.

Of course, the tiniest bump could as easily prevent a collision with Earth. That's the thing about chaos theory. No way to tell.

Click to play embedded YouTube video.

1. Bennu will pass 0.005au from Earth on 23 September 2060. The close approach of 2060 causes divergence in the close approach of 2135.
   
   2. On 25 September 2135 the nominal approach distance is 0.002au from Earth, but Bennu could pass as close as 0.0007au.
   (There is no chance of an Earth impact in 2135.)
   
   3. The 2135 approach will create many lines of variations and Bennu may pass through a gravitational keyhole during the 2135 passage which could create an impact scenario at a future encounter. The keyholes are all less than 55 km wide.

In order to move Bennu by ~0.001au before its next encounter with Earth Bennu's velocity must be sped up or slowed down by ~11 cm/sec.

This would require a momentum impulse of ~7,500,000,000 kg m/sec.

The Paris Gun used to bombard Paris during World War I generated a momentum impulse of ~175,000 kg m/sec.

Hence, a Bennu/Paris Gun would have to discharge over 40,000 times to have a significant effect for the near future.

[gwarstadt]

Is anyone else struck by the fact that the horizontal axis of this non spherical object is exactly half the vertical axis, and its so symmetrical that if you watch an edge as it rotates the profile hardly moves (except for the big rocks)?

[neufer]

Is anyone else struck by the fact that the horizontal axis [sic...you mean radius ?] of this non spherical object is exactly half the vertical axis, and its so symmetrical that if you watch an edge as it rotates the profile hardly moves (except for the big rocks)?

101955 Bennu / 162173 Ryugu

---

Hey Bennu, have we met?
I never forget an asteroid.
Contributed by Phil Plait @BadAstron

Comparing two small asteroids: 550-m-wide Bennu (left), and 1-km-wide Ryugu (right) roughly to scale.

Credit: Ryugu: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST.

101955 Bennu / Earth

---

162173 Ryugu / Earth

---

Note that both retrograde rotating asteroids (i.e., Bennu & Ryugu) have (in the past) regularly passed through Earth's L5 Lagrangian point before then passing sun-wards of the Earth.

Have these asteroids picked up (co-moving) dust trapped in Earth's L5 Lagrangian point (and/or generated a cloud of dust due to larger impacts).

And have such captured dust clouds been immediately given a clockwise rotational spin by passing sun-wards of the Earth

Such retrograde rotating dust rings would then slowly be deposited onto the asteroids so as to define their equators.

[MarkBour]

Better not bump it too hard... Imagine the liability if it was accidentally pushed into an Earth intersecting orbit... The ultimate butterfly effect.

Of course, the tiniest bump could as easily prevent a collision with Earth. That's the thing about chaos theory. No way to tell.

https://www.youtube.com/watch?v=6a6rAa34H9Y

1. Bennu will pass 0.005au from Earth on 23 September 2060. The close approach of 2060 causes divergence in the close approach of 2135.
   
   2. On 25 September 2135 the nominal approach distance is 0.002au from Earth, but Bennu could pass as close as 0.0007au.
   (There is no chance of an Earth impact in 2135.)
   
   3. The 2135 approach will create many lines of variations and Bennu may pass through a gravitational keyhole during the 2135 passage which could create an impact scenario at a future encounter. The keyholes are all less than 55 km wide.

In order to move Bennu by ~0.001au before its next encounter with Earth Bennu's velocity must be sped up or slowed down by ~11 cm/sec.

This would require a momentum impulse of ~7,500,000,000 kg m/sec.

The Paris Gun used to bombard Paris during World War I generated a momentum impulse of ~175,000 kg m/sec.

Hence, a Bennu/Paris Gun would have to discharge over 40,000 times to have a significant effect for the near future.

I just stumbled upon the site: https://cneos.jpl.nasa.gov/ which I find really wonderful. They have a nice set of data and you can run the data in animations for many near-earth objects. I could learn a lot from that site, if I were to give it the time it deserves. What I did learn is that 101955 Bennu has an elliptical orbit similar to Musk's roadster -- running from just a little inside Earth's orbit to nearly Mars' orbit. But I don't think it ever has close encounters with Mars, it is probably not in Mars' orbital plane when it approaches. It can get arbitrarily close to Earth, crossing our orbit, and the close encounters should be about once every 6 years or so.

Back to your momentum calculation. I'll gladly accept that you did the math to determine that it would take an impulse of about 7.5x10⁹ Ns to slow Bennu enough to displace it by 0.001 AU. I assume you figured this displacement in a period that would be about 5 orbits for Bennu = 6 orbits for Earth. (?) And that much displacement ought to be enough to avoid an encounter. The first stage of the Saturn V, I believe, could deliver 9x10⁹ Ns impulse, so that would mean we must have the capability, in theory, to give such a push with conventional tech.

Of course it is a lot easier to build and launch a Saturn V from Florida than to deliver a working Saturn V to an object in space, fully loaded with fuel. So this would require a lot more rockets and would beg a myriad of other questions as well.

(Examples: When would be the best time for such a mission. I assume it would be most efficient to try to intercept it at perigee #(n-1), if we wanted to push it with something between then and perigee #n? If we wanted to move such a pile of rubble, we might rather separate all of the engines of a Falcon-Heavy-like rocket and place them all in a large array. Build something more like a rocket-powered girdle. I wonder if nudging such an object out of our orbital plane is easier than nudging it backward or forward? It would seem a more permanent solution, upon first consideration. Then again, adjusting its perihelion to be > Earth's aphelion is another idea.

[MarkBour]

Is anyone else struck by the fact that the horizontal axis of this non spherical object is exactly half the vertical axis, and its so symmetrical that if you watch an edge as it rotates the profile hardly moves (except for the big rocks)?

The image I am looking at is about 426px tall and 396px wide, so it is relatively close to 1:1, not 1:2.

[MarkBour]

101955 Bennu / 162173 Ryugu

---

Hey Bennu, have we met?
I never forget an asteroid.
Contributed by Phil Plait @BadAstron

Comparing two small asteroids: 550-m-wide Bennu (left), and 1-km-wide Ryugu (right) roughly to scale.

Credit: Ryugu: JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, University of Aizu and AIST.

I might have thought so, once long ago, but now I cannot make this boast.
The other day I ran into 10164472, and completely mistook her for 1829147c. Really embarrassing.

[neufer]

I just stumbled upon the site: https://cneos.jpl.nasa.gov/ which I find really wonderful. They have a nice set of data and you can run the data in animations for many near-earth objects. I could learn a lot from that site, if I were to give it the time it deserves. What I did learn is that 101955 Bennu has an elliptical orbit similar to Musk's roadster -- running from just a little inside Earth's orbit to nearly Mars' orbit. But I don't think it ever has close encounters with Mars, it is probably not in Mars' orbital plane when it approaches. It can get arbitrarily close to Earth, crossing our orbit, and the close encounters should be about once every 6 years or so.

Back to your momentum calculation. I'll gladly accept that you did the math to determine that it would take an impulse of about 7.5x10⁹ Ns to slow Bennu enough to displace it by 0.001 AU. I assume you figured this displacement in a period that would be about 5 orbits for Bennu = 6 orbits for Earth. (?) And that much displacement ought to be enough to avoid an encounter.

• Bennu has an orbital period of ~436.65 days or ~104/87 years
  
  After 5 Bennu orbits (~6 years) the Earth lags by ~0.12 au (too far to matter).
  
  After 87 Bennu orbits (~104 years) the Earth leads by ~0.03 au.

However, I was primarily interested with any possible perturbation with current OSIRIS-Rex activities
affecting the next close approach in 2060 (~104 years after the last relatively close approach in 1956).

• Note: Bennu's orbital period of ~436.65 days changes after each relatively
  close approach making the next close approach after just 75 years (in 2135).

The first stage of the Saturn V, I believe, could deliver 9x10⁹ Ns impulse, so that would mean we must have the capability, in theory, to give such a push with conventional tech. Of course it is a lot easier to build and launch a Saturn V from Florida than to deliver a working Saturn V to an object in space, fully loaded with fuel. So this would require a lot more rockets and would beg a myriad of other questions as well.

Actually, a catapult that could launch 0.1% of Bennu rocks at 110 m/s makes a lot of sense.
(Or having Bruce Willis bury a nuclear charge to blow off the front or back 0.1% of Bennu into space.)

(Examples: When would be the best time for such a mission. I assume it would be most efficient to try to intercept it at perigee #(n-1), if we wanted to push it with something between then and perigee #n? If we wanted to move such a pile of rubble, we might rather separate all of the engines of a Falcon-Heavy-like rocket and place them all in a large array. Build something more like a rocket-powered girdle. I wonder if nudging such an object out of our orbital plane is easier than nudging it backward or forward? It would seem a more permanent solution, upon first consideration. Then again, adjusting its perihelion to be > Earth's aphelion is another idea.

All plans on moving asteroids involve having it sped up or slowed down in order to throw the timing off.

[MarkBour]

...
However, I was primarily interested with any possible perturbation with current OSIRIS-Rex activities
affecting the next close approach in 2060 (~104 years after the last relatively close approach in 1956).
...
Actually, a catapult that could launch 0.1% of Bennu rocks at 110 m/s makes a lot of sense.
(Or having Bruce Willis bury a nuclear charge to blow off the front or back 0.1% of Bennu into space.)
...
All plans on moving asteroids involve having it sped up or slowed down in order to throw the timing off.

Thanks for the answers!

RE: A catapult. If Bennu is truly rubble, I really like the simplicity of the idea, and it would be able to do a little at a time until it got the job done, perhaps not needing fuel? So, it is a brilliant plan on those features, at least.

A catapult would not change the total net momentum, of course. It sounds like it would effectively turn it into a disintegrating "comet". And the way I'm picturing this, it would just make a huge mess out of the launched chunks, as a sort of discrete cometary tail, right? I suppose some of those pieces would have to hit Earth, even as they would allow Bennu's main body to miss. On the bright side, maybe there is something worth mining and trying to get into Earth orbit. Or to even crash into the Moon. Would anyone ever want to play that dangerous game with large chunks? I can scarce imagine the possible long-term effects of such actions.

Anyway, if you send Bruce Willis to do the job, keep in mind that you're going to need to send his whole crew. Non-negotiable. They needed Ben Affleck ("A.J. saved us. He did."), and even more importantly, they needed Steve Buscemi for comic relief.

[neufer]

...
However, I was primarily interested with any possible perturbation with current OSIRIS-Rex activities
affecting the next close approach in 2060 (~104 years after the last relatively close approach in 1956).
...
Actually, a catapult that could launch 0.1% of Bennu rocks at 110 m/s makes a lot of sense.
(Or having Bruce Willis bury a nuclear charge to blow off the front or back 0.1% of Bennu into space.)
...
All plans on moving asteroids involve having it sped up or slowed down in order to throw the timing off.

Thanks for the answers!

RE: A catapult. If Bennu is truly rubble, I really like the simplicity of the idea, and it would be able to do a little at a time until it got the job done, perhaps not needing fuel? So, it is a brilliant plan on those features, at least.

A catapult would not change the total net momentum, of course. It sounds like it would effectively turn it into a disintegrating "comet". And the way I'm picturing this, it would just make a huge mess out of the launched chunks, as a sort of discrete cometary tail, right? I suppose some of those pieces would have to hit Earth, even as they would allow Bennu's main body to miss. On the bright side, maybe there is something worth mining and trying to get into Earth orbit. Or to even crash into the Moon. Would anyone ever want to play that dangerous game with large chunks? I can scarce imagine the possible long-term effects of such actions.

Anyway, if you send Bruce Willis to do the job, keep in mind that you're going to need to send his whole crew. Non-negotiable. They needed Ben Affleck ("A.J. saved us. He did."), and even more importantly, they needed Steve Buscemi for comic relief.

If the catapult could do the job at hand within the first few years then:

• 1) Bennu would be moved by ~0.001 au by 2060 and
  2) the ejecta would all be moved by ~1.00 au in the opposite direction by 2060.

OTOH: If Bruce, Ben & Steve did a high precision job then
the ejecta would be spread out from 0 to 2 au in the opposite direction by 2060.

Either case would be preferable to inaction IMO.

Note that speeding up or slowing down Bennu produces a cumulative effect over the full 4 decades until 2060;
whereas changing the direction of Bennu is only effective for a quarter of an orbit (i.e., 0.30 years).

[Chris Peterson]

...
However, I was primarily interested with any possible perturbation with current OSIRIS-Rex activities
affecting the next close approach in 2060 (~104 years after the last relatively close approach in 1956).
...
Actually, a catapult that could launch 0.1% of Bennu rocks at 110 m/s makes a lot of sense.
(Or having Bruce Willis bury a nuclear charge to blow off the front or back 0.1% of Bennu into space.)
...
All plans on moving asteroids involve having it sped up or slowed down in order to throw the timing off.

Thanks for the answers!

In reality, however, you don't necessarily need anywhere near that impulse... or, you need a lot more. We don't know where Bennu will be a few orbits from now. A trivially small impulse now could have a dramatic effect on its position in 100 years. We wouldn't be able to do anything until a collision was much nearer in time, and then we'd have to do more, of course.

[neufer]

...
However, I was primarily interested with any possible perturbation with current OSIRIS-Rex activities
affecting the next close approach in 2060 (~104 years after the last relatively close approach in 1956).
...
Actually, a catapult that could launch 0.1% of Bennu rocks at 110 m/s makes a lot of sense.
(Or having Bruce Willis bury a nuclear charge to blow off the front or back 0.1% of Bennu into space.)
...
All plans on moving asteroids involve having it sped up or slowed down in order to throw the timing off.

In reality, however, you don't necessarily need anywhere near that impulse... or, you need a lot more. We don't know where Bennu will be a few orbits from now. A trivially small impulse now could have a dramatic effect on its position in 100 years. We wouldn't be able to do anything until a collision was much nearer in time, and then we'd have to do more, of course.

My own plan is to talk Bennu to death.

[BDanielMayfield]

My own plan is to talk Bennu to death.

In furtherance of neufer's (laudable,laughable?) plan I'll pose a hypothetical question, to wit:

Given two asteroids of equal mass and Earth impacting trajectory, one of which is a loose rubble pile and one is a solid body, which poses the greater threat, and which threat would be easier to ward off?

[Whomever]

My own plan is to talk Bennu to death.

In furtherance of neufer's (laudable,laughable?) plan I'll pose a hypothetical question, to wit:

Given two asteroids of equal mass and Earth impacting trajectory, one of which is a loose rubble pile and one is a solid body, which poses the greater threat, and which threat would be easier to ward off?

The rock would be much easier to talk down since there's only one. You couldn't get a whole pile of rocks to agree on anything with each other let alone with a fast talking logophile (except of course their mutual attraction).

[Chris Peterson]

My own plan is to talk Bennu to death.

In furtherance of neufer's (laudable,laughable?) plan I'll pose a hypothetical question, to wit:

Given two asteroids of equal mass and Earth impacting trajectory, one of which is a loose rubble pile and one is a solid body, which poses the greater threat, and which threat would be easier to ward off?

It depends on the mass. Above a certain point, the composition won't make much difference in terms of impact damage. At smaller sizes (maybe a few hundred meters or less) the lower higher porosity object may break up higher, with less mass reaching the ground at a hypersonic velocity. An airburst can cause a lot of local damage, but a large impact affects the entire planet.

Which one is easier to deal with while still in space probably depends upon the method used. Using some kind of thrusters seems likely to be best for a solid body. Fragmenting it with a bomb might be best for a rubble pile. Methods like painting one side or parking a big mass in orbit around it should work about the same for either.