APOD: Tagging Bennu (2020 Oct 22)

[APOD Robot]

Tagging Bennu

Explanation: On October 20, after a careful approach to the boulder-strewn surface, the OSIRIS-REx spacecraft's arm reached out and touched asteroid Bennu. Dubbed a Touch-And-Go (TAG) sampling event, the 30 centimeter wide sampling head (TAGSAM) appears to crush some of the rocks in this snapshot. The close-up scene was recorded by the spacecraft's SamCam some 321 million kilometers from planet Earth, just after surface contact. One second later, the spacecraft fired nitrogen gas from a bottle intended to blow a substantial amount of Bennu's regolith into the sampling head, collecting the loose surface material. Data show the spacecraft spent approximately 5 more seconds in contact with Bennu's Nightingale sample site and then performed its back-away burn. Timelapse frames from SamCam reveal the aftermath.

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[XgeoX]

Awesome! I would guess that they got there money’s worth judging by all that debris that blast of air stirred up. I especially enjoyed seeing that large chunk fly by the camera in I believe frame four of the video.
If they somehow didn’t get enough they have got two more bites at the apple before they have to quit. They can retrieve up to 4.4 pounds of material! They will settle, however, for 2 ounces.

https://www.nasa.gov/sites/default/file ... _kit_0.pdf



The mission’s namesake, the Egyptian God Osiris in a modern badarse imaging.

[MarkBour]

As I read about the TAG maneuver, the gas bottles, and the method of sample collection, it is all very creative. I suppose that up until OSIRIS-REx arrived at Bennu, there were a lot of unknowns, and this technology probably provided a high probability of at least gathering some sample under a lot of scenarios.

But now we've seen Bennu up close, and we've seen some other asteroids/comets up close as well. In the future, I think a high-quality scoop mechanism will work well and will retrieve as large of a sample as we want to carry home. 10 kg, 100 kg, whatever you want to design for.

I'd be happy if some NASA engineer were to explain to me if I'm wrong about that, but the whole asteroid appears to be a loosely packed pile of rubble. In fact, I wonder if one were to latch onto Gargoyle Saxum, how much thrust would be needed to pull it loose.

[Chris Peterson]

As I read about the TAG maneuver, the gas bottles, and the method of sample collection, it is all very creative. I suppose that up until OSIRIS-REx arrived at Bennu, there were a lot of unknowns, and this technology probably provided a high probability of at least gathering some sample under a lot of scenarios.

But now we've seen Bennu up close, and we've seen some other asteroids/comets up close as well. In the future, I think a high-quality scoop mechanism will work well and will retrieve as large of a sample as we want to carry home. 10 kg, 100 kg, whatever you want to design for.

I'd be happy if some NASA engineer were to explain to me if I'm wrong about that, but the whole asteroid appears to be a loosely packed pile of rubble. In fact, I wonder if one were to latch onto Gargoyle Saxum, how much thrust would be needed to pull it loose.

Many asteroids are clearly rubble piles, not solid objects. Bennu has a bulk density of 1.26 g/cm³, despite being made up of material with a density of at least 3 g/cm³. That's a lot of empty space inside.

[Phobos1]

Two comments:
Those are some awfully sharp shards Osiris sampled. It doesn't look like there's been much shifting or micro erosive impacts on the surface.

Is that a plastic tarp around the sampling arm? It sure looks it.

[TheOtherBruce]

Would it be safe to assume that this rubble has the same source as material that lands on Earth as meteorites? I was wondering what could be inferred just from comparing the appearance of these chunks with stuff that's been cooked by entering the atmosphere. Could we guesstimate which bits might be pristine condensations from the protoplanetary disk, and which might be chips off of older, bigger blocks?

[johnnydeep]

As I read about the TAG maneuver, the gas bottles, and the method of sample collection, it is all very creative. I suppose that up until OSIRIS-REx arrived at Bennu, there were a lot of unknowns, and this technology probably provided a high probability of at least gathering some sample under a lot of scenarios.

But now we've seen Bennu up close, and we've seen some other asteroids/comets up close as well. In the future, I think a high-quality scoop mechanism will work well and will retrieve as large of a sample as we want to carry home. 10 kg, 100 kg, whatever you want to design for.

I'd be happy if some NASA engineer were to explain to me if I'm wrong about that, but the whole asteroid appears to be a loosely packed pile of rubble. In fact, I wonder if one were to latch onto Gargoyle Saxum, how much thrust would be needed to pull it loose.

Many asteroids are clearly rubble piles, not solid objects. Bennu has a bulk density of 1.26 g/cm³, despite being made up of material with a density of at least 3 g/cm³. That's a lot of empty space inside.

Gargoyle Saxum is pretty darn big! This says the small lighter rock sitting on Gargoyle Saxum is 6.2 ft long: https://www.asteroidmission.org/2019040 ... -gargoyle/

Angled View of Gargoyle Saxum
This image shows an angled view of one of the darkest boulders on asteroid Bennu, called Gargoyle Saxum. The boulder, located near the asteroid’s equator, has an albedo of 3.3%. The image was taken by the PolyCam camera on NASA’s OSIRIS-REx spacecraft on April 5 from a distance of 1.7 miles (2.8 km). The field of view is 131 ft (40 m). For scale, the small, light-colored rock sitting on Gargoyle is 6.2 ft (1.9 m) long, which is about the height of a person. The image was obtained during Flyby 5B of the mission’s Detailed Survey: Baseball Diamond phase. When the image was taken, the spacecraft was over the southern hemisphere, pointing PolyCam north and west.

Click to view full size image

[Chris Peterson]

Gargoyle Saxum is pretty darn big! This says the small lighter rock sitting on Gargoyle Saxum is 6.2 ft long: https://www.asteroidmission.org/2019040 ... -gargoyle/

Take it as approximately 10 m x 10 m x 20 m, for a volume of 2000 m³. Assume a density of 3000 kg/m³, so a mass of 6 million kg. At Bennu's surface gravity of 98 μm/s², that's a weight of 350 N. (Equivalent to about 36 kg on Earth.)

[johnnydeep]

Gargoyle Saxum is pretty darn big! This says the small lighter rock sitting on Gargoyle Saxum is 6.2 ft long: https://www.asteroidmission.org/2019040 ... -gargoyle/

Take it as approximately 10 m x 10 m x 20 m, for a volume of 2000 m³. Assume a density of 3000 kg/m³, so a mass of 6 million kg. At Bennu's surface gravity of 98 μm/s², that's a weight of 350 N. (Equivalent to about 36 kg on Earth.)

Hmm. So it would seem that Osiris-Rex itself could easily lift that rock away from the rest of the asteroid (provided it's only stuck there by gravity)! At least according to this - https://spaceflight101.com/osiris-rex/o ... -overview/:

OSIRIS-REx is equipped with four Aerojet Rocketdyne MR-107S thrusters installed in a single bank on the base of the spacecraft to be able to deliver thrust for large delta-v maneuvers – the deep space maneuvers setting up for the Earth Flyby, the Asteroid Approach Braking Burn and the Asteroid Departure Burn.

Each of these engines provides a nominal thrust of 275 Newtons with a throttle capability of 85 to 360 Newtons. With the six engines at nominal throttle, OSIRIS-REx has a total thrust of 1,100 Newtons – 112 Kilogram-force.

MR-107S operates at a propellant feed pressure of 7 to 35 bar and a chamber pressure of 4 to 14 bar to create a specific impulse of 225 to 236 seconds. The engine ingests 36 to 155 grams of fuel per second depending on the thrust setting. It has an expansion ratio of 21.5.

Yes, I guess it's all just physics, but I'm still skeptical due to the huge mass, actual weight on the surface of Bennu notwithstanding. I assume separating Gargoyle Saxum from Bennu and getting it back to earth orbit are two completely different problems, the later being much harder to do.

[Astronymus]

Chunks 'o' Space. Looking forward for the results.

[Chris Peterson]

Gargoyle Saxum is pretty darn big! This says the small lighter rock sitting on Gargoyle Saxum is 6.2 ft long: https://www.asteroidmission.org/2019040 ... -gargoyle/

Take it as approximately 10 m x 10 m x 20 m, for a volume of 2000 m³. Assume a density of 3000 kg/m³, so a mass of 6 million kg. At Bennu's surface gravity of 98 μm/s², that's a weight of 350 N. (Equivalent to about 36 kg on Earth.)

Hmm. So it would seem that Osiris-Rex itself could easily lift that rock away from the rest of the asteroid (provided it's only stuck there by gravity)! At least according to this - https://spaceflight101.com/osiris-rex/o ... -overview/:

OSIRIS-REx is equipped with four Aerojet Rocketdyne MR-107S thrusters installed in a single bank on the base of the spacecraft to be able to deliver thrust for large delta-v maneuvers – the deep space maneuvers setting up for the Earth Flyby, the Asteroid Approach Braking Burn and the Asteroid Departure Burn.

Each of these engines provides a nominal thrust of 275 Newtons with a throttle capability of 85 to 360 Newtons. With the six engines at nominal throttle, OSIRIS-REx has a total thrust of 1,100 Newtons – 112 Kilogram-force.

MR-107S operates at a propellant feed pressure of 7 to 35 bar and a chamber pressure of 4 to 14 bar to create a specific impulse of 225 to 236 seconds. The engine ingests 36 to 155 grams of fuel per second depending on the thrust setting. It has an expansion ratio of 21.5.

Yes, I guess it's all just physics, but I'm still skeptical due to the huge mass, actual weight on the surface of Bennu notwithstanding. I assume separating Gargoyle Saxum from Bennu and getting it back to earth orbit are two completely different problems, the later being much harder to do.

Moving it isn't the issue. Moving it enough to be useful for some end probably is. It's a delta-v problem. Say we attach and thrust at a conservative 300 N. That force will result in an acceleration of 0.00005 m/s². How long can we sustain such a burn until we run out of fuel? If we could do it for an hour (which seems doubtful), we would impart a delta-v of 18 cm/s (0.6 km/h). Not sure what to do with that. It would be a long trip back to Earth!

That said, if all you wanted to do was pull it out a few meters to inspect what was behind it, that seems feasible.

[MarkBour]

. . .

Thanks for the calculations and discussion, Chris and Johnny!

So, Gargoyle Saxum would be not too hard to budge, because the gravity on Bennu is incredibly weak. But it would take lots of fuel to do much with anything that large, or to do much to Bennu itself, with a technique of pushing by a rocket.

I enjoyed the article https://www.businessinsider.com/asteroi ... net-2018-6. There I read that any earth visitor larger than a car is a problem (as a rule of thumb). With its approximately 500m diameter, Bennu would have a terrifying impact. The Wikipedia article on Bennu estimates 1200 megatons. https://en.wikipedia.org/wiki/101955_Bennu

[Chris Peterson]

. . .

Thanks for the calculations and discussion, Chris and Johnny!

So, Gargoyle Saxum would be not too hard to budge, because the gravity on Bennu is incredibly weak. But it would take lots of fuel to do much with anything that large, or to do much to Bennu itself, with a technique of pushing by a rocket.

I enjoyed the article https://www.businessinsider.com/asteroi ... net-2018-6. There I read that any earth visitor larger than a car is a problem (as a rule of thumb). With its approximately 500m diameter, Bennu would have a terrifying impact. The Wikipedia article on Bennu estimates 1200 megatons. https://en.wikipedia.org/wiki/101955_Bennu

Figure that an object needs to be larger than 10 m for pieces to make it to the ground still carrying any of their original velocity (and therefore capable of doing significant damage to anything they hit). Figure larger than 100 m to create a significant cratering event, like Meteor Crater in Arizona, which is similar to a small atomic explosion in terms of damage. Just local at the scale of a city. About 1000 m to cause regional damage (over a good chunk of a continent, for instance) and minor worldwide climate effects lasting a few years. A few kilometers and you're getting into the sort of events that create mass extinctions and long term climate change.

[johnnydeep]

Thanks for the calculations and discussion, Chris and Johnny!

So, Gargoyle Saxum would be not too hard to budge, because the gravity on Bennu is incredibly weak. But it would take lots of fuel to do much with anything that large, or to do much to Bennu itself, with a technique of pushing by a rocket.

I enjoyed the article https://www.businessinsider.com/asteroi ... net-2018-6. There I read that any earth visitor larger than a car is a problem (as a rule of thumb). With its approximately 500m diameter, Bennu would have a terrifying impact. The Wikipedia article on Bennu estimates 1200 megatons. https://en.wikipedia.org/wiki/101955_Bennu

Figure that an object needs to be larger than 10 m for pieces to make it to the ground still carrying any of their original velocity (and therefore capable of doing significant damage to anything they hit). Figure larger than 100 m to create a significant cratering event, like Meteor Crater in Arizona, which is similar to a small atomic explosion in terms of damage. Just local at the scale of a city. About 1000 m to cause regional damage (over a good chunk of a continent, for instance) and minor worldwide climate effects lasting a few years. A few kilometers and you're getting into the sort of events that create mass extinctions and long term climate change.

Ha! Just noticed your ID pic

[neufer]

I assume separating Gargoyle Saxum from Bennu and getting it back to earth orbit are two completely different problems, the later being much harder to do.

Moving it isn't the issue. Moving it enough to be useful for some end probably is. It's a delta-v problem. Say we attach and thrust at a conservative 300 N. That force will result in an acceleration of 0.00005 m/s². How long can we sustain such a burn until we run out of fuel? If we could do it for an hour (which seems doubtful), we would impart a delta-v of 18 cm/s (0.6 km/h). Not sure what to do with that. It would be a long trip back to Earth!

• A delta-v of 18 cm/s amounts to up to ~225,000 km by 2040 2060
  this should allow Gargoyle Saxum to be put on a path to intercept Earth in 2135.

<<Bennu will pass 750,000 km from Earth on 23 September 2060, while Moon's average orbital distance (Lunar Distance, LD) is 384,403 km. It will be too dim to be seen with common binoculars. The close approach of 2060 causes divergence in the close approach of 2135. On 25 September 2135, the nominal approach distance is 300,000 km, but Bennu could pass as close as 100,000 km. There is no chance of an Earth impact in 2135. 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.>>

[Chris Peterson]

I assume separating Gargoyle Saxum from Bennu and getting it back to earth orbit are two completely different problems, the later being much harder to do.

Moving it isn't the issue. Moving it enough to be useful for some end probably is. It's a delta-v problem. Say we attach and thrust at a conservative 300 N. That force will result in an acceleration of 0.00005 m/s². How long can we sustain such a burn until we run out of fuel? If we could do it for an hour (which seems doubtful), we would impart a delta-v of 18 cm/s (0.6 km/h). Not sure what to do with that. It would be a long trip back to Earth!

• A delta-v of 18 cm/s amounts to up to ~225,000 km by 2040
  this should allow Gargoyle Saxum to be put on a path to intercept Earth in 2135.

Or to prevent a future collision. There are a variety of methods proposed for doing just that which are effective after applying very tiny delta-v corrections, given enough time.

[BobStein-VisiBone]

A much better animation of the contact (101MB):

https://svs.gsfc.nasa.gov/vis/a010000/a ... _37x_2.gif

(be patient, after the first loop the animation will be quicker and smoother)

Great discussion of the event by Scott Manley (12 min):

https://youtu.be/cmQfWuFbLNg

[TheOtherBruce]

A much better animation of the contact (101MB):

https://svs.gsfc.nasa.gov/vis/a010000/a ... _37x_2.gif

Impressive; looks like the gas blast made a crater a good few feet across. I wonder how easy it will be to spot once OSIRIS-REx gets back to a higher orbit.