This article is from WeChat official account:nut shell (ID: Guokr42), author: haibaraemily, head Figure from: NASA span> p>
At 1:50 am Beijing time on October 21st (20th US time), NASA’s first asteroid sampling return mission OSIRIS- The REx(translated as Pluto) probe, will usher in the most exciting moment of the probe-first attempt to land a small Sampling of the surface of the planet Bennu .
If all goes well, 4 and a half hours later, at around 6:12 AM, Pluto will collect at least 60 grams of Bennu samples-this will be the most samples collected by humans from space since the Apollo mission the amount.
Sample of Hades | University of Arizona/Brittany Enos
And this “Benu” asteroid with a diameter of only about 500 meters, named after the phoenix in Egyptian mythology, was lingering by the Pluto probe.(Careful) After more than a year of circumnavigation and detection, it will finally usher in the “first intimate contact” with the Pluto.
For this day, the Pluto has exercised twice.
August 11, 2020, the “first-person” perspective of the Pluto’s second sampling exercise | NASA/Goddard/University of Arizona
Homework before sampling
“Benu” is a near-Earth asteroid, which means it has the opportunity to be very close to the earth. But it is too small after all, it is only a small bright spot in the ground-based telescope, and no details can be seen.
Bennu (green circle) taken by the 1.5-meter Kuiper Telescope at the University of Arizona in September 2011 | Carl Hergenrother/University of Arizona 
One of the biggest problems in exploring these little guys in the solar system is that people know almost nothing about these “targets” before the probe arrives. Therefore, as soon as you fly to the ground,Obviously it was too reckless and dangerous to start sampling. This delicate task can’t be rushed at all. (after all, the detector is so expensive and brittle).
Planetary scientists and engineers will devise a thorough exploration plan before the probe leaves, so that the probe will spend several months or even a year or two doing homework to fully understand the detection target, and then finally Determine the sampling location, sampling method and other details.
A recent photo of the asteroid Bennu taken by the Pluto | NASA
The plan of the Pluto is to fly around, that is, directly orbit the asteroid and complete various explorations.
On December 3, 2018, the Pluto probe arrived at the asteroid Bennu, and entered the orbit around Bennu on the 31st, flying around only 1.6 to 2.1 kilometers from Bennu.
This is also the first time that a human probe has successfully circled a small celestial body with such a small size and gravity for a long time.
The initial detection arrangement of the Pluto, compared with the next picture, you can see that many schedules have been adjusted later, and the landing sampling time has been postponed from the original August to October | Sinicized from: NASA & APL
For more than a year after that, the Pluto adjusted its orbit little by little, and “strikes” every inch of land on Bennu’s surface at various heights.
Some areas that need to be investigated (such as alternative sampling area), The flying height of Pluto can even be as low as 250 meters.
Benu, get close to you little by little|University of Arizona[ 3]
Through these close observations, Bennu, the small bright spot in the telescope, has become one of the most understood asteroids in the solar system.
A recent photo of the asteroid Bennu taken by Pluto | NASA
The PolyCam camera carried by the Pluto used 2155 photos to compose an ultra-high-definition global image of the asteroid Bennu with a resolution of up to 5 cm/pixel. It is the highest-definition foreign planet world ever acquired by mankind. Image map.
Mosaic of 2,155 photos taken by Pluto from March 7th to April 19th, 2019, from a distance of 3.1~5 kilometers from the surface of Bennu| NASA/Goddard/University of Arizona 
The Pluto also carries a laser altimeter similar to the LiDAR on the iPhone 12 Pro (OLA), which uses 20 million lasers for distance measurement The point data helped the Pluto build a high-resolution 3D terrain model of Bennu’s world.
Left: Pluto’s Laser Altimeter (OLA) | Canadian Space Agency, Right: 3D terrain model of asteroid Bennu obtained by OLA, The higher the red, the lower the blue, the difference is about 60 meters| NASA/University of Arizona/CSA/York/MDA 
The Pluto’s visible light and near-infrared spectrometer(OVIRS) found extensive distribution of hydrated minerals on the surface of Bennu and carbonaceous matter (organic matter and carbonSalt minerals) , This means that the samples collected by the Pluto may give us clues to explain the origin of water and organic matter on earth.
Bennu’s global hydrate and carbonaceous material spectral distribution map obtained by OVIRS. Band depth and band area have a certain indication effect on content, but they are also related to particle size, space weathering and other factors | References  sup>
These detection results have provided strong engineering data support and scientific value evaluation basis for the subsequent landing site selection of the Pluto.
Sampling area surrounded by huge rocks
The Pluto project team never expected that choosing a landing zone would be so difficult.
They might have thought that it would be difficult to select a sampling area as long as the detector went to the ground and did a frantic test! It’s nothing more than too many places with scientific value, you may commit a difficult choice syndrome.
Until the Pluto probe really saw the asteroid Bennu at close range, they were surprised to find that the previous observations and calculations through ground-based telescopes were a bit too good…
The surface of this asteroid is rugged and rocky. Take a look at the densely packed large and small rocks, how to let the Pluto get down (sampling arm)!
The surface of Bennu taken by Pluto’s OCAMS (PolyCam) camera at a distance of 5 kilometers from Benuyo on March 7, 2019. The frame is lower in the middle The light-colored stone is about 7.4 meters wide| NASA/Goddard/University of Arizona 
The cruel reality made it more difficult for the project team to find suitable sampling points, and it also brought more danger and uncertainty to the sampling of the Pluto landing.
but after repeated inspection and selection(how to drop it when you come) As far as possible, the Pluto project team selected 4 alternative landing areas from the surface of Bennu that are still safe and have high scientific value:
The locations of the 4 alternative landing sites, all named after birds| NASA/Goddard/University of Arizona 
Finally, the northernmost “Nightingale”(Nightingale) was elected the first sampling area of the Pluto .
The “Nightingale” sampling area is located in a 140-meter-diameter impact crater in the Northern Hemisphere| NASA/Goddard/University of Arizona 
But even the carefully selected sampling area is still very uncomfortable.
When the Pluto was designed, the project team originally hoped that the flat sampling area should be at least 50 meters in diameter. In this way, even if the detector is not so accurate, it can be somewhat fault-tolerant.
However, the reality is that even the most suitable sampling area for the Nightingale is just a small flat area surrounded by boulders. The landing area that allows safe sampling of the Pluto is only 6 meters in diameter.
The original planned sampling area size (orange circle, 50 meters in diameter) and the actual allowable safe sampling area size (blue circle, 6 meters in diameter), compare The size of the car|NASA/Goddard/University of Arizona 
If it can’t accurately land in this area, the Pluto probably hasn’t collected a sample, and accidentally knocked on a big rock during the landing and then rushed to the street.
Nightingale sampling area, a small flat area surrounded by boulders. Stitched from 345 images of the Hades | NASA/Goddard/University of Arizona 
In order to minimize contact with the big rocks, the Pluto will gather two solar panels into a Y shape during the descent.
The Pluto that shrinks the solar panel sampling: I was terrified.jpg | NASA
But it is not enough.
New landing navigation technology
The Pluto mission originally planned to use laser ranging (LiDAR) as the main navigation positioning method during the landing sampling phase. But in order to be more accurate, NASA finally abandoned LiDAR and switched to another set of alternative navigation and positioning solutions based on optical images-natural feature tracking( Natural Feature Tracking, NFT) technology.
During the landing process, the Pluto constantly took real-time photos of the vicinity of the landing area, compared with the existing three-dimensional terrain and image database, matched the characteristic rocks in the landing area as landmarks, and updated its real-time Location, speed and planned sampling location. If dangerous obstacles are detected in the captured images, the Pluto can also evacuate on its own. .
Natural Feature Tracking (NFT) technology of Pluto | © NASA/Goddard/University of Arizona  Sinicization: @haibaraemily
The Pluto probe has verified the reliability of NFT technology in two landing sampling exercises, and will also use this technology to assist in accurate landing during the actual landing sampling process.
Using Natural Feature Tracking (NFT) technology to descend the Pluto probe | NASA/Goddard/University of Arizona 
Just touch it and go, take away a handful of samples
Faced with the asteroid Bennu, which is only about 500 meters in diameter and has extremely small gravity, the Pluto used a “touch and leave” sampling(Touch and Go , TAG), that is-just touch and go.
The total planned time for the entire contact sampling process of the Pluto is about 4.5 hours, and a total of 3 ignition operations are required to land on the surface of Bennu from the original orbit.
1) At the first ignition, the Pluto departed from the orbit about 770 meters above Bennu’s surface and began to descend;
2) Second ignition: Ignite at a height of about 125 meters, check and adjust the position and speed of the Hades, also known as “check point ignition”;
3) The third ignition: ignition at an altitude of about 54 meters, adjust the flight parameters to match the rotation of Bennu during contact sampling, also known as “matching point ignition”;
4) Landing sampling: Landing on the surface of Bennu and contact sampling. After the sampling arm (TAGSAM) comes into contact with the Bennu surface, it emits a shot of compressed nitrogen to the Bennu surface in less than 16 seconds. The head of the sampling arm captures the Bennu surface material raised by the gas.
Schematic diagram of the head of the sampling arm during the sampling of the Hades | NASA[15 ]
5) The dust is gone: take off and return to the original altitude.
A schematic diagram of the Pluto descending to the sampling point and then ascending. There is no contact sampling step in this animation| NASA Goddard
Have you picked it up? Have you picked enough?
The Pluto plans to collect at least 60 grams of Bennu’s rock and dust samples and bring them back to Earth, possibly up to 2 kilograms. This will be the largest amount of samples humans have collected from space since the Apollo mission.
In contrast, Japan’s JAXA Hayabusa 2 probe, which is on the way back, collected only about 100 mg of samples from another asteroid.
An indication of the mass and volume of samples that may be collected by Pluto| NASA
So the question is, how to make sure that the detector has collected enough samples?
The Pluto project team plans to use “double insurance” to confirm:
1) On October 22, 2020, (US time, the same below), the Pluto will use a SamCam camera to photograph the sampling arm(TAGSAM) head photo, check whether it contains Bennu surface material;
SamCam camera confirmation sample| NASA/Goddard/CI Lab
2) On October 24, 2020, the Pluto will begin to “turn in a circle”-After unfolding the sampling arm, it will rotate to measure whether the collected sample weight meets the standard.
Rotate the “weigh” weight| NASA/Goddard/CI Lab [16 ]
If both tests show that the collection volume is up to the standard, these samples will be sent to the sample return cabin (SRC) to be sent back Earth.
“Packing” | NASA/Goddard/CI Lab 
If no samples were collected from the Nightingale sampling area this time, or not enough samples were collected… It’s okay, it can be saved!
There are also two cans of compressed nitrogen on the Pluto, which means that there are two more opportunities for sampling-It will be in the candidate sampling area “Osprey” as soon as January 2021(Osprey) Try TAG contact sampling again.
“Osprey” sampling area| NASA/Goddard/University of Arizona [ 10]
A new wave of sampling in outer space
The Pluto is scheduled to leave the asteroid Bennu in 2021, and will bring the collected samples back to Earth on September 24, 2023.
The Hayabusa 2, which is already on the way home, will take the lead in sending samples of the asteroid “Dragon Palace” back to Earth in December this year, and then start a new journey.
Pluto and Hayabusa 2 survey schedule| University of Arizona
Before that, China’s Chang’e 5 probe will also arrive first, collecting samples from the moon and returning it to the earth. After decades of silence, sampling and exploration of outer space ushered in another round of upsurge.
Space exploration, the future can be expected!
 NASA to Broadcast OSIRIS-REx Asteroid Sample Collection Activities
 ASTEROID BENNU, AS SEEN FROM EARTH
 OSIRIS-REx: Lowest Altitudes
 Bennu Global Mosaic
 OLA’s Complete 3D Map of Bennu
 Hamilton, V. E., Simon, A. A., Christensen, P. R., Reuter, D. C., Clark, B. E., Barucci, M.A., … & Connolly, H. C. (2019). Evidence for widespread hydrated minerals on asteroid (101955) Bennu. Nature Astronomy, 3(4), 332-340.
 Simon, AA, Kaplan, HH, Hamilton, VE, Lauretta, DS, Campins, H., Emery, JP, … & Golish, DR (2020 ). Widespread carbon-bearing materials on near-Earth asteroid (101955) Bennu. Science.
 Bennu’s Boulders and Limb from Detailed Survey
 NASA’s OSIRIS-REx in the Midst of Site Selection
 X MARKS THE SPOT: NASA SELECTS SITE FOR ASTEROID SAMPLE COLLECTION https://www.asteroidmission.org/?latest-news=x-marks-the-spot -nasa-selects-site-for-asteroid-sample-collection
 OSIRIS-REx: X Marks the Spot-2019 AGU Press Conference
 Nightingale Sampling Area
 Nightingale with Graphic of Spacecraft
 Bennu’s Boulders Shine as Beacons for NASA’s OSIRIS-REx
 TAGSAM Head Collecting Asteroid Sample
 NASA | OSIRIS-REx: Countdown to TAG
This article is from WeChat official account:Nutshell (ID: Guokr42) author: haibaraemily span> p>