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---------------------- 2323 - ASTERIOID - Bennu and Ryugu -
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- A NASA spacecraft that will return a sample of a near-Earth asteroid named Bennu to Earth in 2023 made the first-ever close-up observations of particle plumes erupting from an asteroid’s surface.
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- Asteroid Bennu also revealed itself to be more rugged than expected, challenging the mission team to alter its flight and sample collection plans, due to the rough terrain.
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- This OSIRIS-REx mission began orbiting the asteroid on Dec. 31, 2018. Bennu, which is only slightly wider than the height of the Empire State Building, may contain unaltered material from the very beginning of our solar system., over 4,500,000,000 years old.
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- The discovery of plumes is one of the biggest surprises. And , the rugged terrain went against all earlier predictions of what astronomers expected to find.
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- Shortly after the discovery of the particle plumes on Jan. 6, 2019. Increasing the frequency of observations subsequently detected additional particle plumes during the following two months. Although many of the particles were ejected clear of Bennu, the team tracked some particles that orbited Bennu as satellites before returning to the asteroid’s surface.
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- Bennu is the smallest body ever orbited by spacecraft. Studying Bennu will allow researchers to learn more about the origins of our solar system, the sources of water and organic molecules on Earth, the resources in near-Earth space, as well as improve our understanding of asteroids that could impact Earth.
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- The team didn’t anticipate the number and size of boulders on Bennu’s surface. From Earth-based observations, the team expected a generally smooth surface with a few large boulders. Instead, it discovered Bennu’s entire surface is rough and dense with boulders.
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- The higher-than-expected density of boulders meannt that the mission’s plans for sample collection had to be adjusted. The original mission design was based on a sample site that is hazard-free, with an 82-foot radius. However, because of the unexpectedly rugged terrain, the team hasn’t been able to identify a site of that size on Bennu. Instead, it has begun to identify candidate sites that are much smaller in radius.
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- The smaller sample site footprint and the greater number of boulders will demand more accurate performance from the spacecraft during its descent to the surface than originally planned.
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- The original, low-boulder estimate was derived both from Earth-based observations of Bennu’s thermal inertia, or its ability to conduct and store heat, and from radar measurements of its surface roughness.
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- Now that they have revealed Bennu’s surface up close, those expectations of a smoother surface have been proven wrong. This suggests the computer models used to interpret previous data do not adequately predict the nature of small, rocky, asteroid surfaces. The team is revising these models with the data from Bennu.
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- The team has directly observed a change in the spin rate of Bennu as a result of what is known as the YORP effect. The uneven heating and cooling of Bennu as it rotates in sunlight is causing the asteroid to increase its rotation speed. As a result, Bennu's rotation period is decreasing by about one second every 100 years.
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- Separately, two of the spacecraft’s instruments have made detections of magnetite on Bennu’s surface. This bolsters earlier findings indicating the interaction of rock with liquid water.
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- There is another mission visiting another asteroid at the same time. The Japanese Aerospace Exploration Agency’s Hayabusa 2 is in orbit around Asteroid Ryugu. The Japanese team is a bit ahead of their American counterparts, having arrived roughly six months earlier.
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- Upon approach, the team found the same kind of unexpectedly rough terrain that NASA ultimately found at Bennu. Ryugu is peppered by boulders and rocks several meters in diameter, which threaten the spacecraft as it retrieves samples from the surface.
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- Hayabusa 2 has successfully deployed three landers that floated to the asteroid’s surface, snapping pictures along the way. The mothership has also already completed one of the three expected touchdown maneuvers.
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- During these touchdowns, the spacecraft extends a sample retrieval mechanism, which looks like a 1-meter-long limb that ends in a canister for gathering samples. When the canister is close enough to the surface, a special mechanism within the limb shoots a pinball-sized bullet to the asteroid’s surface, stirring the regolith in the hopes that some of the surface material will end up inside the canister.
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- However, there is no way to know for sure how much they obtained, since the spacecraft has no mechanism to weigh or estimate what’s inside the sample chamber.
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- Ryugu and Bennu are so similar in appearance, it’s hard to tell them apart in images. Both have been called “rubble-pile asteroids,” collections of a variety of different-sized debris held together by gravity.
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- Their low densities imply their interiors are full of hollow spaces, like Swiss cheese. These objects probably came together out of the pieces left over from larger asteroids, which may have come apart during impacts in the early solar system.
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- Both objects are shaped like spinning tops, as their rapid rotation causes a ridge to form along the equator. But while Bennu’s rotation is accelerating, Ryugu appears to have slowed down, and it’s unclear why.
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- Ryugu’s current rotation period is 7.6 hours, but researchers estimate that at some point in its past, it must have rotated more quickly, with a period shorter than 3.5 hours, in order to acquire its current shape.
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- Ryugu is about 900 meters (or about half a mile) wide.
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- There is a big difference in age between the two asteroids. . While Bennu is ancient, with an estimated age ranging from 100 million to 1 billion years, Ryugu appears to be much younger, less than 100 million years old.
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- Ryugu is also darker than Bennu, reflecting on average half the light that that Bennu does. With a reflectivity, or albedo, of 2%, it’s one of the darkest objects known in the solar system.
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- Another unexpected difference is in the amount of water that researchers have found on the two asteroids. While there are hydrated minerals on both objects, Ryugu appears to be much dryer than researchers expected, which suggests that its parent body didn’t have much water either.
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- This is in contrast with Bennu, which has more abundant hydroxyls. These molecules contain hydrogen and oxygen atoms bonded together and are probably associated with clay minerals, hinting at past interactions with water.
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- Unraveling why the two asteroids have different water content will be important for understanding how Earth obtained its water. Solar system formation models often assume that most of the water found on Earth today arrived onboard meteorites and comets both from the asteroid belt and from the outer rims of the solar system. The existence of dry asteroids in Earth’s neighborhood could mean that these models need to be adjusted.
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- The next step for the Hayabusa 2 mission doesn’t sound very high-tech but it’ll be spectacular. On April 5th, just 2 weeks from now, the spacecraft will fire a 5.5-pound projectile at Ryugu In order to create an artificial crater on its surface. The blast aims to make a hole at least 10 meters wide and 1 meter deep.
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- With this maneuver, the team expects to learn more about how asteroids react to impacts, which will help scientists better understand Ryugu’s history and evolution.
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- Information about the impact might also come in handy in case one of these things ever comes too close to Earth, and we have to blast it out of the way.
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- Creating the crater will also enable researchers to peek at what lies below the surface. Two weeks after shooting the cannon, Hayabusa 2 will attempt a second sampling maneuver, aiming for the bottom of the crater.
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- Then, in November or December this year, 2019, the spacecraft will start its one-year return trip to Earth. Let’s hope for happy landings. And I hope I can report more results in the future, stay tuned, there is still more to learn.
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- March 23, 2019
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-------------------------- Saturday, March 23, 2019 --------------------------
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