Transcript – Asteroid Bennu: Selecting Site Nightingale
This is Bennu: a near-Earth asteroid, a remnant from the origins of the solar system, and the target of NASA’s OSIRIS-REx mission.
When OSIRIS-REx arrived in December 2018, it brought Bennu into focus, confirming early radar images that suggested the asteroid was shaped like a spinning top.
But the close-up images also brought a big surprise.
Before OSIRIS-REx arrived, scientists had expected Bennu’s surface to consist largely of fine-grained material, like a sandy beach.
Sand heats up quickly during the day, and cools off quickly at night. In contrast, solid objects like rocks and boulders heat up and cool down more slowly.
Infrared telescope observations had shown that as Bennu rotates, its surface rapidly heats and cools – much like a sandy beach.
But OSIRIS-REx was greeted by a rocky world littered with boulders – the size of cars, the size of houses, the size of football fields.
This unexpected roughness confronted the mission with a major challenge.
The main science goal of OSIRIS-REx is to briefly touch down on Bennu, and grab a sample of fine-grained material for return to Earth.
To protect the spacecraft, the original plan called for touchdown in a boulder-free zone with a diameter of at least 164 feet.
But as it turns out, boulder-free sites of that size don’t exist.
Following arrival, mission planners began looking at safe zones that are just a few parking spaces wide, with enough loose material to provide a sample.
In mid-2019, they identified four candidate sites, and named them after birds that can be found in Egypt: Osprey, Kingfisher, Nightingale, and Sandpiper.
Mission planners evaluated each site based on its safety to the spacecraft, the ease of getting to the site, the amount of sampleable material that it contains, and the science value of the material itself.
After carefully evaluating these criteria, they chose Nightingale as the primary sample collection site and Osprey as the backup.
Nightingale is located near Bennu’s north pole. It sits inside a small crater measuring 66 feet in diameter.
Nightingale contains mostly fine-grained material, and has multiple areas for sample collection.
It is also the darkest of the candidate sites, and has high color variation, suggesting a diverse mineralogy.
Osprey is also located within a 66-foot-wide crater, just north of Bennu’s equatorial bulge, and may contain rock types from both the northern and southern hemispheres.
Osprey has the strongest signal for carbon-rich materials of all four sites, and contains a dark patch at its center that is of high scientific interest to the mission.
Now that the primary and backup sites have been chosen, OSIRIS-REx is performing additional reconnaissance flights to prepare for the sample collection event.
Later in 2020, the spacecraft will descend to Bennu’s surface, briefly touch down, and collect up to four-and-a-half pounds of loose material.
After collection, OSIRIS-REx will carefully stow the sample and make the long journey back to Earth.
In late 2023 it will return the sample, delivering pristine material from the origins of the solar system that will be studied on Earth for decades to come.