Asteroid Bennu as seen by the Osiris-REx spacecraft. (Image courtesy NASA/Goddard/University of Arizona)

On Oct. 20, NASA’s Osiris-REX spacecraft successfully acquired a sample from the surface of asteroid 101955 Bennu.

Launched in 2016, Osiris-REx (an acronym for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has been in orbit around Bennu since 2018. Since then, over 3,500 citizen scientists have been mapping the surface of this asteroid using images returned from the spacecraft.

These volunteers have made over 14 million annotations of Bennu’s surface features, supplying the information that ultimately led to the selection of the “Nightingale” sample site. The project, run by the Planetary Science Institute and CosmoQuest, turned out to be incredibly difficult. Bennu is not a big rock in space, but rather a loosely bound collection of pebbles and small rocks with a diameter of about 565 meters and a mass of 78 billion kilograms.

78 billion kilograms seems like a lot, but from a gravitational perspective, its barely enough to capture a small spacecraft. Osiris-REx orbits Bennu at a speed of .11 mph. If the spacecraft travelled much faster than that, it would fly away from the asteroid.

In contrast, the International Space Station orbits the Earth at 17,500 mph. The mass of the Earth is about 5,973,600,000,000,000,000,000,000 kg.

From another viewpoint, 78 billion kg is a potentially devastating mass. One of the main reasons why we are studying Bennu is that it has a 1-in-2,700 chance of impacting Earth between 2175 and 2199. If such an impact were to occur it would release energy equivalent to 1,450 megatons of TNT.

For comparison, the atomic bomb that destroyed Hiroshima was equal to about .015 megatons of TNT. Incredibly, an impactor the size of Bennu strikes the Earth about every 100,000 years. It would be a very bad day, but it wouldn’t cause mass extinction or destroy human civilization.

The mission hopes to obtain at least 60 grams worth of material. We will determine the mass of the sample by measuring the moment of inertia of the spacecraft. By applying a known amount of thrust to spin the spacecraft, we can calculate from its rotational acceleration how the mass and mass distribution of Osiris-REx has changed, and therefore the size of the sample collected. Despite some signs to the contrary, we humans are remarkably clever.

Scientists will study the sample for decades to come when it is returned to Earth in 2023.

A video of Osiris-REx collecting a bit of Bennu is available online. It is amazing.

The Morning Sky

Venus continues to light your way in the early morning eastern sky, slowly making its way through the constellation Virgo. The Pleiades cluster, high in the west before sunrise, is an exceptional binocular target.

The Evening Sky

Halloween brings with it a “blue moon.” The second full moon of the month should ward off any ghouls or ghosts on Halloween night. Mars dazzles in the east after sunset. The red planet is visible all night long, appearing to move from east to west over the course of the night due to the rotation of the Earth.

Saturn and Jupiter dominate the southern sky as darkness falls. Jupiter reveals a few of its moons when viewed through binoculars while Saturn presents a life-changing view through a small telescope.

Dan Price is a NASA/JPL Solar System Ambassador and informal educator. He encourages you to search for a citizen science project online that you and your family can become involved in. Have a question about astronomy or space science? Send an email to and it might be featured in a future column.

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