Alan Stahler: Building-block for a planet |

Alan Stahler: Building-block for a planet

Alan Stahler
Asteroid Bennu – less than half-a-mile across – from 15 miles away.
Submitted Photo

Walking on the moon is different from walking on Earth … walking on an asteroid is more different yet. Besides the need for a spacesuit, it comes down to gravity — the force that glues us to Earth, that gives us weight.

The gravity of a planet, moon or asteroid relates directly to its mass — the more matter a body contains, the stronger its gravity.

The moon is less massive than Earth, so its gravity is less. Something (or someone) who weighs 150 pounds on Earth would weigh only 25 pounds on the moon.

Asteroid Bennu is way smaller than the moon — not even half-a-mile across. That 150-pounder (on Earth) would weigh less than an ounce on Bennu. If you’re out for a stroll on Bennu — walking all the way around this tiny world would cover little more than a mile) — don’t sneeze. The sudden, jerky motion would launch you off the surface, out into space.

Asteroid Bennu is unusual in that it has escaped the asteroid belt, the region between Mars and Jupiter that hosts uncounted chunks of rock, left over from the birth of the solar system — rock that never got pulled in to join one or another of the newly-forming planets. Bennu orbits the sun in an orbit similar to Earth’s, and crosses Earth’s orbit every half-dozen years. If could, conceivably, one day collide with our planet.

If all goes well, we’ll soon have a small piece of asteroid Bennu to study here on Earth.

The spacecraft OSIRIS Rex, launched in 2016, arrived within a million-or-so miles of asteroid Bennu late last year. Navigation duties were then handed-off to the optical navigation team: Bringing the spacecraft closer to Bennu, and inserting it into orbit around the asteroid, would now be based on information from its on-board cameras.

I spoke recently with the mission’s optical nav lead, Coralie Adam.

Bennu’s minuscule gravity makes placing a spacecraft in orbit around the asteroid a bit dicey. Tiny forces that can normally be ignored — or, at least, put on the back-burner — become significant.

If you want to push something, you can push it with your hands or with a stick, or you can throw rocks at it, or push it with a blast of water from a hose.

You can also push on something with a beam of light.

You cannot push very hard with light — you can barely push at all, which is why we normally don’t notice it. But light does have just a bit of a kick. And, given the weak gravity of asteroid Bennu, that kick must be factored in. Otherwise, according to Adam, “Solar radiation pressure would be pushing us toward the asteroid when we were on the day-time side of the asteroid (the sun behind the spacecraft) and away when we’re on the night-time side (the sun behind the asteroid). That would degrade our orbit very quickly.”

“We chose to go into a stable orbit—– it’s called a frozen orbit.”

The trick behind the frozen orbit is that it stays always sidewise to the sun. “That allows us to stay … stable … without degrading over time.”

Over the next couple of years, OSIRIS Rex will drop closer and closer to the asteroid, to eventually TAG — touch-and-go — in 2020, snag a sample, and return it to Earth in 2023. Asteroids likely hold clues to how our Earth was formed.

I’ll speak again with Coralie Adam, as she navigates the spacecraft to TAG.

Al Stahler enjoys sharing nature with friends and neighbors.  His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at

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