ŷ���ڱ���Ƶ

ŷ���ڱ���Ƶ the Event��

Exploring the Asteroid Bennu: Will it ever hit Earth?

NASA’s spacecraft OSIRIS-REx rendezvoused with Bennu, an asteroid orbiting the sun more than 200 million miles from Earth, in late 2018. Since then, the spacecraft has studied the object in more detail than any other asteroid in the history of space exploration. Bennu belongs to a class of smaller bodies that scientists call “rubble pile” asteroids, loosely held-together mounds of debris. It is a large rubble pile, as wide as the Empire State Building is tall.

Using OSIRIS-REx’s own navigational instruments and other tools, Scheeres and his colleagues spent nearly two years mapping out the ebbs and flows of Bennu’s gravity field. When OSIRIS-REx first arrived at Bennu, they observed, tiny bits of material, some just the size of marbles, seemed to pop off the asteroid and into space. In many cases, those particles circled Bennu before falling back down to the surface. Members of the mission’s radio science team at JPL were able to witness how the body’s gravity worked first-hand. It was a little like someone was on the surface of the asteroid and throwing these marbles up so they could be tracked. The gravity field of the asteroid could be inferred from the trajectories of those particles.

Combined those records of Bennu’s gravity at work with precise measurements of how the asteroid tugged on the spacecraft over a period of months, it was possible to study the core of the asteroid. The team discovered some under-dense regions around its equatorial bulge at its equator and at its core. The under-dense region at its center could be large enough to contain a couple of football fields.

The asteroid has a rotational rate of 4.3 hours, and its spin may be responsible for that void. It is spinning faster over time due to the effect of sunlight on its surface. That increasing momentum could be slowly pushing material away from the asteroid’s center and toward its surface. Bennu may spin itself to pieces in a few million years.

But much before then, Bennu has a 1/1800 chance of impacting Earth. Venus could also be hit, but if Earth becomes the target, a collision could occur between 2178 and 2290. If all of its matter stayed together, the energy of the collision could be as great as 1200 megatons.

How may it be possible to nudge such a loosely held-together collection of rubble away from such a collision?

ŷ���ڱ���Ƶ the Speaker

When Professor Daniel Scheeres received his Ph.D. from the University of Michigan in 1992, he was focused on celestial mechanics, primarily a theoretical endeavor. After getting his degree, he went to the Jet Propulsion Laboratory, where he worked on spacecraft navigation for the NEAR (Near Earth Asteroid Rendezvous) mission. During his JPL experiences, he realized that “all the theoretical stuff that I’d learned in my Ph.D. could be applied to the problem of orbiting asteroids” and became intrigued by the field of asteroid navigation and its plethora of “cool questions to answer.”

A primary focus of his research is determining the orbital mechanics of small bodies (such as moons and asteroids) with applications to planetary and asteroid missions. Currently, Scheeres is serving as the Radio Science Lead and Co-Investigator for NASA’s OSIRIS REx Asteroid Sample Return Mission.

In recognition of Professor Scheeres’ extensive contributions to the study of astrodynamics around small bodies, the Asteroid (8887) 1994LK1 was renamed (8887) Scheeres.Asteroids, with their irregular shapes and densities, do not have uniform mass distributions. Radio science data can be used to measure its mass distribution. Scheeres explains:“The coolest thing you could find with radio science data is whether the center of the body is under-dense or over-dense, or if the crust is over-dense [etc.]. Each of these scenarios is tied to a specific theory of asteroid evolution. Even though they are little bodies, asteroids experience complex physics.”

Asteroids have the potential to pose significant societal repercussions. “Asteroids hit the earth all the time. If a big enough one hits, it would reset world civilization. At some point, we will see a big enough asteroid to make us worry- we will then have to figure out a way to deflect it.”

ŷ���ڱ���Ƶ the Series

The ŷ���ڱ���Ƶ Boulder Retired Faculty Association (UCBRFA) presents the distinguished professors of the University of ŷ���ڱ���Ƶ, a��lecture and presentation series featuring some of our finest professors and��their extraordinary research and scholarly work.