抖阴旅行射

Banner image: Artist's depiction of the OSIRIS-REx spacecraft swooping toward the asteroid Bennu to collect a sample of material from its surface. (Credit: NASA/Goddard/University of Arizona)

New findings from NASA鈥檚 suggest that the interior of the asteroid Bennu could be weaker and less dense than its outer layers鈥攍ike a cr猫me-filled chocolate egg flying though space.

The results appear in a study 听and led by the University of Colorado Boulder鈥檚 OSIRIS-REx team, including professors Daniel Scheeres and Jay McMahon. The findings could give scientists new insights into the evolution of the solar system鈥檚 asteroids鈥攈ow bodies like Bennu transform over millions of years or more. 听

OSIRIS-REx rendezvoused with Bennu, an asteroid orbiting the sun more than 200 million miles from Earth, in late 2018. Since then, the spacecraft, built by Colorado-based Lockheed Martin, has studied the object in more detail than any other asteroid in the history of space exploration.

So far, however, one question has remained elusive: What鈥檚 Bennu like on the inside?

Scheeres, McMahon and their colleagues on the mission鈥檚 radio science team now think that they have an answer鈥攐r at least part of one. Using OSIRIS-REx鈥檚 own navigational instruments and other tools, the group spent nearly two years mapping out the ebbs and flows of Bennu鈥檚 gravity field. Think of it like taking an X-ray of a chunk of space debris with an average width about the height of the Empire State Building.

鈥淚f you can measure the gravity field with enough precision, that places hard constraints on where the mass is located, even if you can鈥檛 see it directly,鈥� said Andrew French, a coauthor of the new study and a former graduate student at 抖阴旅行射 Boulder, now at NASA鈥檚 Jet Propulsion Laboratory (JPL).

What the team has found may also spell trouble for Bennu. The asteroid鈥檚 core appears to be weaker than its exterior, a fact that could put its survival at risk in the not-too-distant future.

鈥淵ou could imagine maybe in a million years or less the whole thing flying apart,鈥� said Scheeres, a distinguished professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences.

Evolution of asteroids

Of course, that鈥檚 part of the fun of studying asteroids. Scheeres explained that Bennu belongs to a class of smaller bodies that scientists call 鈥渞ubble pile鈥� asteroids鈥攚hich, as their name suggests, resemble loosely held-together mounds of debris.听

Asteroids also change over time more than people think.听

鈥淣one of them have sat out there unchanging since the dawn of the solar system,鈥� Scheeres said. 鈥淭hey鈥檙e being changed by things like sunlight affecting how they spin and collisions with other asteroids.鈥�

To study how Bennu and other similar asteroids may change, however, he and his colleagues needed to take a peek inside.

This is where the team got lucky. When OSIRIS-REx first arrived at Bennu, the spacecraft spotted something unusual: Over and over again, 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鈥檚 radio science team at JPL were able to 鈥攁 bit like the apocryphal story of Isaac Newton inferring the existence of gravity after observing an apple falling on his head.听

鈥淚t was a little like someone was on the surface of the asteroid and throwing these marbles up so they could be tracked,鈥� Scheeres said. 鈥淥ur colleagues could infer the gravity field in the trajectories those particles took.鈥�

Squishy center

In the new study, Scheeres and his colleagues combined those records of Bennu鈥檚 gravity at work with data from OSIRIS-REx itself鈥攑recise measurements of how the asteroid tugged on the spacecraft over a period of months. They discovered something surprising: Before the mission began, many scientists had assumed that Bennu would have a homogenous interior. As Scheeres put it, 鈥渁 pile of rocks is a pile of rocks.鈥澨�

But the gravity field measurements suggested something different. To explain those patterns, certain chunks of Bennu鈥檚 interior would likely need to be more tightly packed together than others. And some of the least dense spots in the asteroid seemed to lie around the distinct bulge at its equator and at its very core.

鈥淚t鈥檚 as if there is a void at its center, within which you could fit a couple of football fields,鈥� Scheeres said.

The asteroid鈥檚 spin may be responsible for that void. Scientists know that the asteroid is spinning faster and faster over time. That building momentum could, Scheeres said, be slowly pushing material away from the asteroid鈥檚 center and toward its surface. 听Bennu, in other words, may be in the process of spinning itself into pieces.

鈥淚f its core has a low density, it鈥檚 going to be easier to pull the entire asteroid apart,鈥� Scheeres said.

For the scientist, the new findings are bittersweet: After measuring Bennu鈥檚 gravity field, Scheeres and his team have mostly wrapped up their work on the OSIRIS-REx mission.听

Their results have contributed to the mission鈥檚 sample analysis plan鈥攃urrently in development. The returned sample will be analyzed to determine the cohesion between grains鈥攁 key physical property that affects the mass distribution observed in their study.

鈥淲e were hoping to find out what happened to this asteroid over time, which can give us better insight into how all of these small asteroids are changing over millions, hundreds of millions or even billions of years,鈥� Scheeres said. 鈥淥ur findings exceeded our expectations.鈥�

The University of Arizona leads science operations for OSIRIS-REx. NASA鈥檚 Goddard Space Flight Center in Maryland manages the overall mission.

Other coauthors on the new study include researchers at the Jet Propulsion Laboratory, Smithsonian Institution, The Open University, Northern Arizona University, KinetX Aerospace, Inc., NASA Goddard Space Flight Center, University of Maryland, Johns Hopkins University, York University, University of British Columbia, Southwest Research Institute, Universit茅 C么te d鈥橝zur and University of Arizona.