“The Hubble images basically show us that this comet has the same kind of physical structure as comets in our own solar system,” said Jennifer J. Wiseman, a senior project scientist with the Hubble Space Telescope mission.

Comets in our solar system are like fossil records of its earliest days. Because they spent most of their lives far from the sun, they retain the most primitive material from the solar system’s early years. By studying Comet Borisov, scientists are getting some of their closest observations yet into the formation of another star system.

“The planetary system that this object came from originally was possibly not that different from our solar system,” said Heidi B. Hammel, a planetary astronomer with the Association of Universities for Research in Astronomy.

The Hubble observations have also yielded surprises about the size of the comet’s core. At its upper limit, it has a radius of about 500 meters, or a third of a mile, and only a couple hundred meters at its lower limit. That makes it about one-fifteenth the size that astronomers previously thought.

Because Comet Borisov is so small, it’s possible the space rock could break apart, Dr. Jewitt said. As the icy rock travels near the sun, it heats up and ejects more and more material. This process, called outgassing, can accelerate the spinning of the comet. The smaller the comet, the faster its spin can be accelerated, which could potentially cause it to explode. Because Comet Borisov just whisked by the sun in early December, there’s still a chance it could experience such a fate in the next few months as it tries to exit our solar system.

“It’ll be interesting because we’ll be able to see what comes out from the inside of one of these interstellar bodies as it breaks down,” Dr. Jewitt said. “I’m hoping for an explosion.”