Long-period comets, which require at least hundreds of years to complete one orbit around the Sun, spend most of their life thousands of times further away from the Sun than the Earth.
Comets produce gas and dust as they approach the Sun, which astronomers call cometary activity. This activity slows down comets that travel close to or inside Earth’s orbit.
As comets pass through the more distant region beyond Saturn, astronomer Nathan Kaib of the University of Oklahoma discovered the same comet-fading behavior.
“Long-period comets, those that take at least hundreds of years to go around the Sun once, spend most of their lives thousands of times further from the Sun than the Earth is,” said Kaib, lead author.
“However, sometimes they develop highly elliptical orbits and, in turn, make regular incursions toward the Sun and its nearby planets. As these comets approach the Sun, its intense heat turns their icy surfaces into gas.
This cometary activity is what gives comets their striking appearance in the sky and makes them relatively easy for astronomers to find. As extreme heating from the Sun steadily depletes their surface ice supply, the activity of comets passing near Earth diminishes, or fades, over time.”
This fading process also occurs among comets moving through the outer solar system near or beyond Saturn’s orbit, according to Kaib’s research. His findings are startling because such comets receive far less warmth from the Sun than those closer to Earth. In fact, unlike closer comets, the Sun’s warmth is so feeble on these comets that water-based ice cannot evaporate.
Kaib demonstrated that the gravity of the large planets compresses the orbits of distant comets, causing them to make smaller excursions away from the Sun in between journeys through the outer solar system, by running computer simulations of comets moving near the outer solar system’s giant planets.
“We should therefore expect that the outer solar system has many more comets on these shrunken orbits compared to those on larger orbits,” he added. “Instead, astronomers see the opposite; distant comets with shrunken orbits are almost entirely absent from astronomers’ observations, and comets with larger orbits dominate our census of the outer solar system. Rapid comet fading that occurs during this orbit-shrinking explains this paradox, since it will effectively make older comets invisible to astronomers’ searches.”
Because faraway comets are difficult to examine due to their distance, astronomers focus their knowledge of comets on those that orbit close to Earth. Kaib’s discovery shows that many near-Earth comets’ physical features may be altered by travels through the outer solar system before they are identified.
“Fading among distant comets was discovered by combining the results of computer simulations of comet production with the current catalog of known distant comets,” added Kaib. “These distant comets are faint and extremely difficult to detect, and comet-observing campaigns have taken great pains to build this catalog over the past 20 years. Without it, this current work would not have been possible.”
Kaib anticipates that the Legacy Study of Space and Time, a 10-year effort to survey the southern sky at Chile’s Vera C. Rubin Observatory, would significantly enhance comet discoveries.
“The comet fading characterized in my work will be critical to properly understanding and interpreting this imminent deluge of newly discovered comets,” he said.
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