From the October 2012 issue

Could we detect a comet or asteroid hitting the Sun?

Steve Scherr, El Dorado Hills, California
By | Published: October 29, 2012
Comet C/2011 W3 (Lovejoy) narrowly escaped death in December 2011 when it passed only 87,000 miles (140,000 kilometers) from the Sun. // NASA/SDO
Yes. A comet or asteroid impacting the Sun’s surface (called the photosphere) would explode due to the pressure of the body traveling through the solar medium. The explosion’s brightness and effects would depend on the object’s mass, but the event likely would be similar to a solar flare — with a fireball, X-ray or ultraviolet emission, and even possible ripples in the photosphere. Sun-observing spacecraft, like the Solar and Heliospheric Observatory, the Solar TErrestrial RElations Observatory, or the Solar Dynamics Observatory, might detect the blast. An impact by a large asteroid or comet — say tens of miles wide — would contain more kinetic energy than even the largest solar flares and likely would be observable from amateur telescopes on Earth’s surface.

Any of these impacts, however, is unlikely. For a comet or asteroid to reach the photosphere, it must first traverse the Sun’s increasingly hot and dense atmosphere. The extremely high temperatures (thousands of degrees) so near our star would force components of a comet or asteroid to turn directly to gas. As the object penetrated farther into the solar atmosphere, denser gaseous material would ablate the rock. Thus, as a comet or asteroid approached the Sun, it would lose mass quickly and might not even survive long enough to hit the photosphere.

Finally, because everything in our solar system normally orbits the Sun without hitting it, any object from the solar system on a path to impact our star would have interacted with another body to change its orbit that much. Or, the impacting object could be a body ejected from another planetary system. — Matthew Knight, Lowell Observatory, Flagstaff, Arizona