Comet or Asteroid?

What is the difference between a comet and an asteroid? The typical response is that an asteroid is rocky and a comet is icy/gassy.  Further than this, asteroids typically orbit closer to the Sun than Neptune, and comets orbit beyond this loose dividing line.  But as with everything in nature, there are often exceptions to the rule.

‘Comet’ C/2014 S3 (PANSTAARS). Credit: ESO/M. Kornmesser

C/2014 S3 PANSTAARS is classified as a weakly active comet, originating in the Oort cloud with an orbital period of 860 years.  As it approached the Sun, astronomers noticed that it was lacking the characteristic comet tail, resulting from the blast of solar radiation upon approach to the Sun.   On closer inspection of it’s spectrum, astronomers from the University of Hawaii have categorized it as an S-type asteroid, a type of object usually found in the inner portion of the asteroid belt between Mars and Jupiter.

Even though it is classified as a comet, it is made of the same basic materials as the inner solar system.  This means it was likely ejected from the solar system through gravitational interactions over 4.5 billion years ago, and has since resided in the Oort cloud, staying cold and preserving the raw materials from the formation of the solar system.

Lead author Karen Meech of the University of Hawaii’s Institute for Astronomy says “We already knew of many asteroids, but they have all been baked by billions of years near the Sun. This one is the first uncooked asteroid we could observe: it has been preserved in the best freezer there is.”

By estimating the number of these ‘rocky comets,’ astronomers can infer the properties of the early solar system, such as where the gas giant planets formed.  If they formed closer to the Sun, more rocky bodies would have been ejected into the Oort cloud for us to find today.  It ultimately helps us figure out which solar system model is correct.


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