MadSci Network: Astronomy

Re: How are the surface and core temperatures of comets and asteroids measured?

Date: Tue Jan 1 17:12:12 2002
Posted By: Kisha Delain, Staff, Chandra Mission Planning, Harvard-Smithsonian Center for Astrophysics
Area of science: Astronomy
ID: 1008457497.As

Most temperatures in an astronomical setting are measured by the peak
wavelength of light that the object emits.  This does give us a feel for
the surface temperature only, of course, since it is the surface of the
object which we see emitting the light.  The radiation is called "blackbody

Asteroids are very cool objects, so the peak of their radiation would be in
the infrared.  If we were to take a spectrum, and then fit it with a
blackbody model we could see what temperature the asteroid is at.  
[Moderator's note: asteroids are often used as "calibration sources" for
infrared telescopes!]

Comets in general are also cool; however they do heat up as they approach
the sun.  Still, they also emit the majority of their radiation in the

In contrast, the sun's surface is at 5800K and the peak of its blackbody curve
is in the optical - greenish yellow, to be more precise, and that is probably
why our eyes have evolved to see best at those wavelengths.

The core temperatures of such objects are not directly measured but
calculated by complicated radiative transfer codes. 

A website which has a java applet that shows you blackbody spectra and
discusses them briefly is at:
(There are other java applications at:

Another java applet which has some of the parameters that would go into a
model for the calculation of the interior temperature is the
build-an-asteroid webpage at:

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