Imagine the Universe!
Imagine Home  |   Ask an Astrophysicist  |  
Ask an Astrophysicist

The Question

(Submitted May 18, 1997)

I am 34 years old and have recently completed Astronomy 101 in college. I am writing a novel in which scientists are studying a comet whose tail will cut across the orbital path of Earth. I wish to be accurate about the instruments and measurements to be used to determine our rate of velocity and possible chemical composition of this tail.

The Answer

With recent detections of Comet Hyakutake in the x-rays, we high-energy astronomers are just beginning to become more deeply acquainted with the instrumentation needed to study comets. You can find an article about the x-rays detected from Hyakutake in

However, the real experts are the planetary scientists. So I'd recommend you take a look at the JPL home page:

We admire and appreciate your desire to be accurate in your novel !

Jim Lochner
for Ask an Astrophysicist

P.S. One of my colleagues, Casey Lisse, has provided further information about instrumentation used to study comets and the results about the composition of the tail. Here's his reply. (We think he mis-remembered the name of the Japanese satellite that flew by Halley. It was Suisei. Another Japanese satellite that flew by Halley but didn't get close was Sakigake.)

I hope this is helpful to you.

Jim Lochner (with Japanese satellite help from Koji Mukai)

What a question! The range of instruments now used to study comets is huge. Comet astronomers are using everything from the VLA and BIMA in the radio to CSO and JCMT in the sub millimeter to the NASA/IRTF and UKIRT and KECK (and now HST NICMOS) in the infrared to HST and KECK and all the other optical telescopes in the world (including amateur small telescopes in the backyard), to HST and EUVE (IUE is dead) in the UV to ROSAT and ASCA and BeppoSAX and RXTE in the x-ray. Orbits are determined by multiple observations over weeks in the optical, typically, and solving Kepler's equations - we have two premier scientists doing this, Brian Marsden at Harvard-SAO and Don Yeomans at JPL. Occasionally we get an in situ flyby, like the ICE, Vega, Giotto, and Sukui flybys of P/Halley, and the upcoming ROSETTA, DS-1, and STARDUST missions to P/Wirtannen and P/Wild2.

Our understanding of the composition of comets is a long one, too - the short answer is that comets are bid dirty snowballs, very roughly ~30% dust (rock forming elements like silicates) and the rest, ~70%, volatile materials like water ice and methane ice and ammonia ice and carbon monoxide ice and carbon dioxide ice. Comets are usually a few km in radius (Hale-Bopp was VERY large), and some of the smallest bodies in the solar system, while also having some of the largest structures in the solar system, the dust/ plasma tails and dust trails.

For more information, I suggest the following:

The intro to my thesis, published 1992 , UMD, available on microfilm
The wonderful books "The Mystery of Comets" By Fred Whipple, and a
"History of Comets" by Gary Kronk
The web pages by Kronk and JPL for comets; both are accessible from my
web page at UMD,

Hope this helps!


Questions on this topic are no longer responded to by the "Ask an Astrophysicist" service. See for help on other astronomy Q&A services.

Previous question
Main topic
Next question

Imagine the Universe is a service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA's Goddard Space Flight Center.

The Imagine Team
Acting Project Leader: Dr. Barbara Mattson
All material on this site has been created and updated between 1997-2012.

DVD Table of Contents
Educator's Index