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CHAPTER 37

Infrared Views of

the Solar System

from Space

Mark V. Sykes

Planetary Science Institute

Tucson, Arizona

1. Introduction 5. Asteroid Physical Properties


2. A New View of the Zodiacal Dust Cloud and Its Sources 6. Pluto and Beyond


3. A Ring of Dust Around the Earth’s Orbit 7. An Exciting Future


4. Comets and Their Nature

S

ince 1983, a series of telescopes operating in the ther-
mal infrared have been launched into Earth orbit and
now heliocentric orbit. The images and other data returned
have resulted in the discovery of new phenomena in the so-
lar system and a new perspective on the processes within it.
These observations have focused on comets, asteroids, and
interplanetary dust because the major planets and Earth’s
Moon were too bright to be observed.

1. Introduction

At night we see objects in the solar system by the sunlight
they reflect. The Moon, planets, comets, and (with the help
of telescopes) asteroids and distant Kuiper Belt objects are
visible to the extent that they efficiently reflect that light,
coupled with their apparent size. Small particles and dust
are basically invisible with the exception of the zodiacal light
seen near before sunrise and after sunset at certain times of
the year and the interplanetary particles that give off light as
they burn up as meteors in the Earth’s upper atmosphere.
At thermal wavelengths, the sky is dramatically different
(Fig. 1). The otherwise invisible dust now dominates the
view and “familiar” phenomena like comets have a very
different appearance that has changed our understanding
of their nature.

At thermal wavelengths, we are looking at the objects

themselves as sources of light, instead of reflected light

from another source like the Sun. All objects in the uni-

verse radiate heat. The energy distribution of this radiation

with wavelength is a function of the temperature of the

source. The Sun, at a temperature of more than 5000 K,

radiates primarily at visual wavelengths and appears yel-

low. Colder sources radiate at longer wavelengths. Thus,

the heating element of an electric stove appears orange-

red.

Objects in space (e.g., asteroids, comets, and planets)

also radiate, but they do so at wavelengths much longer

than can be detected by the human eye. This region of the

spectrum (generally beyond 5μm to the submillimeter) is

referred to as the thermal infrared. Analysis of the thermal

radiation from an object can tell us much about its composi-

tion and other physical properties including thermal inertia

and grain size distribution and characteristics.

Observing this radiation from ground-based telescopes

is complicated by thermal emission from the telescope itself

and the atmosphere, both of which are much brighter than

the astrophysical sources being observed. This has been

compared to observing a star in the daytime with the tele-

scope on fire.

Techniques that allowed objects within tiny patches of

sky to be observed by ground-based and aircraft-borne

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