Solar System Dust 623
FIGURE 2 Interplanetary dust
particles collected in the stratosphere
by NASA’s cosmic dust program. Three
grains are of chondritic composition
and of various degrees of compactness,
and there is one Fe–S–Ni sphere
(lower right). The widths of the
photographs are 15μm (first and third
photos, clockwise from upper left) and
30 μm (second and fourth photos).
(Courtesy of NASA.)FIGURE 3 Comparison of meteoroid sizes and masses covered
by different observational methods.
impacts have been returned from space and analyzed. High-
flying aircraft have collected from the stratosphere dust that
was identified as extraterrestrial material and that was an-
alyzed by the most advanced microanalytic tools. Modern
space-based infrared observatories now allow the observa-
tion of the thermal emission from interplanetary dust in the
outer solar system.2.1 Meteors
Looking up at the clear night sky, one can record about 10
faint meteors (or shooting stars in colloquial language) per
hour. Once in a while, a brighter streak or trail of light or
“fireball” will appear. Around the year 1800, the extraterres-
trial nature of meteors was established when triangulation
was used to deduce their height and speed. This technique
is still used in modern meteor research by employing specif-
ically equipped cameras and telescopes. About 50 years ago,
radar techniques were also developed to observe faint me-
teor trains even during daylight.
Visible meteors result when centimeter-sized mete-
oroids enter the Earth’s atmosphere at a speed greater
than 10 km/s. At this speed, the energy of motion, which is