Sky & Telescope - September 2024 - 17

Comet Tsuchinshan-ATLAS has never approached the Sun
before, its durability has never been tested.
The Nucleus
The physical body of a comet - its nucleus - is usually
between 1 and 10 km (between 0.6 and 6 miles) across and
no more than half ice; the rest is rock and dust. The ice is
mostly water (90%), but astronomers have also observed ices
of carbon dioxide, carbon monoxide, methane, and ammonia.
Since comets are relics of the solar nebula, their composition
provides direct evidence that volatiles made up most of
the protoplanetary disk.
The first detailed observations of a comet's nucleus were
made by NASA's Deep Impact spacecraft in 2005. This daring
mission flew to Comet 9P/Tempel 1 and shot it with an
impactor. In studying the subsequent debris, scientists found
materials such as clays, carbonates, and crystalline silicates.
They observed more dust and less ice than expected, and the
dust itself seemed more like a fine powder than dirt. In fact,
the impactor ejected so much material, and the crater was so
much bigger than expected, that scientists concluded that the
nucleus must be mostly empty space - less like solid ground
and more like a snowbank.
Rosetta found the same during its years-long journey
with Comet 67P/Churyumov-Gerasimenko. Joining up with
the nucleus in 2014, Rosetta was the first orbiter to follow
a comet for several years, including through perihelion. Its
lander, Philae, is the only craft so far to collect data directly
from a comet's surface.
" There was an instrument on Rosetta called CONSERT,
which was a little bit like an ultrasound but with electromagnetic
waves, " Altwegg says. " It was made to study the interior,
the nucleus. And they got some very good results. "
Astronomers long characterized comet nuclei as " dirty
snowballs, " until observations of Comet Halley in the 1980s
showed its composition was rockier than expected. Likewise,
Rosetta's CONSERT instrument (short for Comet Nucleus
Sounding Experiment by Radiowave Transmission) established
that a significant portion of 67P's interior is carbonaceous
rock and dust.
But how are comet interiors structured, exactly? Do they
have solid-rock cores surrounded by ice? Or perhaps multiple
alternating layers? Or are they more like gravel all the way
through? If so, how does the ice mix with the other material
- does it smoothly saturate on the level of individual grains,
like a slushy? Or does the ice surround bigger pebbles and
rocks, like a soda with ice cubes?
" The best thing to learn about how the dust and ice are
mixed would be to dig down - about 10 meters, at least, " says
Altwegg. Her dream mission would be a drilling campaign,
extracting an ice core to bring back to Earth for study. " That
would be wonderful. Because for me, the biggest mystery
about comets is still the interior. "
Touchdown missions like Deep Impact and Rosetta are
complex and rare endeavors; a robotic mining operation,
THE NUCLEUS ESA's Rosetta mission captured this close-up of
Comet 67P/Churyumov-Gerasimenko from 29 km (18 mi) away. Its surface
is in parts smooth or slumped with scattered boulders and in other
parts rough and jagged.
like the one Altwegg is hoping for, is currently beyond our
capabilities. For the near future, at least, remote observations
of the atmospheres and tails of comets like C/2023 A3 will
continue to be our primary source of information about their
nature and composition.
The Coma
In the near-absolute zero temperatures of the Oort Cloud,
comets are dark and inactive. It's only when they enter the
warmer region of the planets that their ices start to sublimate,
transitioning directly from a solid into gas. The emerging
gases also eject dust from the surface. Theatrics ensue as all
of this material forms an atmosphere around the comet's
nucleus called a coma.
The gaseous envelope will begin to glow as it approaches
the Sun. Usually the coma appears whitish or yellow; however,
it can sometimes take on a greenish hue, perhaps due
to sunlight irradiating diatomic carbon and cyanogen in the
nucleus. Sky-watchers were even able to observe this interesting
effect with the naked eye, when Comet C/2022 E3, a
long-period comet with an orbit of 50,000 years, came within
0.28 astronomical unit (a.u.) of Earth in 2023.
NASA acquired the first close-up of a coma in 2004 using
its space probe Stardust. Stardust's primary mission was to
capture dust grains from the short-period comet 81P/Wild 2.
Among the material the probe captured, scientists found
glycine, the protein building block that Rosetta also found on
67P later on. Stardust's find marked the first time scientists
saw glycine in cometary material.
The comae of pristine comets, such as C/2023 A3, differ
from those of short-period comets like Tempel 1, Churyumov-Gerasimenko,
and Wild 2. The latter are all Jupiter-family
comets and have visited the Sun before, whereas pristine
sk yand te l escope .o r g * SEPTEMBER 2024 17
ESA / ROSETTA / NAVCAM
http://skyandtelescope.org
Sky & Telescope - September 2024

Table of Contents for the Digital Edition of Sky & Telescope - September 2024
