Sky & Telescope - September 2024 - 14

Ancient Messengers
Ion tail
Comet 67P. This means that comet impacts could have supplied
the building blocks for life on Earth.
Comet Tsuchinshan-ATLAS didn't get the chance to make
such an important contribution, though - it was stuck in the
outskirts of the solar system.
Nucleus
Coma
Dust tail
THE PARTS OF A COMET A comet begins as a mixture of rock and
ices. As it flies closer to the Sun, those ices begin to sublimate, surrounding
the nucleus in a gaseous coma. Tails may also form, as the
Sun's solar wind and radiation pressure drive away gas ions and dust,
respectively. The slower dust grains make a broad, curved tail, while the
faster-moving ions shoot out in a tail that's straight and narrow.
land), one of Rosetta's project leaders. " They have different
isotopic ratios. This means they have two different origins.
For 50 years scientists asked why, and we now have an explanation:
impacting comets. "
Rosetta scientists found that two of xenon's heaviest
isotopes were scarcer in the gas surrounding Comet 67P/
Churyumov-Gerasimenko. This same deficit exists in Earth's
air (S&T: Mar. 2023, p. 34), providing a definitive link
between the impact-delivery of volatiles and Earth's current
atmosphere. Volatiles are substances that can be easily vaporized,
such as hydrogen, methane, ammonia, water, nitrogen,
carbon dioxide, sulfur dioxide, and, yes, xenon.
Comets delivering xenon to early Earth could also have
contributed prebiotic material such as phosphorus and
the amino acid glycine, which Rosetta also detected on
FAST FACTS
Origin: Oort Cloud
Size: about 10 km wide
Speed: 67 km/s (150,000 mph) at perihelion
Orbit: near-hyperbolic, retrograde, and inclined
139° from the ecliptic plane
Perihelion: Sept. 27, 2024
Closest approach: Oct. 12, 2024, 0.47 a.u. from Earth
Potentially visible to unaided eye: late September
Magnitudes over time:
January 2023: 13.5
Early May: 10
October (predicted): magnitude 0?
Fate: Long-period comet, with a perihelion every
80,000 years, or an interstellar object
14 SEPTEMBER 2024 * SKY & TELESCOPE
Comet Origins
After the planets formed, most of the remaining planetesimals
settled into two reservoirs: a donut-shape region beyond
Neptune and a more distant, spherical collection of icy bodies
called the Oort Cloud that encloses the entire solar system.
While we've observed several thousand trans-Neptunian
objects, especially in the region called the Kuiper Belt, we
have not directly seen the Oort Cloud. In fact, it's so vast and
far away, and the objects there so small, dark, and sparse, we
wouldn't even know it was there at all if it weren't for all the
pesky comets it sends our way. Such as Comet TsuchinshanATLAS,
for example.
For the last 4 billion years, C/2023 A3 has been in a kind
of suspended animation. It has been frozen solid in a void so
desolate, it might never have encountered another object in
the whole lifetime of the Sun if not for a chance encounter
with gravity. It is impossible to know the exact circumstances
that altered the course of our comet, but galactic tides are the
leading explanation. The Sun's gravitational influence is weak
in the Oort Cloud, so bodies out there are subjected to a tug
of war between our host star and the Milky Way itself. The
competition can change the small objects' trajectories over
time. Other theories for the perturbation of Oort Cloud bodies
include interactions with dwarf planets and passing stars.
Whatever the reason, Comet Tsuchinshan-ATLAS has been
nudged into a nosedive that's taking it through the inner
solar system, close enough to the Sun that it will pass just
outside the orbit of Mercury.
RARE VISIT Comet Tsuchinshan-ATLAS, which
comes from the outer edge of the solar system, is
now on its way to its closest approach with the Sun
on September 27th. Its steep incoming angle means
that it won't come near the gas giants; however, their
gravitational nudges could still alter its orbit.
Saturn
Jupiter
Mercury
Venus
Mars
Earth
PARTS OF A COMET: CASEY REED / S&T; ORBIT: BEATRIZ INGLESSIS / S&T
Sky & Telescope - September 2024

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