COMET:^
NASA/^ ESA/^ D.^ J
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ITT(UCLA);NOBEL^ I
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TIONS:^ NOBEL
M
EDIAON AUGUST 30, 2019, an amateur
astronomer discovered another inter-
stellar object — the second after
‘Oumuamua (S&T: Feb. 2018, p. 10).
This one’s a comet that will pass nearest
the Sun and Earth in December.
Gennady Borisov captured Comet
2I/Borisov using a 0.65-meter tele-
scope at the MARGO observatory near
Nauchnij in Crimea, when it was about
3 astronomical units (a.u.) from the
Sun. Unlike ‘Oumuamua, which was
spotted after its perihelion, the new
comet was still inbound. It comes clos-
est to the Sun on December 8th, passing
within 2 a.u. Its closest approach to
Earth follows on December 28th.What sets Borisov (and ‘Oumua-
mua) apart from solar system comets
is the eccentricity of its orbit. Planets,
asteroids, and comets have ellipti-
cal orbits, with eccentricities between
0 and 1. But Borisov’s eccentricity is
more than 3, indicating a hyperbolic
orbit. That is, it’s not gravitationally
bound to the Sun. Moreover, its high
velocity — which will reach a peak of
44 km/s (100,000 mph) at perihelion
— precludes an origin within the solar
system. The comet appears to be coming
from Cassiopeia in the direction of the
galactic plane.
Borisov was around 18th-magnitude
at discovery, and early images showed afaint but distinct coma and the barest
hint of a tail — cometary activity that
‘Oumuamua lacked (S&T: Oct. 2018,
p. 20). By October, Borisov had reached
16th magnitude, and it’s expected to
peak at 15th magnitude in December.
From early December through early
January 2020, when Comet Borisov is
expected to be brightest, it will travel
about 0.8° per day, from the central
part of Crater southward to Centau-
rus. The fi rst 10 days of December will
be best for amateur imaging and even
visual attempts (using at least an 8-inch
telescope under pristine skies) before the
full Moon interferes on December 12th.
Moon-free nights return on Decem-
ber 22nd.
In September, astronomers obtained
spectra of Comet Borisov using the
4.2-meter William Herschel Telescope
on La Palma, Spain, and the 8.2-meter
Gemini North telescope on Maunakea,
Hawai‘i. The object’s slightly reddish
surface color, its 2-kilometer-wide
nucleus, and the properties of its dust
and gas resemble aspects of long-period
solar system comets. The results appear
October 14th in Nature Astronomy, and
more observations are forthcoming.
■ BOB KING & NOLA TAYLOR REDD-^ Find a chart for Comet Borisov at
https://is.gd/borisov.
COMETS
Second Interstellar Visitor Discovered
ASTRONOMY & SOCIETY
Nobel Prize Honors
Exoplanet, Cosmology
DiscoveriesTHE ROYAL SWEDISH ACADEMY
OF SCIENCES has awarded the 2019
Nobel Prize in Physics to James Peebles
(Princeton) and to Michel Mayor and
Didier Queloz (both at the University of
Geneva, Switzerland). The prize, which
will be split in half, honors discoveries
that offer new perspectives on our place
in the universe.
Peebles is being honored for his theo-
retical contributions to our understand-
ing of the Big Bang, as well as the role
that dark matter and dark energy play
in shaping our universe.When Robert Wilson and Arno Pen-
zias at the Bell Telephone Laboratories
in New Jersey found a persistent buzz in
their radio experiments — a discovery
that won them the 1978 Nobel Prize
in Physics — Peebles and his colleagues
had already predicted the existence of
background radiation. This radiation,
they theorized, was initially trapped
in the primordial soup of ions in the
hot, early universe. But 370,000 years
after the Big Bang,
the soup had cooled
enough to combine
into neutral atoms,setting the photons free. This cosmic
microwave background (CMB) now fi lls
the universe.
Peebles worked for decades to under-
stand tiny fl uctuations in the CMB,
which turn out to encode information
about the universe’s earliest years. As
astronomers were fi nding evidence for
the existence of dark matter and dark
energy in their observations of stars,
galaxies, and galaxy clusters, PeeblesThe Hubble Space
Telescope obtained
this image of
Comet 2I/Borisov
on October 12,
2019, when the
object was 2.8 a.u.
from Earth.uJames Peebles (left)
shares the 2019 Nobel
Prize in Physics with
Michel Mayor (center) and
Didier Queloz (right).10 JANUARY 2020 • SKY & TELESCOPE
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