Sky and Telescope - July 2017 - 12
Fireball over ALMA
A New Take on Sounds
A BAD HAIR DAY might give you the
temporary superpower of hearing meteors. Richard Spalding (Sandia National
Laboratories) and colleagues sought an
explanation for occasional but persistent
reports of observers hearing hisses, pops,
or pings from bright meteors. The team's
study, appearing February 1st in Nature's
Scientiﬁc Reports, suggests the sounds
might be triggered by light's effect on
certain materials, including human hair.
In principle, observers should not
hear sounds simultaneously with seeing
ﬁreballs' ﬂashes. Not only do meteors
occur in the tenuous upper atmosphere,
which transmits sound poorly, but
they're also distant, roughly 80 km (50
miles) up. Any noise would reach you
noticeably after seeing the ﬂash.
Spalding's team proposes that the
sounds originate much closer to observers. Fireballs' strong, millisecond-long
ﬂashes are intense enough to induce
radiative heating in dielectric materials such as dry leaves, clothing, or even
hair, via what's called the photoacoustic effect. The irradiated surfaces heat
the air next to them, producing tiny
pressure oscillations - in other words,
sound. A ﬂickering ﬁreball around −12
in magnitude (about as bright as the full
Moon) could induce an audible sound of
roughly 25 decibels, slightly louder than
a whisper's 10 to 20 decibels.
This scheme even explains why people
with frizzy hair are more likely to "hear"
meteors. "Hair near the ears will create
localized sound pressure, so it is likely
to be heard," the authors write. "Also,
hair has a large surface-to-volume ratio,
which maximizes sound creation."
■ DAVID DICKINSON
On the Web: Learn more and listen to a rendition of "Greensleeves" created
using the photoacoustic effect at https://is.gd/audiblemeteors.
Rosetta Sees Changing Face of Comet 67P
RESEARCHERS HAVE used data from
the European Rosetta mission (S&T:
May 2017, p. 14) to link outbursts on
with dramatic surface changes. Comets are notorious for sudden outbursts,
even fragmenting, as they near the Sun.
Scientists have had only a poor under-
Aswan cliff, before and after (from different orientations)
J U L Y 2 0 1 7 * SK Y & TELESCOPE
Runaway Stars in Orion
A RUNAWAY PROTOSTAR in the Orion
Nebula suggests a tussle occurred
between four stars 540 years ago.
The protostar, labeled Source X, lies
in the vicinity of the Kleinmann-Low
Nebula, the most active part of the
star-forming Orion Nebula complex.
By comparing Hubble Space Telescope
images from 1998 and 2015, Kevin Luhman (Penn State University) and colleagues found that Source X is moving
at more than 55 km/s (120,000 mph)
across the sky, away from the same
origin point as two other high-velocity
stars: the Becklin-Neugebauer (BN)
object and Source I. BN and Source I are
zooming in opposite directions at 26
km/s and 10 km/s, respectively.
The three objects might once have
been part of a four-star system, Luhman's team reports in the March 20th
Astrophysical Journal Letters. When two
of the stars came too close together, they
formed a binary or merged (Source I),
sending BN and Source X ﬂying. The
new measurement of Source X's motion
accounts for the system's missing kinetic
energy and seals the deal.
■ MONICA YOUNG
standing of why. But on July 10, 2015,
Rosetta's navigation camera caught
sight of a large plume of dust coming
from the Seth region on Comet 67P.
Five days later, the spacecraft detected
a freshly exposed escarpment along
Aswan, a cliff face that's 134 m (440 ft)
high. The albedo of the exposed material is 40%, the same as dry sand, making it six times brighter than the older,
Based on a visual count of boulders
at the cliff's bottom, Maurizio Pajola
(NASA Ames) and colleagues estimate
that around 10,000 tons of material
were involved in the landslide, with
about 100 tons released in the plume.
The study, published March 21st in
Nature Astronomy, supports previous speculation that landslides might
explain comet outbursts.
■ DAVID DICKINSON
ME TEOR: ESO / C. M A LIN; ROSE T TA: ESA / ROSE T TA / MPS / UPD / L A M / IA A / SSO / IN TA / UPM / DASP
/ IDA / M. PA JOL A