Sky and Telescope - October 2018 - 6

FROM OUR READERS

Solo Artist

Howard Banich replies: I'm impressed that you took the time to
dig into the Gaia DR2 release to look for
companions to the Dumbbell's central star
and came up with two possibilities. It would
be really cool if they are true companion
stars, and I hope someone does point a big
scope with a high-resolution spectrograph
at M27's central star to ﬁnd out one way or
another. It would be awesome if it turns out
to be a multiple star!

It took me a few glances over a couple of
days at the "new, pancake-shaped artist's concept of `Oumuamua" (S&T: July
2018, p. 8) to ﬁnally see it: `Oumuamua
is the Millennium Falcon, albeit rusty.
Coincidence? I think the new, pancaked-shaped artist has a sneaky sense
of humor. (What was wrong with the
old pancaked-shaped artist, anyway?)
I'll deﬁnitely be keeping a closer eye
on all your content.

Martian Greenwich

u Perhaps this strange visitor from another
solar system isn't such a stranger after all.

Another Approach to LPCs

Astronomer Seeks
Companion

(S&T: July 2018, p. 66) of an unseen
stellar companion deﬁnitely piqued my
curiosity, since I seemed to recall that
there was already a suspected mainsequence star companion near the
central star of this nebula.
I ﬁred up the CDS's Aladin software
and overlaid proper motions from the
Gaia DR2 database onto Pan-STARRS
imagery of M27 to try to ﬁnd this
previously suspected companion. I was
surprised to see not one but two other
stars within 8 arcseconds of the central
star with proper motions and parallax
measurements essentially identical to
that of the central star, within Gaia's
measurement error. One is an early
K-type dwarf that I believe was already a
suspected companion, and the other is a
19th-magnitude probable red dwarf that
might be a companion as well, which I
think would be a new ﬁnd.
However, radial-velocity measurements are necessary in order to determine the probability that these other
stars are indeed companions. If they
are, then it means that the progenitor
system to M27 could have been somewhat analogous to the Alpha Centauri
triple-star system.
Does anyone have a big telescope
with a high-resolution spectrograph to
follow up?

The mention in "Messier 27: The First
Planetary Nebula" by Howard Banich

Jeffrey Neubauer
Tucson, Arizona

Thank you for giving due mention to
long-period comets in your feature article about the impact threat (S&T: June
2018, p. 12). Too often these days planetary defense is equated with defense
against rogue asteroids exclusively.
One feature of the acknowledgment
of the cometary threat remains puzzling,
however, and indeed disturbing. Alan
Harris is quoted as saying that, because
of the short warning time for an LPC
heading our way, "The best you can do is
evacuate and things like that."
However, a typical LPC is larger than
the minimum size of an object that
scientists have agreed presents "the
risk of civilization-ending catastrophe."
So unless the evacuation Harris has in
mind is to another habitable planet, a
different approach is called for.
The obvious alternative, which has
been advocated by other scientists in
this area such as Brent Barbee and
Joseph Nuth, is to design, test, and
build a deﬂection infrastructure prior to
the discovery of any such object . . . in
other words, beginning right now.
Joel Marks
Milford, Connecticut

OCTOBE R 2 018 * SK Y & TELESCOPE

Your special issue on Mars prompts me
to ask something I've always wondered:
Who established the 0° point for longitude on Mars, and when? In the article
on "canals" (S&T: July 2018, p. 28),
both Schiaparelli's and Flammarion's
maps have it in the same place, just
where it is on modern maps. Does this
location predate them? What's more,
the point doesn't seem to focus on any
prominent Martian feature, and there's
no "Greenwich," so what was the basis
for placing it where it is?
Jan A. Maas
Alexandria, Virginia

Contributing Editor Bill Sheehan
replies: In 1830, while making the

observations that led to the ﬁrst map of
Mars, German astronomers Wilhelm Beer
and Johann Heinrich von Mädler used the
3.75-inch Fraunhofer refractor in Beer's
private observatory in the Berlin Tiergarten.
They were struck by the appearance of a
small round patch "hanging from an undulating ribbon," which because of its small
size and conspicuous form they regarded
as a convenient reference point for determining the rotation period of Mars. It was
designated with the letter "a" and served as
the prime meridian on their map. (Curiously,
later, in 1840, when Mädler was in Dorpat
and had the use of its 9.6-inch Fraunhofer
refractor, he was unable to recover the
feature and doubted the permanency of the
Martian markings of which he had been sure
in earlier years. Was the area covered by a
dust storm as it is now?)
As astronomers with better telescopes
studied the planet, they discovered the
round spot was actually more complex in
structure. Rev. William Rutter Dawes in the

Jeff Martin * San Diego, California

Sky and Telescope - October 2018

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