Sky and Telescope - April 2015 - 63

S&T: J. KELLY BEATTY

SOFIA was grounded the following year for a series
of equipment and electronics upgrades. Undaunted, I
strapped in for a ﬂight in March 2013. But an hour after
takeoﬀ, low oil-pressure warnings in two engines forced
an early return to Palmdale.
In three years of trying, I am 0-for-5 - and now my
Flight Attempt Number 6 isn't going well either.
Fortunately, standing idle on the tarmac nearby is
one of NASA's other 747s, one once used to ferry Space
Shuttles piggyback across the country. Mechanics raid its
cockpit for the needed part and make the swap in record
time. The plane's tires ﬁnally lift oﬀ the runway at 8:56
p.m. - just minutes before the ﬂ ight would be scrubbed.

Time Management

Actual ﬂight path
CA

Pallas

VY CMa

M82

Planned ﬂight path

Palmdale

AZ
S&T: LEAH TISCIONE, SOURCE: USRA / KARINA LEPPIK

Arcturus

Mexico

Restricted
airspace

S&T: J. KELLY BEATTY

We're airborne, but we've lost nearly 2½ hours of ﬂying time - and compromises must be made. While the
mechanics were preoccupied with the ﬂap sensor, science
ﬂight planner Karina Leppik and copilot Wayne Ringelberg had huddled tensely around a computer display of
possible reroutings. Their eventual Plan B preserves the
observations of VY CMa, Pallas, and SN 2014J, along with
an hour of staring at Arcturus as a calibration target. Losing out, tonight at least, are U Mon and NGC 3501.
The plane makes a beeline for the coast and, after
threading a narrow corridor between restricted airspaces,
heads out over the Paciﬁc Ocean for the ﬁrst of ﬁve long
legs that will take until nearly dawn to complete. Everyone
settles in for the all-nighter ahead.
After passing through about 30,000 feet, Grashuis
makes the call to slide open the giant barrel door in the
fuselage. I expect to hear a deep-throated whoosh or feel
some vibration to signal that stratospheric air is rushing past a huge hole in the plane's side at Mach 0.85. But
nothing like that happens - in fact, the only indication
that the door really has opened is a graphic on one of the
computer monitors.
A glance aft at the big bulkhead shows the focal plane,
counterweight, and FORCAST moving slightly, up and
down, side-to-side. But in reality the 17-ton telescope's
pointing is rock-solid, steadied to 0.35 arcsecond in some
modes; the gentle nodding actually betrays the slight
pitch, yaw, and roll of the plane around it.
The 2.7-m primary mirror has an eﬀective aperture
of 2.5 m because of its large secondary and other factors, and the overall optical path is f/20. The atmospheric
seeing out the door typically limits resolution to 2 to 4
arcseconds even at altitude. That would be a problem for
a visual observer back on Earth, but deep in the infrared,
it's less than the diﬀraction limit for a telescope of this
size at wavelengths greater than 15 microns.
We zigzag our way over the open ocean, completing
the VY CMa and Pallas legs. There's a deceptive calm in
the cabin, as 13 scientists and operators stare intently into
their consoles. Tight time constraints mean the observing

Top: Science ﬂight planner Karina Leppik and copilot Wayne Ringelberg work
out a new ﬂight plan after takeoﬀ was delayed by more than 2 hours. Middle:
Because the telescope stares out the port (left) side of the fuselage, SOFIA's
pilots must execute a convoluted ﬂight plan to put all the night's targets within
view. This plot shows the original itinerary (red) and the abbreviated routing
(blue) necessitated by a delayed takeoﬀ. Green polygons denote restricted
airspace. Bottom: Parked near SOFIA's hangar is one of two 747 aircraft that
NASA once used to ferry Space Shuttle orbiters across the country.

Sk yandTelescope.com April 2015

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Sky and Telescope - April 2015

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