Sky and Telescope - March 2018 - 25

LE AH TISCIONE / S&T, IN-PL A NE SOURCE: N ASA , OU T- OF-PL A NE SOURCE: G. RICK ER E T A L. /
JOUR NAL OF ASTR ONOMICAL TELESCOPES, INSTRUMENTS, AND SYSTEMS 2014

TESS's main raison d'être, or Level 1 Science Requirements in
formal NASA-speak, is to deliver to the community 50 planets
with sizes less than 4 Earth radii and with measured masses.
ideal orbit. This is not a trivial problem. The Hubble Space
Telescope's orbit, just 550 km (340 miles) above Earth, is relatively easy to get to but is bad for survey astronomy. Earth is
bright and hot, and its radiation can easily reﬂect into a telescope or onto a spacecraft, contaminating images. Furthermore, Hubble's orbit has a day-night cycle like we do on the
ground, and not only does the changing sunlight level require
careful shielding to avoid thermal variations in instruments
(which can mess with things like focus), it's also bad for the
continuous observations needed to discover transits.
Another common choice, the Earth-Sun gravitational
balance site called the second Lagrangian point, is less than
ideal, too. At 1.5 million km from Earth's nightside, the
location would challenge our ability to download in a costeffective way the reams of survey data that TESS will amass.
Presenting a similar problem is an Earth-trailing solar orbit
like that of Kepler or the Spitzer Space Telescope, which are
both slowly drifting away from our planet with time.
After extensive investigations, the MIT, Aerospace Corporation, and Orbital ATK collaborators on the TESS team
devised a never-before-used orbit that ﬁts the bill. The orbit
avoids the frequent dips into Earth's shadow that Hubble
experiences, and it's not as communications-constrained as
L2 or a drift-away solar circuit. Known as P/2, TESS's path
will be a highly elongated, 13.7-day Earth orbit in resonance
with the Moon that will carry it nearly as far away from our
planet as our natural satellite is. The lunar resonance means
that, when at its farthest point, the spacecraft will always
be roughly 90° ahead of or behind the Moon in its orbit -
although there's quite a bit of wiggle room in this separation.
TESS will spend most of its time far from Earth, then speed
up as it approaches Earth and downlink data while zipping by.

Once the data come down extensive computer codes, built
upon those used to analyze the Kepler and its later K2 mission observations, will tackle the data using systems located
both at NASA Ames and at MIT. Next is the fun part of the
job. Assisted by software, human vetters will pore over the
data to identify which signals most likely come from planets
transiting their host stars, versus those that come from binaries, stellar "junk" from variability, or instrumental noise.
Looking through the thousands of light curves that trigger
software, out of the tens of thousands of light curves per
observing sector, is like a page-turning mystery novel: One
never knows what might be uncovered on the next page. Vetters search for clues in the light curve that indicate the signal

Transfer
Orbit
P/2 Mission
Orbit
Lunar
Flyby
Phasing
Orbits

Moon
Orbit
Orbit
Insertion

TESS

Planet Pipeline
TESS's launch window opens in late March 2018. It will shoot
skyward from Cape Canaveral aboard a SpaceX Falcon 9
rocket. Once it's safely in space, the team will work nonstop
during a two-month commissioning phase to make sure TESS
is working as planned. Then the real excitement will begin.
To complete its survey, TESS will slice the celestial sphere
from pole to equator in long, thin strips of sky that match the
24°x96° dimensions of its four-camera view. Each hemisphere is split into 13 of these segments, with overlap at the
poles. The spacecraft will stare at a single strip for 27 days,
then move on to the next one. In its ﬁrst year, TESS will tile
the southern hemisphere; in the second year, the northern
hemisphere. Over a two-year nominal mission, it will observe
nearly all of the sky: Only small gaps around the ecliptic will
remain untouched.

Earth

Moon
p RING AROUND THE EARTH The TESS spacecraft will follow a unique
orbit, looping out of the Moon's orbital plane (top) so as to keep both
Earth and its natural satellite out of TESS's field of view as much as possible. The orbit also occupies a 2:1 resonance with the Moon, keeping
the spacecraft roughly 90° from the Moon (above).
sk yandtelescope.com * M A RCH 2 018

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Sky and Telescope - March 2018

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