Australian Sky & Telescope - April 2018

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LEAH TISCIONE /

S&T

, IN-PLANE SOURCE: NASA, OUT-OF-PLANE SOURCE: G. RICKER ET AL. /

JOURNAL OF ASTRONOMICAL TELESCOPES, INSTRUMENTS, AND SYSTEMS

2014

ideal orbit. This is not a trivial problem. The Hubble Space

Telescope’s orbit, just 550 km 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 cost-

effective 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.

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

24×96° 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.

Once the data come down, extensive computer codes,

built upon those used to analyse 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 will search for clues in the light

Moon

Orbit

Orbit

Insertion

Transfer

Orbit

Phasing

Orbits

Lunar

Flyby

P/2 Mission

Orbit

TESS

Moon

Earth

SRING 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).

TESS’s main raison d’être, or Level 1 Science Requirements in

formal NASA-speak, is to find for the community, 50 planets

with sizes less than 4 Earth radii and with measured masses.