Discover 4

72 DISCOVERMAGAZINE.COM

FROM TOP: NASA; ROEN KELLY/DISCOVER, AFTER RICKER ET AL. (2014); SAO/ORBITAL ATK

shape of the surfboard itself varies

as well, getting wider or narrower

while maintaining a constant length.

Ricker and his team eventually

learned these oscillations could be

kept under control, which hadn’t been

established in the 2001 paper. “If you

set up the initial conditions right,”

Ricker says, “these oscillations will stay

bounded — or stable, in other words.”

Their colleagues at Goddard arrived

at the same conclusion, using different

numerical methods. P/2 would work.

NASA officially approved the

mission in 2013 after Ricker and

Persinger earned the support of a

scientific review panel. “TESS would

not have been selected for flight by

NASA without the P/2 high-Earth

orbit,” says Persinger. It’s an essential

part of the mission.

HERE COME THE NEW ORBITS

While the TESS crew makes final

launch preparations, other astronomers

are eyeing its unique orbit.

Randall Smith of the Smithsonian

Astrophysical Observatory has

adopted a slight variant, a P/4 lunar

resonant orbit, for the proposed X-ray

observatory Arcus. It would make four

trips around Earth for every one the

moon takes. And Lynx, a proposed

successor to the Chandra space

telescope, ranked “a TESS-type orbit”

as among its best options. “We’re

following in TESS’ footsteps here,”

Smith says. “These lunar-assist orbits

are ‘the new hotness,’ to steal a phrase

from Men in Black.”

That’s perhaps an apt comparison for

an orbit designed to help us spot the

closest, most likely places for alien life.

P/2 and other lunar-assisted orbits may

well become useful for future missions,

but TESS — to borrow another phrase

from Men in Black — was still the first

to “make this look good.”^ D

Steve Nadis, a contributing editor to Discover

and Astronomy, plays handball in Cambridge,

Massachusetts, where he also lives.

a much higher altitude, away from
Earth’s radiation and heat. You can
see better up there, just as the views are
better from the top of a skyscraper than
they are from the second floor. Plus, it
takes only a modest amount of rocket
fuel to reach the P/2 orbit, thanks to
the moon’s gravitational boost. It’s the
perfect path for TESS to take.

MINDING YOUR P’S AND 2’S
But sorting out the intricacies of
TESS’ orbit, along with other vital
aspects of the mission, has been a long
haul. Ricker started working on the
project a dozen years ago, and NASA
rejected the first TESS proposal in

2008. He tried again with the help of
      Randy Persinger, an Aerospace Corp.
      consultant, and other colleagues.
      By 2010, they had most details
      figured out, but still hadn’t settled on an
      orbit. There were a lot of requirements:
      It had to be big enough, and of long
      enough duration, to provide continuous,
      unobstructed looks at specific stars,
      yet still close enough to Earth that the
      mission’s smaller rocket could reach it.
      Luckily, another member of the
      TESS collaboration showed them a
      2001 paper that contained the first
      serious discussion of a high-Earth, P/2
      orbit. Ricker and Persinger quickly
      realized that this untested, “crazy

orbit” might afford them extended

stargazing times — but at a cost. “We

probably read the paper a dozen times,”

Persinger said. “This orbit had never

been tried before, but we bet the future

of TESS on it.”

The concern was that the orbit could

be unstable, since it oscillated in several

different ways. Imagine the orbit as a

pliable surfboard balanced on a ball

that represents Earth, the board’s edge

serving as TESS’ flight path.

The oscillations mean the surfboard

will periodically swivel from side to side

and up and down, wildly changing the

direction it points to. And just as the

surfboard can roll forward or backward

over the ball, so too can the orbit’s

overall position change relative to

Earth, varying by huge margins — up

to 10 times Earth’s radius. Finally, the

The TESS satellite

will follow

an innovative

new orbital path

as it searches

for exoplanets.

Closest approach

(67,000 miles)

When TESS gets

close to home,

it will send data.

Final orbit

3.5 initial

orbits

Moon’s orbit

Lunar flyby

(248,500 miles)

Farthest point

from Earth

(232,000 miles)

A high orbit,

outside the plane

of Earth and the

moon, will mean

long stretches

of unobstructed

viewing.

P/2: A Magic Orbit

Earth

The lunar-assisted orbit for the proposed X-ray

observatory Arcus, inspired by TESS, would

circle Earth four times for every lunar trip.

Out

There