70 DISCOVERMAGAZINE.COMNASA’S GODDARD SPACE FLIGHT CENTEROrbital
Tra i lbla zer
NASA’s latest planet hunter
is about to forge a new path
through space.
BY STEVE NADIS→
NASA will be making history
again, soon.
Sometime this spring, if all goes as
planned, a SpaceX Falcon 9 rocket will
carry the Transiting Exoplanet Survey
Satellite (TESS) into space. Once
in “high-Earth” orbit, the satellite’s
instruments will scan the entire sky,
hoping to find small planets outside
our solar system. The main targets
are potentially habitable worlds that
are relatively nearby, within a few
hundred light-years.
But the mission’s scientific objectives
aren’t the only historic part: TESS also
stands out because of the orbital path
it will follow around Earth, blazing
a course through space that no craft
has ever flown. Thanks to the orbit’s
elongated elliptical shape, says TESS
principal investigator George Ricker
of MIT, “we can stay away from Earth
during observations and get close to
Earth to transmit our data, once every
13 or so days.”
These and other orbital attributes
will get TESS exactly where it needs to
be — with relatively little expenditure
of energy and money. That has caught
the attention of scientists planning
future space missions. It’s a unique
orbit that, if not groundbreaking, is
certainly “spacebreaking.”THE GOLDILOCKS ORBIT
Upon its launch from Cape Canaveral,
TESS will circle our planet 3.5 times
before zipping around the moon,
whose gravity will then propel thesatellite into its intended orbit. The
orbit is named “P/2” because TESS
will circle Earth every 13.7 days, half
the period of the moon’s 27.3-day
orbit; TESS will loop around Earth
twice for every one trip the moon
takes. The satellite’s trajectory is also
inclined, tipped up at an angle of
about 40 degrees from the plane of the
Earth and moon.
“It’s kind of a magic orbit, which
should give us long, unbroken
observations for 300 hours at a stretch
because the Earth and moon aren’t in
the way,” Ricker explains. “It’s also a
novel orbit that we had to develop and
prove mathematically.” Joining him
in this endeavor were specialists at the
NASA Goddard Space Flight Center,
which is responsible for guiding themission, and at the Los Angeles-based
Aerospace Corp.
Making preparations for a never-
before-tried orbit might seem like a
normal part of mission planning, but
there’s surprisingly little variety in
satellites’ paths. “For space astronomy,
there are about half a dozen orbits that
are typically used,” notes Ricker, but
none of the standard ones seemed quite
right for TESS.
The Wilkinson Microwave
Anisotropy Probe, for example, used a
so-called L2 orbit, as will the upcoming
James Webb Space Telescope. L2 is
an orbit around the sun that moves
in lockstep with Earth, from almost
a million miles farther out. But
this remote orbit wasn’t a practical
option because it requires a bigger,
more powerful rocket than TESS’
budget would allow.
Ricker’s team also rejected a
low-Earth orbit, such as the one the
Hubble Space Telescope follows. The
TESS satellite would have had to pass
through a high-radiation zone, part of
the Van Allen radiation belts, multiple
times each day, which could take a toll
on the craft’s sensitive equipment. Heat
given off by Earth could also interfere
with TESS’ measurements.
The P/2 orbit keeps the satellite atMaking preparations
for a never-before-tried
orbit might seem like a
normal part of mission
planning, but there’s
surprisingly little variety
in satellites’ paths.
The Transiting Exoplanet
Survey Satellite will soon
look for planets from a
revolutionary new orbit.Out
There