8 AUSTRALIAN SKY & TELESCOPE February | March 2019
AFTER A NINE-YEAR historic mission,
the Kepler space telescope has
finished its job. Exhausted of fuel and
hobbled with inoperative reaction
wheels, Kepler’s exoplanet hunting
days are over.
“As NASA’s first planet-hunting
mission, Kepler has wildly exceeded
all our expectations and paved the
way for our exploration and search for
life in the Solar System and beyond,”
said Thomas Zurbuchen (NASA) in an
October 30th press release.
Launched on March 6, 2009, Kepler
took up station in an Earth-trailing
heliocentric orbit. The telescope’s
initial mission was to stare at a patch
of sky overlapping the constellations
Lyra, Draco and Cygnus, looking for
rhythmic dips in starlight that betray
the presence of transiting planets as
they passed in front of their suns.
To this end, Kepler monitored about
150,000 stars during its 3.5-year
primary mission (upping to nearly
half a million stars during its entire
career) and ultimately turned up
2,899 exoplanet candidates and 2,
confirmed worlds — more than two-
thirds of all planets known in the
galaxy.
The slew of Kepler’s discoveries
suggests that 20% to 50% of starsThe Kepler era comes to an end
in the Milky Way have small, rocky,
Earth-sized planets that could
maintain liquid water on their
surfaces. Kepler also showed us that
the most common sort of world is one
not seen in our Solar System — super-
Earths bigger than Earth but smaller
than Neptune. Just what these worlds
are like remains to be seen.
Kepler also revealed miniature
planetary systems, such as the
eight planets in the Kepler-
system orbiting a Sun-like star or
the six worlds whipping around the
Kepler-42 system, both of which fit all
their worlds in a much smaller space
than our system does and make our
own look sparse in comparison.
The exoplanet-hunting task is now
passed to the Transiting Exoplanet
Survey Satellite (TESS). Launched in
early 2018, TESS will cover most of the
sky during its initial two-year survey
and is expected to add thousands
more worlds to our catalog of known
exoplanets.
Less than three decades ago, no
exoplanets were known. Kepler will
now follow Earth in its orbit around
the Sun, a testament to the pioneering
effort to uncover worlds beyond our
Solar System.
■ DAVID DICKINSONSunset for Dawn
SHORT ON FUEL, the end has come
for NASA’s Dawn spacecraft, the first
and only mission to visit the dwarf
planet Ceres and the asteroid Vesta. It
was the first mission to orbit more than
one body beyond Earth and the Moon.
It was NASA’s first deep space mission
to use ion propulsion. And Dawn was
also the first spacecraft to visit a dwarf
planet, beating the New Horizons flyby
of Pluto by just a few months.
Launched in 2007, Dawn eventually
arrived at asteroid 4 Vesta on July 16,- Dawn revealed the misshapen
world in dramatic detail, mapping it
from pole to pole while probing it from
core to surface. One key finding was
that Vesta seems to be a remnant of the
rocky planetesimals from the early days
of the Solar System. But it turned out
that the exploration of Vesta was just a
prelude for the excitement to that was
to come.
Dawn fired up its ion engines and
departed Vesta on September 5, 2012,
for a 30-month transit to Ceres,
arriving March 6, 2015. On approach
to Ceres — the largest body in the
asteroid belt — Dawn spotted several
bright patches on the world’s surface.
These proved to be briny salt deposits
of hydrated magnesium sulfate and
ammonia-rich clays, remnants of water-
ice eruptions from the world’s interior.
Dawn gave us key insights into the
cryovolcanic activity erupting on the
surface of Ceres and a look at an active
dwarf planet.
The 11-year mission came to an
end when the spacecraft missed two
scheduled communications on October
31 and November 1, 2018, leading
mission scientists to conclude that the
spacecraft had finally run out of fuel.
Without steering ability, Dawn can no
longer aim its main communications
antenna back at Earth. The probe will
now remain in orbit around Ceres, in
line with planetary protection protocols
guaranteeing Dawn won’t crash into the
dwarf planet for the next few decades.
■ DAVID DICKINSON
With its exoplanet-hunting mission now over,
the Kepler space telescope (illustrated) trails
Earth in orbit around the Sun.
NASANEWS NOTES