Sky & Telescope - February 2019 - 12

MISSIONS

MISSIONS

The Kepler Space Telescope Comes to an End

Sunset for Dawn

AFTER A NINE-YEAR historic mis-

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 16th,
2011. Dawn revealed the misshapen
world in dramatic detail, mapping it
from pole to pole while probing it from
core to surface (S&T: Nov. 2011, p. 32).
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 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 waterice 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 (S&T: Dec. 2016, p. 16).
The 11-year mission came to an end
when the spacecraft missed two scheduled communications on October 31st
and November 1st, leading mission scientists to conclude that the spacecraft
had ﬁnally run out of fuel. Without
steering, 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.

sion, the Kepler space telescope has
ﬁ nished 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 2,681 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 stars
in the Milky Way have small, rocky,

With its exoplanet-hunting mission now over,
the Kepler space telescope (illustrated) trails
Earth in orbit around the Sun.

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 - superEarths bigger than Earth but smaller
than Neptune (S&T: Mar. 2017, p.
22). Just what these worlds are like
remains to be seen.
Kepler also revealed miniature
solar systems, such as the eight planets in the Kepler-90 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 (S&T: Mar. 2018, p. 22).
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 DICKINSON

■ DAVID DICKINSON

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FE B RUA RY 2 019 * SK Y & TELESCOPE

N ASA

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Sky & Telescope - February 2019

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