The Astronomy Book

310

In the early 19th century, it
allowed scholars to decipher the
hieroglyphic writing system, thus
unlocking the meaning of many
ancient Egyptian writings. Philae
refers to an obelisk with multiple
inscriptions that was used in a
similar way. Comets are remnants
left over from the formation of the
solar system, so these names were
chosen because the Rosetta and
Philae missions at comet 67P
were intended as a way to unlock
new knowledge of the primordial
material that formed the planets.

Comet cruise
Rosetta took a circuitous route
to the comet, using three flybys
of Earth and one of Mars (a
risky maneuver, skimming its
atmosphere only 150 miles [250 km]
up) to boost speed through gravity
assists. This process took five
years, after which Rosetta had
enough speed to fly through the
asteroid belt (getting a very close
look at some asteroids), and out
beyond the orbit of Jupiter. There,
it began to swing back around,

UNDERSTANDING COMETS

and was soon bearing down on

67P at great speed. For the journey

to deep space, Rosetta had been

powered down to save energy,

but it powered back up and

contacted Earth on schedule as it

neared the comet in August 2014.

Rosetta’s controllers then began

a series of thruster burns to make

Rosetta zigzag through space and

slow from 2,540 ft/s (775 m/s) to

26 ft/s (7.9 m/s). On September 10,

the spacecraft went into orbit

around 67P, offering the first look

at the target world.

Bumpy landing

Comet 67P is about 2.5 miles (4 km)

long and turned out to be more

irregular in shape than expected.

From some viewpoints, the comet

looks like a vast rubber duck, with its

two lobes, one larger than the other,

connected by a narrow neck. (It is

assumed the comet was formed

from two smaller objects making

a low-speed impact.) The surface

was riddled with boulder fields

and ridges, and the Rosetta team

struggled to find a clear location

to set down the Philae lander.

We are on the surface of the

comet! Whatever we do has

never been done before. The

data we get there is unique.

Matt Taylor

Rosetta Project scientist

Rosetta received gravity assists from both Earth and Mars en

route to Comet 67P. As it swung around the planets, their gravitational

fields threw the spacecraft forward at greatly increased speed.

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Rosetta captured this image of
comet 67P/Churyumov−Gerasimenko
on July 14, 2015, from a distance of
96 miles (154 km), as the comet
neared its closest point to the sun.

A landing zone on the “head” of

the comet was selected, and at 8:35

GMT on November 12, 2014, Philae

was released from Rosetta. It took

almost eight hours to confirm that

Philae was on the surface, much

longer than expected. The lander

was designed to touch down at a

slow speed—slower than an object

dropped from shoulder height

on Earth—and attach itself to

the ground using harpoons fired

from the tips of its legs. However,

something had gone wrong. It is

November 13, 2007

2nd Earth gravity assist

June 8, 2012

Enters deep space hibernation

February 25, 2007

Mars gravity assist

1st Earth

gravity assist

Launch

March 2, 2004

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