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They orbit between us and
our star, hidden within the sun's

glare, often visible only at
sunrise or sunset, Venus, the

first and brightest star in
the evening sky, and Mercury

fleeting across the solar
disk. They are half of our solar

system's terrestrial planets,
yet we know so little. As

we begin to take a closer look
at our companions, they are

posing more questions than answers.

This is Mercury, the innermost
planet closest to our sun.

Mercury's days are longer
than its years, and it has an

elliptical orbit from 47
million kilometers at its closest

point to 70 million kilometers.
Difficult to observe from

Earth due to the sun's light,
Mercury is an enigma of the

solar system's evolution.
Well, Mercury is really a weird

planet. The normal terrestrial
planets, all the rest, have a

relation between how big they
are and how dense they are.

Mercury is not following that
rule, it's much more dense

than what you would expect
for its size. That is not normal.

So something went wrong in
the formation of Mercury that we

don't know. Only two probes
have been sent to investigate

the planet, and only one
has achieved orbit. Mercury is

difficult to get to because
of its orbital speed and the

gravitational influence of
the sun. Mercury for us is a

planet of extremes. So you
have temperatures like 400, 450

degrees on the surface during
the day, and imagine that's

like being in a pizza oven
on Earth. So it's really hot

there, and temperatures at
about minus 175 during the night.

Mercury orbits the sun
every 88 days in an atypical

elliptical orbit. At its nearest
approach to Earth, it is 77

million kilometers away.

The first close-up imagery
of Mercury came from the US

probe Mariner 10 in 1974.
With insufficient fuel to slow the

craft, it was not possible
to put the probe into orbit.

Instead, it orbited the sun,
passing by Mercury three times.

The flybys provided two
interesting observations. Firstly,

Mercury has a magnetic
field similar to Earth, which is

unusual due to the very slow
spin of the planet. Secondly,

the surface of the planet
showed extensive cratering and

very little volcanic or crustal
movement. Two contradictory

observations of why the
planet has such a magnetic field.

The second and most recent
probe is MESSENGER. Launched in

2004, it made several planetary
flybys of Earth, Venus and

Mercury to slow down to
the correct speeds. It took six

and a half years to reach
Mercury and enter orbit in 2011.

Because of this short distance
from our central star, the

temperature of the planet is
very, very high. And when you

want to fly around Mercury
with a satellite, you find

yourself in a very special
kind of situation where you get a

lot of heat coming from the
planet itself that behaves like

a thermal mirror, and then
from the side you get the heat

from the sun. So you have
two heating sources which

may destroy a normal
spacecraft. Another NASA mission,

MESSENGER, is getting
operational in a very loose Mercury

orbit because, as I said, the
thermal situation is such that

you better stay away from the planet.

Its primary mission was
completed within a year, mapping

the entire globe with about
100,000 images. With fuel in

reserve, the mission life
of the probe was extended.

After 10 years, MESSENGER
continued to send back information

and observations, magnetic
field data, the magnetosphere,

the effects of solar wind
plasma and studies of the

atmosphere of Mercury.
It is so low in density, it is

constantly blown off the planet
as an exosphere, revealing

the likes of sodium, calcium
and magnesium in a trailing

tail away from the sun.

Spectrographic imaging of
the surface material has thrown

up more questions about
the high metallic density of the

planet. The observations
do not fit the standing theories

of the planet's evolution,
which will have to be completely

overhauled.

One other interesting detail,
at the north and south polar

regions, MESSENGER has
detected the presence of water ice in

the permanently shadowed interiors
of craters, just like on our Moon.

In 2014, the probe was
tasked to fly much closer to the

surface, as close as 50
kilometers. There, it imaged the

surface at much greater
resolution, hunting for more

detailed signs of the
planet's geological history.

Left unaided, the probe
continued to descend and ultimately

impact on the surface. There
was, however, sufficient fuel

in reserve for three trajectory
alterations to increase

altitude and give the probe
a short time extension to

continue its work into 2015.

A third mission to Mercury is
underway by the European Space

Agency and the Japanese agency
JAXA. BepiColombo is named

after the Italian mathematician
Giuseppe BepiColombo, who

first developed the Gravity
Assist Maneuver for NASA with

the Mariner 10 probe. It
consists of two orbiters, the

ESA Mercury Planetary Orbiter
and the Japanese Mercury

Magnetospheric Orbiter. What
scientists want to do is try to

understand the nature of the
planet itself, what material is

on the surface, we would like
to measure temperatures, we

would like to see the interaction
with the solar wind, then

Mercury has a magnetic field,
which is like Earth, a dynamo

field, we would like to understand
that. The Japanese MMO

will focus on the magnetic
field environment around the

planet, while the ESA probe
will focus on the planetary

surface.

Like the previous two probes,
BepiColombo will use Earth

and Venus to slow the probe's
speed down to drop closer to

Mercury's orbit. This
trajectory will take approximately

seven years to accomplish.
With solar electric motors to

maintain deceleration,
then a conventional

rocket engine will be
used for orbital insertion.

The orbital life of the probes
in the harsh environment is

expected to last one to two
years. The two craft carry a

suite of the most advanced
instruments, including a laser

altimeter and magnetometer,
infrared and ultraviolet

spectrometers, imaging X-ray
and high-resolution stereo cameras.

BepiColombo is designed to
answer specific questions about

this planet. Why is the planet
so dense? How did it form? Is

the planet tectonically
active? Why can no iron be

identified on the surface? In
the absence of any ionosphere,

how does the magnetic
field interact with the

solar wind? How is the
magnetic field generated?

We have a theory to understand
how planetary systems form.

Now, the theory was based
on explaining the solar system,

and it was fine. It worked. But
now, when we have discovered

new planets around other
stars in the galaxy, the extra

solar planets, they don't
fit at all. So something is

wrong. And the special
cases, those that are difficult to

understand in their details,
like Mercury, are very helpful.

By finding answers to some of
these questions, BepiColombo

will help us understand how
the solar system was formed 4.5

billion years ago.

It is the first and brightest
star you see at night.

Our sister planet, in
some ways, our closest

planetary neighbor in
both distance and diameter.

However, Venus is difficult to reach, and
a very strange world when you get there.

In the early days of the solar
system, Venus and the Earth

must have been very similar.
But then something happened,

and they took a different
route in its evolution. Venus is

the only planet in the solar
system that needs more time to

rotate once around its own
axis than it rotates around the

sun. There must have been
a major disaster in the early

history of the planet, where
it collided with a big other

object, and this made it stop
its rotation. The problem is

that without the protection,
without the rotation, the

planet lost its water, and so
it's completely dry. And this,

in the end, led to a horrible
greenhouse effect that caused

the temperature to rise to
the levels that we see. One of

the main differences between
Venus and the Earth is simply

that Venus doesn't have a
magnetic field. So the way it

interacts with the star is
completely different. If you want

to understand how did Venus
get that way, why does Venus not

have any ocean, you
need to understand the

interaction between
the star and the planet.

The Soviet Union and the United
States sent probes with some

success. Three Mariner
spacecraft made brief flybys, and the

Soviet Veneras entered the
atmosphere and touched down, some

surviving for minutes. They
managed to send back tantalizing

images of an almost serene vista.

NASA sent two more probes,
Pioneer Venus 1 and 2, the latter

depositing five small probes
onto the surface. The Soviets

continued their program
of Venera probes with four

successful landers and orbiters.
A further two Russian Vega

missions deposited atmospheric
probes as they swung by

en route to Halley's Cormich.
The atmosphere of Venus is

incredibly interesting because
it's so different from the

Earth's atmosphere, and we'd
like to understand what causes

these differences, because
this might well help our

understanding of climate
change on planet Earth. There's a

very dense atmosphere, it's
97% of carbon dioxide, very

strong greenhouse effect, and
the temperature down on the

surface is more than 450
degrees Celsius, and the surface

pressure is 92 bars, it's
almost 100 times more than the

Earth. So it's a very, very
unpleasant place to be. High up

in the clouds, about 100
kilometers from the surface, the

wind speeds are very, very
high. The wind is traveling at

300 kilometers an hour, and
it travels around Venus every

four to five days. So that's
one of the things we don't

understand, why at the surface
the winds are very gentle,

and high up the winds are
very, very fast. So it's a very

different atmosphere to the
Earth's atmosphere. Zero and

liftoff of space shuttle Atlantis.
In 1989, the NASA probe

Magellan was launched from
the payload bay of Atlantis en

route to Venus, its five-year mission
to radar map the entire planet's surface.

The radar managed to
peer through the dense

atmosphere and reveal
the true face of Venus.

What it showed was a young
surface with few craters, and

mostly covered with volcanic
activity, lava flows and large

lava plains.

Surprisingly, there was little
evidence of wind erosion, and

the surface plate tectonics
were dominated by global rift

zones, unlike Earth. To date, that
was the last Venus probe from NASA.

It was 12 more years
before Venus came under

scrutiny again. This time
by ESA's Venus Express.

To conserve fuel for a long
mission, the probe utilized

aerobraking in the Venusian
atmosphere to make orbit.

Very interesting discoveries
of Venus was a huge vortex, a

huge cloud at one of the poles
of Venus that in some ways

looks very similar to a
hurricane on the Earth. But this is

a long-standing event that's
there all the time. We don't

understand how, how it's
formed, why it's there, and how it

will evolve. When we arrived
at Venus eight years ago, we

detected winds at 300 km per
hour, very fast. But what has

happened during these years
until now, they have actually

increased. We now see
winds of 400 km per hour,

and we can't really explain
why that has happened.

More questions were raised by
the planet's absent magnetic field.

Imagine that we're flying
from pole to pole on the night

side, so we're feeling the
plasma around us, and then

suddenly it just, the ionosphere
disappears, and then it

sort of comes back. And this
is what an ionospheric hole is.

It's like a chasm in the
sky, a big hole where the

ionosphere is just absent.
The Pioneer Venus Orbiter

basically measured inside
these things, and we saw that

there is very little plasma,
and there's all this magnetic

field. And from that, they
sort of suggested that this is

essentially a magnetic
structure that's sitting in the night

side. I wanted to see if I
could go looking for these things

with Venus Express. What
we saw is essentially this really

exciting, strong, great
magnetic field line that we

expected, and really boring
plasma signatures. It shows us

that this is a magnetic
structure, right? This is a tube of

interplanetary magnetic field
that, well, PVOs saw it coming

out of the ground, and now
we've seen it way out in the

tail. So these things really
continue out to much higher

altitudes than we previously
have found them at.

As Venus Express neared
the end of its mission, scientists

made some risky aerobraking
maneuvers to bring the probe

much lower in orbit to make
close-up observations of the

planet. These maneuvers
involved daring plunges into the

hostile atmosphere only 130
kilometers above the planet's

surface. For the aerobraking,
we wanted the most area

possible to get the most
friction possible, but that also

generates heat. And so what
we did was we went into the

atmosphere in this direction,
because this face of the

rocket, originally when it was
launched, was most able to

take the forces and the
temperatures. We also turned the

solar panels so that the side
with the actual solar arrays

was in the back to maximize
the area as we went through the

atmosphere, to maximize the
amount of friction, and to get

the most amount of braking.
What we saw that was a little

unusual is the variability in
the pressure, as if there were

waves within the atmosphere,
and so that possible wave-like

structure was not expected,
and analyzing that data will

keep scientists busy for a
little while yet. Venus Express

carried a suite of seven
instruments, magnetometer,

spectrometer, and several
cameras to study the atmospheric

winds and surrounding space
environment. Venus Express

outlived its planned two-year
mission by another seven, and

only recently was it directed
to ditch into the planet's

atmosphere.

The Japanese have made
the most recent attempt to reach

Venus with Akatsuki, whose
planned observations are to

include cloud and surface
imaging with infrared cameras.

Their main focus is the
Venusian meteorology, including

confirming the presence
of lightning and any current

volcanic activity.

The probe failed to enter
orbit on its first attempt, and

JAXA are hopeful when the
probe's heliocentric orbit returns

it to Venus that a second
attempt will succeed very soon.

There are many plans afoot to
return to Venus. Even a manned

mission has been studied by
various groups. One such study

conducted for NASA produced
HAVOC, a high-altitude Venus

operational concept. It would
rely upon two spacecraft sent

into Venus' orbit, the first
unmanned atmospheric descent

stage and the second orbital
return craft carrying the crew.

They would dock, transfer
crew to the descent stage and

proceed to de-orbit. Once
into the upper atmosphere, a

parachute would deploy,
slowing the descent and allowing

the deployment of a dirigible
craft to inflate and carry the

crew at high altitude.

From there, they would
study the planet below.

The return journey for
the crew would begin

with a high -altitude
launch back into orbit,

where they would dock
with the orbiter, transfer crew

and equipment, and then
make the journey back to Earth.