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The 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 t
o our Sun.

Mercury's days are longer then 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.

- Mercury is really a weird planet.

The normal terrestrial
planets, all the rest,

have a relation between how big they are

are 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, first, is a planet of extremes.

So, you have temperatures
like 400, 450 degrees

on the surface during the day.

I imagine that's like being
in a pizza oven on Earth.

So, it's really hot there,
and temperatures have

about minus 175 degrees at 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 closeup 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's a 30-day high.

And when you want to fly around
Mercury with a satellite,

you find yourself in a very
special thermal situation

where you get a lot of heat coming

from the planet itself that behaves

like a thermal meter.

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, that 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
a 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.

BepiColumbo is named after
the Italian mathematician

Giuseppe Bepi Colombo 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,

the other 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 xray and
high-resolution stereo cameras.

BepiColumbo 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 failure 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 exosolar planets,

they don't fit at all.

So, something is wrong.

And the special cases, those that are

difficult to understand in the details,

like Mercury, are very helpful.

By finding answers
to some of these questions,

BepiColumbo 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 Earth must
have been very similar.

But then something happened,

and they took a different
road 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 outer object,

and this made it stop its rotation.

The problem is that resulted protection,

the resulted 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 today.

- One of the main differences
between Venus and the Earth

is simply Venus that 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.

The 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 One and Two.

The later 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

on route to Halley's Comet.

- 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.

- It's a very dense atmosphere

to 97% of carbon dioxide.

Very strong greenhouse effect,

and the temperature down on the surface

is more than 450 degrees Celsius.

And the 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

on route to Venus.

It's 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 in 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.

It's there all the time,

and we don't understand how.

How it's formed, why it's there,

and how it would evolve.

- When we arrived at
Venus eight years ago,

we detected winds

at 300 kilometer per hour, very fast.

But, what has happened
during these years until now,

they have actually increased.

We now see winds of 400
kilometers per hour,

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

More questions were raised

by the planet's absent magnetic field.

- Venus doesn't have a magnetic field.

So, Venus is just this big
rock with this atmosphere,

I mean ionosphere, sitting in space.

And the supersonic solar wind

with the interplanet magnetic field,

whacks into Venus.

And it can't penetrate
through the ionosphere.

So, the interplanetary
field lines are so piled up

ahead of Venus, like cars on a freeway.

This solar wind whacking into it

essentially sort of
strips off the atmosphere,

and it literally forms this
long tail, like a comet,

but stretching out into space.

Pioneer Venus Orbiter
was the first spacecraft

to really thoroughly explore
that night-side region

where the atmosphere is
always escaping away.

And it discovered this
really mysterious phenomena

which has yet to be explained.

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,
straight magnetic field line

that we expected, and really
boring plasma signatures

that shows us that this is
a magnetic structure, right?

This is a tube of
interplanetary magnetic field

that well, PVO saw it
coming out of the ground.

And now, we've seen it
way out in the tails,

and 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 closeup
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 spacecraft

which had been attached to 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 rays

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 of
a Venusian meteorology

including confirming the
presence of lightening

and any current volcanic activity.

The probe failed to enter
orbit on its first attempt,

and JAXA are hopeful that when

the probe's heliocentric
orbit returns it to Venus,

that a second attempt
will succeed very soon.

There are many plans of
folks 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 spacecrafts 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 deorbit.

Once into the upper atmosphere,

a parachute would deploy,
slowing the descent,

and allowing the deployment
of a divigable craft

to inflate and carry the
crew at high altitude.

Looking good HAVOC.

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.

There, they could dock
with an Orion capsule

for the return to Earth.