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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.