(mysterious music)
Some of our solar system's planets
have been visited by scientific probes
less frequently than others.
(mysterious music)
The outer gas giants, Uranus and Neptune,
are so distant,
they're hard to reach.
Uranus is 20 times further from the Sun than Earth
While Neptune is 30 times further.
(mysterious music)
Both have only been seen at close range
by NASA's Voyager 2 spacecraft.
Mercury is so close to the Sun
that any probe sent in its direction
must take a circuitous path to offset
the sun's immense gravitational influence.
The Mariner 10 probe flew past Mercury in 1973
and the MESSENGER probe went into orbit
around Mercury in 2011.
(mysterious music)
Venus presents different problems.
Though it's our closest planetary neighbor
and easier for spacecraft to reach,
dense cloud hides its features
and its surface has hellish conditions.
(mysterious music)
The Russian Venera craft have landed,
but in the hostile environment,
they could only survive for minutes.
(space craft swoosh)
(suspense music)
(explosion)
(intense music)
Roughly twice every century,
the planet Venus passes between the Earth and the Sun.
Called the Transit of Venus,
it was closely observed in 1769.
Astronomers realized
that careful timing of Venus's passage
across the face of the Sun
would allow them to calculate the distance to the Sun,
which in turn would unlock far more
accurate methods of navigation.
(serious music)
In 1961, the Soviet Union launched Venera 1,
the first Venus probe.
(spacecraft firing)
It passed Venus as intended,
but mission control had lost contact with it.
(spacecraft engine rumbling)
The following year,
NASA launched Mariner 1 to Venus.
(spacecraft engine rumbling)
{Man on radio] We're not on trajectory.
A coding error led to control problems
with the launcher.
Destruct command.
It was destroyed minutes after liftoff.
(spacecraft explosion)
(indistinct) blew-up.
Because convenient launch opportunities
only occur in 18 month cycles,
NASA had a second probe ready to launch.
Mariner 2 was essentially a range of spacecraft
designed to go to the moon.
These were the early days of the space race
and the United States was desperate to catch up
with the Soviet Union.
Lead times was short
and the Jet Propulsion Laboratory did not have time
to complete its original design.
(spacecraft engine rumbling)
In August 1962, Mariner 2 was launched.
The ranger spacecraft
launched toward the moon
had all failed.
Mariner 2 on its way to Venus
was functioning,
but its transmissions were weak
and due to a launch anomaly,
it was off course.
After a week,
instructions for a complex course correction
were transmitted to the spacecraft
About an hour later,
Mariner executed the maneuver
which involved a roll turn,
followed by a pitch turn
and finally, a main engine burn.
(light upbeat music)
(engine rumbles)
It worked well,
but several days later
the craft lost lock on the Sun and the Earth,
it's two attitude reference points.
It corrected itself before ground control
could diagnose the problem.
Next, the signal strength increased to its normal level,
but a short in a solar panel
left it low on power.
At this time, although both America
and the Soviet Union had been sending
probes toward the planets,
nothing had succeeded.
Mariner 2 lost several telemetry sensors
and it began to overheat.
It continued limping toward Venus,
but some of the spacecraft problems
were solving themselves.
Mariner 2 was now close enough to the Sun
that it could function effectively
on just one solar panel.
It passed slightly less than 35,000 kilometers
above Venus's cloud tops.
It could detect no planetary magnetic field
and it recorded temperatures across the planet
approaching 500 degrees Kelvin.
Clearly, landing on the surface
would present problems.
(fanfare music)
But America wanted to focus
on their first real success in space,
finally doing something that the Soviets had not.
(fanfare music)
Mariner 2 was the first successful
interplanetary probe,
and in California, the home of JPL,
they celebrated.
The next major advances in the exploration of Venus
were made by the Soviet Union.
The objective of the Venera series
was to land on the surface of Venus.
Designed as understood
that not only were the surface temperatures
hot enough to melt lead,
but that the atmospheric pressure
was many times that of Earth.
The landers they built,
looked more like diving bells than spacecraft.
(spacecraft engine rumbling)
In June 1967, Venera 4 was launched.
The vehicle consisted of a carrier craft
with instruments used during
the cruise phase to Venus,
and a spherical landing module
that could communicate independently.
After entering to the atmosphere,
Venera 4's parachute opened.
It sent back data for 93 minutes,
but stopped 28 kilometers above the surface.
Yet its electronics hadn't been overwhelmed by the heat,
it had simply run out of power.
Extrapolations from its final measurements
showed a surface temperature of 500 degrees Celsius
and a pressure of 75 atmospheres,
far higher than anyone expected.
The Venera program strengthened its landers
and fitted smaller parachutes
to reduce descent time.
Launched in January 1969,
Venera 5 and 6 learnt more about
the chemical makeup of the atmosphere,
but neither remained functioning at the surface.
The Venera series continued,
refining the technology
and making incremental improvements
to mission duration,
adding to the knowledge about Venus.
(spacecraft engine rumbles)
In 1975, Venera 9 was launched.
It was a new design
consisting of an orbiter lander combination,
with the orbiter
able to act as a relay station for signals
transmitted from the surface.
Four months after launch,
the orbiter and the lander,
encased in a spherical shell,
separated.
(theatrical music)
It entered the atmosphere two days later
while the mother craft
became the first probe to go into orbit around Venus,
photographing parts of the surface in ultraviolet.
The new lander had a ring shield
that could replace a parachute
during the latter stages of the descent
through the dense atmosphere.
(theatrical music)
Venera 9 transmitted the first
black and white pictures from the surface.
There were designed fault,
meant a second camera could not eject its lens cap.
Three days later and 2000 kilometers away,
a twin craft, Venera 10 landed.
It took pictures too,
but the same design fault
left the lens cap stuck in place.
Both landers had been pre-cool
while still in space
and circulating cooling fluid
kept the craft operating on the blistering surface
for more than an hour.
In 1983, two more Venera craft arrived at Venus.
Equipped with synthetic aperture radar,
they made the first series attempt
to map the surface beneath the cloud layer.
Over eight months,
they mapped from the north pole
down to 30 degrees north.
(man speaking in foreign language)
Nine, eight, seven, six.
(engine blaring)
NASA had taken a minor role
in the early exploration of Venus.
(spacecraft engine rumbling)
But in 1989, the space shuttle Atlantis
lifted off carrying the Magellan probe.
(spacecraft engine rumbling)
Magellan was bound for Venus.
Like the Venera craft before it,
Magellan would use radar
to map the surface of the planet.
It was the first interplanetary spacecraft
launch from the space shuttle.
Following the cruise of 15 months,
Magellan arrived at Venus
and entered an elliptical orbit.
To keep costs down,
the probe had been built
from an agglomeration of spare parts
leftover from previous NASA missions.
After some software problems, it began mapping.
The images it relayed
remained the highest resolution pictures we have
of the surface of Venus.
Pictures of low volcanic blisters emerged
and lava channels where evidence
of an extremely active surface.
The thick atmosphere has prevented all,
but the largest meteors,
reaching Venus' ground
and few impact craters were visible.
Yet evidence of plate tectonics
that sculpts the Earth surface
was not obvious.
After mapping Venus,
Magellan changed its orbit
and plotted the planet's gravitational anomalies.
On Venus, localized changes in gravity
correspond to surface features.
On Earth, this is not the case.
A new naked picture of Venus emerged.
The surface appears to have been
completely remade around half a billion years ago.
Yet while volcanoes and lava channels
are common features on Venus,
Magellan could not find evidence
that volcanic activity still happens on the planet.
In 2006, the European Space Agency's Venus Express
went into orbit around Venus.
Its focus was the long-term analysis
of the planets atmosphere.
During its eight year mission,
it registered a sharp rise
in the atmosphere's sulfur dioxide.
This could be due to changes in wind patterns,
but it could also be a sign of volcanic activity.
Researchers also saw increases in infrared radiation
coming from three different volcanic locations,
more circumstantial evidence
of current volcanic activity.
Finally, the infrared teams
saw short-term temperature changes
that fluctuated over just a few days.
It appears that volcanoes may still be active on Venus.
The mission ended in 2015
with a series of swoops into the upper atmosphere
that verified unexpected ripples in the mesosphere.
Very little in the way of Venus exploration
has happened since Venus Express.
There're elaborate plans exist
for future missions to Venus,
nothing at this stage has been funded,
yet many missions still pass close to Venus
to use its gravitation to alter their flight paths.
In 1974, Mariner 10 was the first spacecraft ever
to use the gravitational slingshot effect
on its way to Mercury.
Italian Mathematician Giuseppe Colombo
devised the maneuver
as a way to save fuel
and to fly pass Mercury not once,
but several times.
The technique is now commonplace.
10 days after launch,
Mariner 10 executed instructions
for a routine course correction.
(fire blazing)
This appeared to go well,
but after the burn,
when the craft attempted to reorient itself,
there was a problem.
Mariner 10 knew where it was pointing
because it's tracking sensor
could lock onto the star Canopus,
but a flake of paint
that had come from the spacecraft
was confusing the system.
An automated backup procedure found Canopus again,
but flaking paint was an issue
for the rest of the mission.
To reach Mercury,
a spacecraft must approach the Sun
and its immense gravity presents a problem.
Voyages to outer planets
are constantly slowed by solar gravity,
but with the inner planets,
a spacecraft constantly accelerates.
Mariner 10 used Venus's gravity
to reduce its speed
and it approached Mercury at an acute angle.
Mariner 10 did not have enough fuel
to go into orbit around Mercury,
but its sun-centered path allowed the probe
to make three close passes.
Its first pass revealed a moon-like planet
with a heavily cratered surface.
Though Mercury is the smallest planet,
it's the most dense.
It has a large iron rich core.
Prominent escarpments were seen.
Here, Discovery Scarp cuts through to craters.
It falls three kilometers.
It's thought that these cliffs are the result
of cooling and shrinking of the core.
Mariner 10 continued to suffer technical problems.
It's tape recorder kept sticking,
there were restrictions in the rates
of data transmission
and limited attitude control
meant flight engineers were using solar pressure
on the high gain antenna
and solar panels to compensate,
yet the mission continued.
Mariner 10 could only map about 45% of Mercury's surface,
as the same hemisphere faced the Sun
during each of its passes.
Mariner 10 discovered a very thin atmosphere
primarily of helium.
Several months after its third and final pass of Mercury,
it ran out of fuel.
It still orbits the sun.
Main engine start,
tWo, one and- (fire explosion)
It was more than 30 years
before the next mission to Mercury.
In 2004, MESSENGER was launched.
It was designed to go into orbit around Mercury,
which presented a number of design constraints.
It featured a large, woven, ceramic sun shield,
but it did not have a dish antenna.
It would rely on a phased array
that could be electronically pointed.
After a year in space,
MESSENGER was back at Earth
using its gravitation to modify its orbit.
Even though it was not a large spacecraft,
it had a powerful engine for course corrections
and orbit insertion.
It continued on to pass Venus twice
to lose speed as it drew closer to the Sun.
Three and a half years after launch,
MESSENGER approached Mercury,
but this was not the of its journey.
The probe made two more passes of Mercury
before finally going into orbit
after almost seven years in space.
Mission engineers had the extra problem
of always requiring the probe sun shield
to be pointed toward the sun.
Because it was in orbit,
MESSENGER was able to complete the mappings
started by Mariner 10.
The planet's dominant feature
is the Caloris basin.
It's an ancient crater
more than 1,500 kilometers across.
Mariner 10 saw some of the area,
but the rest have been in darkness.
This map of the southern polar region
uses color to represent illumination.
Because Mercury's axis is not tilted,
sunlight cannot penetrate deep craters near the poles.
It was in these areas
that MESSENGER discovered substantial amounts of water ice.
MESSENGER received several mission extensions,
but in 2015 it crashed into Mercury
after running out of fuel.
A new mission
is already on its way to Mercury, BepiColombo,
named after the designer of Mariner 10's trajectory.
It's a joint effort
between JAXA, the Japanese space agency
and the European Space Agency.
It will take seven years to reach Mercury.
The Voyager 2 spacecraft
is the only probe to have made close approaches
to the two outer ice giants,
Uranus and Neptune.
Launched in 1977 with its twin, Voyager 1,
it was able to take advantage of a rare alignment
of the four outer planets,
enabling it to make close observations of each one.
In 1986, Voyager approached Uranus.
In the distant past,
it must have been hit by another massive body
that knocked its axis sideways.
Uranus has an east and a west pole
and for half it's orbit,
one side sees continual sun
while the other remains in darkness.
It has rings which follow
its north/south equator.
Voyager 2 discovered 11 new moons
and a misaligned magnetic field.
Images that the Voyager captured
showed Uranus as a bland, featureless planet,
but this was because of its particular season.
With images from the Hubble Space Telescope,
we now know that at certain times
clouds and planetary weather appear in the atmosphere.
Uranus' largest moon, Miranda,
was observed in detail for the first time.
So chaotic is it surface
that researchers thought
that it must have been blown apart
by some cosmic impact,
with the fragments reforming.
Now it's thought that tectonic forces,
initiated by the gravitation of Uranus,
are responsible for the moon's jumbled appearance.
As Voyager 2 left Uranus,
backlighting from the sun revealed
two new rings encircling the planet.
The spacecraft was now heading toward Neptune,
the solar system's last planet.
In the three years it would take to get there,
ground engineers began preparing for unique challenges.
(suspense music)
Neptune is 30 times further from the Sun
than the Earth,
and the light intensity is 1000th what it is here.
For photography, time exposures would be necessary,
yet Voyager 2 was traveling so fast
that images would spear
without special preparation.
Engineers calculated
just how much the craft would have to swivel
while exposures were made
to compensate for the probes movement.
In June 1989,
Voyager 2 began returning distant images of Neptune.
Across the world,
people had realized that the data sent back
to Earth by this spacecraft
was transforming our understanding of the solar system.
(indistinct chatter) (man clapping)
This was before the internet age.
Researchers of the Jet Propulsion Laboratory
clustered around TV sets to watch as data
and images came in line by line.
Neptune is a more conventional planet than Uranus.
It's axial tilt is 30 degrees
and it revolves in the same direction as Earth.
While Neptune is slightly heavier
than its fellow ice giant Uranus,
it has a slightly smaller diameter.
And though it is further from the Sun than its neighbor,
Neptune emits more heat than Uranus.
The planet has an internal heat source
that drives more dynamic weather patterns.
Voyager 2 measured wind speeds at Neptune
in excess of 2000 kilometers per hour,
the fastest in the solar system.
There were cirrus clouds in the atmosphere
and the probe recorded pictures of a great dark spot
similar to Jupiter's great red spot.
It was an anti-cyclone
in the Southern hemisphere
as large as the Earth.
In 1994, when Hubble tried to find the same feature,
it had disappeared,
but a new dark spot was forming in the Northern hemisphere.
Voyager 2's last observations within the solar system
where of Neptune's largest moon, Triton.
Unlike all other moons in the solar system,
Triton has a retrograde orbit,
indicating that it was not formed
at the same time as the planet,
but that it had been captured.
As Voyager 2 moved beyond the planets,
its cameras will switched off to save power.
Both Voyagers continue away from the solar system
measuring the influence of the solar wind.
This remains the only mission to the ice giants.
(indistinct chatter)
On January the 19th 2006,
an Atlas V was launched.
(spacecraft engine rumbles)
It was a very powerful rocket
with an unusually small payload.
New Horizons left Earth orbit faster than any other probe.
It was headed for the Kuiper Belt
at the outer edge of the solar system,
in particular, Pluto.
In a little more than a year,
New Horizons reached Jupiter
where it received a gravitational assist
that cut three years from its flight time to Pluto.
After it passed Jupiter,
the spacecraft went into hibernation,
simply sending an all is well transmission
once a week.
It took New Horizons more than nine years
to reach Pluto.
Since it had departed,
Pluto had lost its status as a planet.
With a discovery of more objects
of similar size in the Kuiper Belt,
it was decided that to be a planet,
a body had to clear its orbit.
Pluto's features surprised everyone.
Here was a living planet
shaped by tectonic forces,
but instead of rock,
the mountains were made of ice and frozen methane.
And Pluto has a thin atmosphere,
mainly of nitrogen.
The probe continued on over Charon, Pluto's largest moon.
It's icy surface has deep canyons
and some evidence suggests that it has ice volcanoes.
Charon is about half the size of Pluto
and the two orbit each other.
From Pluto, Charon would appear motionless in the sky.
As the New Horizons probe sped away from Pluto
into deep space,
it began the slow process of transmitting its recorded data
back to the Earth.
At these distances,
it takes signals four and a half hours to reach Earth
with data coming in at one kilobit per second.
It took 469 days for all the Pluto information
to be received back on Earth.
Early in 2019, New Horizons passed
through trans-Neptunian object, Ultima Thule.
And with a mission extension,
it continues exploring the outer reaches
of the solar system.
(exciting music)