Mercury Exploration: The First Planet From The Sun

Mercury is our solar system’s smallest planet. It’s only slightly larger than Earth’s moon. And like the moon, it has almost no atmosphere. As a result, craters from debris impact litter Mercury’s surface.

Mercury and its neighbor Venus are the only planets in our solar system without any moons. So let’s dive right and learn some fun facts about Mercury, the first planet from the Sun.

Mercury Exploration: The Second Planet From The Sun
Rendering Credit: Wikimedia

Why Doesn’t Mercury Burn Up Since It’s So Close To The Sun?

Since Mercury is sun-scorched, why doesn’t the planet burn up? Mercury’s dayside superheats to around 800 degrees Fahrenheit (430°C). But at night, it plummets hundreds of degrees below freezing, down to -290°F (-180°C). It’s so cold that ice may form in some surface craters.

While the Sun superheats one side of Mercury, it’s not hot enough to melt the dense planet. The radius of its metallic inner core is 1,289 miles (2,074 kilometers). That’s almost 85% of the planet’s entire radius.

Evidence exists that part of the inner core is molten or liquid. It is similar to the Earth’s solid inner core, surrounded by a fluid, molten iron, and nickel outer core. Mercury’s outer shell is similar to Earth’s rocky mantle and crust.

What Spacecraft Made Visits To Mercury?

While humans haven’t traveled to the planet, Mercury exploration is still critical. Past and ongoing investigations seek to answer some of our most pressing Mercury questions.

Mariner 10

Mariner 10 was the first spacecraft to make flybys of Mercury, collecting data as it passed.

  • Three flybys in 1974 and 1975
  • The spacecraft took more than 2300 images.


The first spacecraft to orbit Mercury was the MESSENGER.

  • MErcury Surface, Space ENvironment, GEochemistry, and Ranging mission
  • Launched from Cape Canaveral, Florida, on August 3, 2004.
  • January 14, 2008, orbited at a distance of 125 miles.
  • October 6, 2008, orbited at a distance of 124 miles.
  • September 29, 2009, orbited at a distance of 124 miles.


The European Space Agency’s BepiColombo completed two flybys of Mercury with plans for more.

Photo Credit: NASA
  • Flyby dates: Oct 1, 2021; June 23, 2022; June 20, 2023; Sept 5, 2024; Dec 2, 2024; Jan 9, 2025
  • Arrival at Mercury: December 5, 2025
  • Beginning of routine science operations at Mercury: Expected in February 2026
  • Mercury exploration answers sought:
    • Where did it form?
    • Is there water on Mercury?
    • Is the planet dead or alive?
    • Why is Mercury so dark?
    • Why does Mercury have a magnetic field?

How Long Would Humans Survive On Mercury?

Because of its temperature extremes and the planet’s solar radiation, it’s unlikely that life as we know it could exist on Mercury. Humans could not survive at all. 

Venus is warmer

Although Mercury is closer to the Sun, it is cooler than Venus. Venus has a denser atmosphere and higher albedo, the highest in the solar system at 0.90. So that means it reflects more of the Sun’s heat than it absorbs. But once the heat gets reflected, it struggles to pass through Venus’ thick atmosphere. So heat gets trapped.

Mercury is colder

In terms of temperature, Mercury is the opposite of Venus. Mercury’s thin atmosphere and low albedo allow it to release or absorb most of the heat it receives. Mercury receives four times more of the Sun’s energy and absorbs almost nine times more than Venus. But it’s still cooler than its neighboring planet.

Photo Credit: NASA’s Messenger

Rotation around the Sun

Mercury rotates very slowly. One complete rotation takes 59 Earth days. Mercury travels around the Sun more quickly than any other planet, taking only 88 days to travel around the Sun. Because of this slow spin and fast rotation around the Sun, Mercury only has one sunrise every 180 days. 

Standing stationary on the planet, you’ll be in the dark for six months out of every twelve Earth months.


Although Mercury is inhabitable to life as we know it, scientists are still gathering data about the planet. Planetary scientists hope that BepiColombo will provide answers about how Mercury was formed, its magnetic field, and its core.

Uranus Orbiter and Probe Mission 

The Uranus Orbiter and Probe is scheduled to launch in 2031/2032. A gravity assist from Jupiter will slingshot the Orbiter and Probe toward a 2044/2045 Uranus arrival. 

The Uranus Orbiter and Probe mission may bog down here on Earth due to budgetary issues or competing NASA missions before the launch date. If the 2031 or 2032 launch date is missed, it will not be possible to obtain a gravity assit from Jupiter. 

Lacking a gravity assist, the Uranus Orbiter and Probe will arrive at Uranus in 2053.

Uranus Orbiter and Probe Mission 
Photo Credit: NASA

Why Visit Uranus?

Funding competition is fierce between space exploration projects to determine which projects will move forward. Some projects proceed after a preliminary feasibility study. 

Planetary Scientists would like to explore both ice giants Uranus and Neptune. Budgets, timelines, and politics come into play.  

  1. Finance
    1. Spacecraft flight time to Uranus with a gravity assist is roughly a decade.  
    2. Flight time to Neptune is approximately twelve years. 
    3. Time is money. Estimates add $200 million to the baseline project if Neptune is selected instead of Uranus. 
    4. Spacecraft requires a larger rocket to reach Neptune. 
  2. No significant exploration
    1. There have been zero dedicated missions to Uranus.  
    2. Most of our knowledge of Uranus is from the Voyager 2 mission.  
    3. Voyager 2 flew by Uranus in January 1986 on its way out of the solar system. 
  3. Similarities to other exoplanets
    1. Astronomers have found over five thousand exoplanets in other star systems that appear very similar to our “ice-giants.”  
    2. Lacking exploration and a baseline understanding of our solar systems’ ice-giant planets makes comparison impossible

The National Academies of Sciences, Engineering, and Medicine voted in 2022 that NASA’s primary planetary science project for the next decade should be a mission to Uranus. The Uranus project is the number one priority flagship mission, and that’s a big deal.

Uranus' Atomsphere
Photo Credit: NASA with The Hubble Telescope

NASA Mission Classification

NASA has three types of mission classification. The Discovery, New Frontiers, and Solar System Exploration/Flagship missions. The program classification is for small, medium, and large space exploration projects.

Discovery Program

  • Addresses vital scientific questions about the solar system
  • 450 million dollar budget cap, excluding specific hardware/software/analysis and project partner contributions
  • New launch every thirty-six months

New Frontiers Program

  • Addresses high-priority goals that the planetary science community has established
  • 850 million dollar budget cap, excluding particular hardware/software/analysis and project partner contributions
  • New launch every sixty months
    • Dragonfly will launch in 2026 to explore Saturn’s moon, Titan

Solar System Exploration Program/Flagship Missions

  • High-priority, “large” missions based on the planetary science community input
  • Big budget. Previous missions have cost billions of dollars.
  • NASA leads the project
Photo Credit: NASA

Uranus Mission Objectives

The spacecraft arriving at Uranus will have two components. An Orbitor that will collect data as it circles Uranus. The Orbitor will release a Probe into Uranus’ atmosphere.

  1. Orbitor-Will orbit around Uranus. Data collection from Uranus and its moons.
    1. Magnetic fields
    2. Why is the planet so cold?
    3. Rotation. Uranus has an axis rotation of approximately 98 degrees. (Earth is about 23 degrees.) How/Why did this happen?
    4. Ring composition. Uranus has 13 rings, but little is known about them. We know that they’re red, blue and grey. What’s the elemental composition? How were the rings formed?  
    5. Moons-
      1. Do the largest moons have oceans under their icy crusts?
      2. What is their regolith composition?
  2. Probe-Will be deployed through the Uranus atmosphere to determine
    1. Atmosphere composition
    2. Atmospheric winds source?
    3. Wind speed as a function of depth
    4. Thermal stratification 

Uranus Orbiter and Probe-Technical development

The National Academies of Sciences, Engineering, and Medicine selected the Uranus mission based on NASA’s preexisting space exploration capabilities and the technology available today. No new technology needs development for inclusion in the Uranus Orbitor and Probe. It’s a “shovel-ready” mission.  

Mission planning, spacecraft manufacturing, and concrete mission objects must be defined to begin planning for a specific launch date.  

Rendering Credit: iStockPhoto

Wrap up!

A project that requires several decades of active engagement, both operationally and financially, will require Herculean efforts to reach fruition. NASA must soon decide if they’re up to the task.  

Triton: Neptune’s Largest Moon

99.5% of all the mass orbiting around Neptune is part of the moon, Titan.  Neptune’s largest moon, Triton, was discovered on October 10, 1846. English astronomer William Lassell found it just 17 days after the discovery of Neptune itself. Triton is the seventh-largest moon in the solar system and the only large moon with a retrograde orbit. 

Triton and Neptune
Photo Credit: NASA

Triton’s Characteristics

With a composition similar to Pluto and its retrograde orbit, scientists think Triton was a dwarf planet from the Kuiper Belt.

The Moon’s Size

Triton is a rocky, icy world with a diameter of about 1,700 miles (2,700 kilometers.) To put that in perspective, it’s about 4.7 times smaller than Earth but nearly the size of our moon. Triton is also larger than the dwarf planet Pluto.

Because of its large size, astronomers believe Neptune’s other moons probably formed after Triton’s capture. Moreover, they believe the capture event would have significantly disrupted or destroyed any satellite system around the planet. So the moons we see today are smaller and likely newer than Triton.

The Moon’s Makeup

Triton is full of volcanic activity caused by the Sun’s heat. 

  • A thin atmosphere of icy nitrogen and methane 
  • Rock and metal core. 
  • The surface consists of smooth volcanic plains. 
  • Mountains and craters are formed by frozen lava flows.

The moon’s density is twice that of water, higher than most other outer planet moons. The higher density likely signals a very rocky core.

Neptune's Moons
Photo Credit: NASA

The Moon’s Orbit

Triton’s retrograde orbit means it travels in the opposite direction to the planet’s rotation. This orbit direction is likely due to Triton being a captured object rather than a moon formed with Neptune.

Another orbital fact is that, like Earth’s moon, Triton and Neptune share a synchronous rotation. So one side of Triton’s face is constantly exposed to Neptune. 

But unlike our moon, Triton’s orbital inclination means its poles alternate turns facing the Sun. Since Neptune takes 165 Earth years to travel around the sun, each of Triton’s polar regions spends about 80 years soaking up the solar heat.

Is There Life on Triton?

Does Neptune’s moon, Triton, bear consideration as humans continually seek the possibility of life in outer space? Probably not.

The moon has mild winds, similar to our moon. But due to the thin atmosphere, you probably wouldn’t feel them. However, when Voyager 2 flew over Triton in 1989, it measured the moon’s temperature at -391 degrees Fahrenheit (-235 degrees Celsius.) So the surface is too cold for life as we know it to exist. 

The spacecraft measured the moon’s atmospheric pressure at 20,000 times less than Earth’s surface pressure. And finally, Triton has shallow gravity, about eight times lower than ours.  

If life on Triton were possible, it wouldn’t be like life on Earth. For example, even if you lived on Triton’s “sunny pole” for its eighty-year stretch, you would still receive no benefit from the sun’s warmth. 

On the other hand, if you lived in an area of Triton that does experience night and day, the Sun would only be a tiny blip on your horizon. And because of Triton’s retrograde orbit, you would see the faraway Sunrise in the west and set in the east, opposite to what we see on Earth.

Triton's Surface
Photo Credit: NASA

How The Moon Got Its Name

This moon’s name comes from the Greek demi-god and merman, Triton. Half man and half fish, he was the son of Amphitrite and Poseidon (Neptune.) Since all Neptune’s moons have names from the Greek water gods and nymphs, Triton fits.

Additionally, Triton was known simply as “the satellite of Neptune” until Neptune’s second moon, Nereid’s discovery in 1949. 


Triton is Neptune’s first discovered and largest moon. Its freezing temperatures make it uninhabitable to life as we know it, even though its surface shows smooth areas. This icy moon likely began as a dwarf planet before Neptune forced it into her orbit.

Hippocamp’s Unlikely Discovery

Hippocamp, also known as S/2004 N 1, is a small natural satellite of Neptune. Hippocamp’s unlikely discovery came in July 2013 by a team of astronomers analyzing 150 archival images from the Hubble Space Telescope.

Photo Credit: NASA

How Was Hippocamp Found?

Dr. Showalter’s group of scientists used a technique called “stellar occultation,” which measures a star’s brightness while the star is behind another object. By measuring how much the intensity of the star’s brightness dimmed, the team could determine the size and shape of the blockage.

Using both stellar occultation and “image stacking” of the Hubble images, the group was able to detect a small object orbiting Neptune. The object is estimated to be orbiting at about 12,000 miles (20,000 kilometers) from Neptune. 

Astronomers used Newton’s laws of motion to estimate the location of Hippocamp when they collected images.  

  • Estimate the “location” of the Hippocamp in each Hubble image
  • Stack a new image (with software algorithms) based on vector mapping from the “last” estimated position of Hippocamp to the new place.
  • Adjust the “image center” to the subsequent estimated position and stack it again
    • It’s similar to timed exposures on a telescope with an equatorial mount. The software algorithm is the mount. 

How The Moon Got Its Name

Provisionally designated S/2004 N 1, Hippocamp got its name in conjunction with Neptune’s Greek nomenclature. All moons in Neptune’s orbit have titles from Greek mythology’s lesser gods and nymphs.

So the moon “Hippocamp” is born of mythological sea monsters. These creatures had a horse’s upper body and a fish’s lower body. Nereid nymphs rode them. And a team of hippocampus pulled the Greek sea god Poseidon’s (or the Roman Neptune’s) chariot.

Neptune's Rings
Photo Credit: NASA


Further observations by the team revealed that Hippocamp is a tiny moon with a diameter of only about 20 miles (32 kilometers.) It’s about 1/1000th the size of its nearest moon neighbor, Proteus. 

Additionally, Hippocamp likely chipped off Proteus after a comet hit it billions of years ago. Hubble images show a significant impact crater in Proteus named the Pharos Crater. Planetary scientists believe a small piece of the moon from that impact is now what we see as Hippocamp. 

Hippocamp’s Unlikely Discovery

This moon resides about 3 billion miles from Earth. It is also very faint, reflecting only about 1% of the light that reaches it, making it difficult to detect using traditional techniques. 

  • Hippocamp is so small and dim that it’s approximately 100 million times beneath the faintest star we can see without a telescope from Earth. 
  • It’s too faint to detect in a single Hubble Telescope picture.

Hippocamp’s unlikely discovery was a significant achievement, as it marked the first finding of a new Neptune moon in more than 20 years. Before this discovery, the last new moon found around Neptune was Proteus in 1989.

The Moon’s Features

Since its discovery, astronomers have studied Hippocamp using various telescopes and spacecraft, including the Hubble Space Telescope and the Voyager 2 probe, which flew past Neptune in 1989. These studies provided more information about the moon’s size, shape, and surface features, as well as its orbit and other characteristics.

Photo Credit: ESO/P. Weilbacher (AIP)


Hippocamp is likely a remnant from the early solar system, formed from the same material as Neptune and its other moons, especially Proteus. Additionally, like our own moon, Hippocamp is slowly spiraling away from Neptune.


This moon orbits Neptune once every 22 hours and 48 minutes of Earth time. Since it’s close to Proteus, Hippocamp is influenced by its significant gravitational pull. Larger moons typically eject smaller moons from their gravitational orbits or absorb them completely. However, these two moons coexist in close proximity.


A seemingly literal knockoff from the larger moon, Proteus, Hippocamp’s unlikely discovery came from analyzing Hubble Space Telescope images. Dr. Mark Showalter and his astronomy team found Neptune’s newest moon in 2013. Since then, they have continued learning more about this tiny moon.  

How many other moons reside in the solar system waiting for us to discover them?

Enceladus- Saturn’s Breakthrough Ocean Moon

Photo Credit: NASA

Few moons create excitement like Enceladus! The ice-covered moon is the most reflective object in the solar system. Cassini flybys turned planetary science on its head! The data analysis is ongoing nearly a decade after Cassini left Enceladus.  

Gravitational Flexing

The interior core of Enceladus increases in temperature as the moon responds to the gravitational forces of Saturn.  

  • Enceladus is tidally locked.
  • One side of the moon always faces Saturn
    • Highest gravitational forces from Saturn
  • One side of the moon faces away from Saturn
    • Lower gravitational forces from Saturn
  • Gravitational intensity changes during orbit
  • The different gravitational forces create friction, which creates heat.

South Pole


The south pole has large crust fractures. Cassini detected approximately 101 geysers located within the fractures. The geysers are ejecting “something fluid” hundreds of miles into space at about 800 miles per hour (1287 km per hour). A portion of the plume falls back to the surface of Enceladus. The other part enters into orbit around Saturn in the E-Ring.

Rendering Credit: Planet Volumes


Scientists expected Enceladus’s surface temperature to be approximately -315℉ (-192℃). Roughly 80% of the sunlight is reflected from Enceladus. Scientists expected the poles to be colder than the Equatorial temperature. 

Infrared spectrometer data of the entire south pole shocked the scientists with an average temperature of -304℉ (151℃). Data subsets are much warmer at -261℉ (-163℃). Why are the temperatures so much different than expected?

The crust is thinner

Doppler data analysis reveals that the average crust/ice shell thickness is between 11 and 14 miles (18 to 22 km). The South Poles’ crust/ice shell thickness is much closer to 3 miles (5 km). Scientists believe that there’s an ocean underneath the south pole!


Scientists couldn’t perform ground-level testing of the ocean inside Enceladus, but they could analyze the ejected matter from the underwater sea. Cassini flew through the plume of vapor seven times over a decade.

  • Mass spectrometer data analysis reveals that the vapor plume contains methane, ammonia, nitrogen-bearing organic compounds, oxygen, and carbon.  
  • Liquid water and rock interact at temperatures greater than 200℉ (90℃) and create Silica Nanograins. Silica nanograins equal hydrothermal activity. 
  • Hydrothermal sites in the earth’s oceans contain organisms like tubeworms and crabs.
Enceladus rendering
Rendering Credit: NASA

Mission time!

NASA relies on the scientific community to define and prioritize scientific questions. Once per decade, the National Academy of Science (NAS) generates recommendations for NASA. 

Two thousand four hundred members, including 190 Nobel prize winners, create recommendations for NASA. 

In 2022 the NAS recommended that NASA develops the Enceladus Orilander. The projected launch date is 2038, landing on Enceladus’s surface in 2050.  

The planned mission will orbit for 18 months and then collect data on the surface for 24 months. Plume data will be collected and analyzed: 

  1.  Inside the E-Ring 
  2. On the surface as the plume drops material onto the Orilander.
  3. Below the surface with plume content that’s “dug up” with a probe.

Wrap up!

Enceladus is exciting! The building blocks of life might all be ready for discovery in an ocean buried under ice.   

Why Don’t We Explore Venus?

Photo Credit: NASA

An Overview of the Conditions on Venus

The physical environment on Venus is so horrific that humans can’t visit. Sulfuric rain, high temperatures, and crushing pressure do not equal a good place to visit. 

Galileo observed the size and shape of Venus change over several months as it circled the sun. Watching through his telescope in 1610 Galileo discovered that Venus was a planet and not a star.  Venus is discernible with the naked eye. The earliest humans that walked the Earth looked to the night skies and saw Venus.

Venus Firsts

  • Mariner 2: Venus is the first planet visited by a spacecraft.  Mariner 2 passed within 21,607 miles (34,773 km) of the planet on December 14, 1962.  Mariner 2’s final data broadcast was on January 3rd, 1963.  Today Mariner 2 orbits the sun.
  • Venera 4 descent module transmitted atmospheric data in 1967
  • Venera 7 landed on the planet in 1970 and transmitted data for 23 minutes.
  • Mariner 10: Ultraviolet (UV) images captured for the first time.
  • Verena 9/10: Captured first black and white images on the surface of Venus.  Spacecraft survived for one hour.
  • Verena 11/12: 1978 Spacecraft transmitted data for two hours.
  • Venera 13/14: Transmission color images of the surface of Venus.
  • 1980s-1990s: Radar mapping of the planet.
  • Present: The Akatsuki is studying the chemistry and physics of the atmosphere. 
  • Future: Three new missions will begin within the next decade.
  • DAVINCI: Orbiter and Probe (2029)
  • VERITAS: Orbiter (2031)
  • EnVision: Orbiter (2032)
Photo Credit: NASA

Why hasn’t there been more exploration of Venus?

Venus is closer to Earth (24 million miles/38 million km) than Mars (34 million miles/54.5 km).  So why hasn’t there been more exploration of Venus? 

Even though Venus is our closest next-door neighbor, she isn’t very friendly!

Temp Low-225°F/-123°C719°F/655 °C
Temp High70°F/20°C864°F/462 °C
Pressure (bar)0.01651892
CloudsIce CrystalsSulfuric Acid

Reasons to Not Visit Venus

  1. The temperature is hot enough to melt Lead, Zinc, and Aluminum.  
  2. The pressure on the surface of Venus will kill humans very quickly.
    1. Good news! You’ll last about five seconds due to the pressure.
    2. Bad news! The surface temperatures will kill you in roughly one second.
  3. The pressure on the surface will collapse your spacecraft. You’ll need a spaceship akin to a submarine that can handle enormous pressures.  (Plus, there’s still that pesky temperature to deal with!)
  4. The clouds on Venus contain Sulfuric Acid. 
  5. There are wind storms on Mars, but Mars has a shallow atmospheric pressure. Venus’s thick atmosphere creates very intense windstorms
venus NASA
Photo Credit: NASA

Why Should We Explore Venus?

Venus is Earth’s closest neighbor.  Scientists believe that billions of years ago, Venus was very much like Earth.  Venus is called Earth’s “twin” due to their similarities.

Examination of Venus’s formation process allows comparison between planets.  Venus ended up as a hellish planet, yet Earth ended up as a planet filled with life.  What made the difference between the two planets?