How Many Robotic Explorers Have We Sent to Mars?

It is specifically because of those efforts that our perceptions of Mars has gone from the stuff of myths and legends and became the stuff of real science.

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So far, all missions to Mars have been performed by robots, in the form of orbiters, landers, and rovers. This is expected to change in the not-too-distant future; but so far, the trend has been consistent. And compared to other celestial bodies, exploring Mars is challenging. So why the fascination and why do we keep going back?

Just as importantly, why are we hoping to send human explorers there in the future? And why are some people hoping to make Mars their permanent home?

Reasons for Mars Exploration:

There are many reasons why Mars is a popular target for observation and exploration. For one, there is its proximity to Earth. Every two years or so (ranging from 764 to 812 days), Mars and Earth will be at the closest points in their orbit to each other. This is known as an "opposition" since the positions of Mars and the Sun will be opposite to each other in the sky.

But even at this point, the distance between Mars and Earth ranges considerably - from between 54 and 103 million km (34 and 64 million mi). The closest recent approach took place back in 2003 when Earth and Mars were only 56 million km (3,4796,787 mi) apart, which was the closest they’d been in 50,000 years.

The next closest approach will take place on July 27th, 2018, when Earth and Mars will be at a distance of 57.6 million km (35.8 mi) from each other. Regardless of this variation in distance, it is during opposition that Mars is most visible in the night sky. Because of this, humans have been able to observe it with relative ease for millennia.

It is also at these times that it is most convenient to send exploration missions there. Depending on the nature of the mission and the speed with which the craft is launched, it can take as few as 150 days or as many as 300 days (5 to 10 months) to get a robotic mission to Mars.

To be clear, Venus is the closest planet to Earth. The point where these two planets are closest to each other is known as an inferior conjunction, where Venus lies between the Earth and the Sun. This occurs every 584 days, at which point Venus and Earth reach an average distance of 41 million km (25.5 million mi). 

For missions launching during an inferior conjunction, it takes 97 to 153 days (roughly 3 to 5 months) to reach Venus. For these reasons, one has to wonder why so many missions have been sent to Mars and relatively few to Venus. Herein lies the other big reason why Mars is so attractive for scientists and researchers. This one is so important, it deserves its own category.

Similarities Between Earth and Mars:

To put it mildly, Venus' environment is hellish and horrible. In fact, if there were a competition to see which celestial body is most similar to Hell, Venus would win, hands down. On average, surface temperatures are hot enough to melt lead (462 °C; 863.6 °F) and atmospheric pressure is enough to crush your bones - 92 bar, or 92 times that of Earth’s atmosphere.

On top of all that, the atmosphere is toxic to all life as we know it, composed predominantly of carbon dioxide and containing thick clouds of sulfuric acid. For this reason, no probe that has been sent into Venus' atmosphere has been able to survive for more than two days, and those few that made it to the surface only lasted from about 20 minutes to just over two hours.

Comparatively speaking, the Martian environment is cool and much more accommodating. Granted, compared to Earth its a frigid, desiccated world that makes Antarctica look balmy by comparison, but it does have a number of "Earth-like" features that have kept astronomers and planetary scientists coming back for more.

First of all, you have Mars' similar composition. Like Earth, Mars is a terrestrial planet, meaning that it is composed predominantly of silicate minerals and metals that are differentiated between a core, a mantle, and a crust. Like Earth, it has polar ice caps that are composed of water ice, with a significant amount of dry ice (frozen carbon dioxide) present in the southern ice cap.

Also, a day on Mars (or sol) is only slightly longer than a day on Earth - 24 hours, 39 minutes and 35 seconds, to be exact. A year, meanwhile, lasts about 687 days (or 668.6 Martian days), which is almost twice as long as a year on Earth. Nevertheless, the seasons on Mars function much the same as they do on Earth.

Mars also has seasonal patterns that are similar to Earth, though they last about twice as long. For instance, Spring in the northern hemisphere coincides with when Mars it aphelion, which makes it the longest season on the planet (roughly 7 Earth months). Meanwhile, summer will last a good six months, while Fall and Winter last over 5 and over 4 months, respectively.

In the south, the length of the seasons is only slightly different, though they are a bit more extreme in terms of temperature. This similarity in seasonal change is due in part to the fact that Mars' axis is tilted in a similar fashion to Earth's (25.19° to its orbital plane compared to Earth’s tilt of approx. 23.44°).

It’s also due to eccentricity in Mars' orbit, which varies from 249.2 million km (154.8 million mi) at perihelion to 206.7 million km (128.4 million mi) at aphelion. This variation in distance also leads to significant variations in temperature. While the planet’s average temperature is -46 °C (51 °F), this ranges from -143 °C (-225.4 °F) at the poles to 35 °C (95 °F) during midday at the equator.

This works out to a variation in average surface temperature that is quite similar to Earth’s – a difference of 178 °C (320.4 °F) versus 145.9 °C (262.5 °F). These highs in temperature are what allow for water to flow (albeit very intermittently) on the surface, another thing Mars has in common with Earth. 

But most importantly of all, scientists now know that long ago, Mars was a lot more like Earth. While today, its atmosphere is about 0.5% as dense as Earth's and extremely cold and dry, it was once much thicker and warmer. In addition, water once flowed on its surface in the forms of rivers, lakes, and even an ocean that covered much of the northern hemisphere.

Because of these similarities, the study of Mars is a priority for scientists because of how it can shed additional light on how Earth formed many billions of years ago. And because of the way the Martian landscape has been preserved, scientists are also able to study the ancient history of this planet and learn more about what was taking place in the Solar System at this time.

In short, the study of Mars could reveal things about how all the rocky planets formed and evolved, how water was distributed throughout the Solar System billions of years ago, and maybe even how life itself emerged on Earth - and maybe whether or not it has any cousins on other planets and bodies.

Early Missions:

The exploration of Mars began in earnest during the 1960s. And much like the first satellite into space or the first crewed mission, the Soviets picked up an early lead. Over time, the United States caught up and overtook them by sending missions that were greater in number and technical complexity.

These missions dispelled previously-held beliefs about Mars and its capacity to support life. It also led to new theories about Mars' formation, evolution, and geological history - ones which are still being explored today.

 Mars 2, 3, 6, and 7:

Between 1960 and 1969, the Soviet Union launched nine probes to Mars, all of which failed. Three of these failed at launch, three more failed to reach near-Earth orbit, one failed while attempting to achieve a trans-Mars trajectory, and the remaining two failed during the interplanetary orbit.

By the early 1970s, the Soviets achieved a measure of success and even a few firsts with their Mars probes - each of which consisted of a flyby spacecraft and a lander. The Mars 2 and Mars 3 probes, launched in 1971, managed to reach Mars and captured many pictures of the planet-wide dust storm that was taking place at the time.

Both probes also deployed their landers, which met with limited success. The Mars 2 lander crashed on the surface but was still the first robotic mission to impact on the surface of another planet. The Mars 3 lander fared better, achieving a soft landing on the surface and transmitting for 20 seconds before losing contact with mission controllers (for unknown reasons).

In 1973, the Soviet Union sent four more missions to Mars: the Mars 4 and Mars 5 spacecraft and the Mars 6 and Mars 7 orbiter/lander missions. All missions (except Mars 7) sent back data, with Mars 5 sending back the most - 60 images before contact was lost. The Mars 6 lander transmitted data during its descent, but crashed on the surface, while the Mars 7 lander failed to separate properly during orbit and was lost.

Mariner 4, 6, 7, 9:

NASA, meanwhile, made its own attempts to reach Mars during the 1960s and 70s with the Mariner program. The first two were Mariner 3 and Mariner 4, two identical flyby spacecraft that were launched in 1964. The former failed during launch, but the latter managed to make it to Mars and snap the first up-close images of another planet in 1965.

These pictures provided radically more accurate data about the planet, showing its impact craters and its very thin and cold atmosphere. In addition, no magnetic fields or radiation belts were detected, all of which indicated that life would have a much harder time surviving on Mars than previously thought.

In 1969, two more probes were sent - Mariner 6 and Mariner 7 - and managed to conduct successful flybys while gathering information on the planet's atmosphere and surface. The two probes also took hundreds of pictures, which failed to note the "canals" that were long thought to be part of the surface.

Mariner 9 probe, which reached Mars in 1971, became the first spacecraft to successfully enter orbit around the planet. It's arrival coincided with the planet-wide dust storm that was also being observed by Mars 2 and Mars 3, so the probe was diverted to Mars' larger moon Phobos (and took pictures of it) while mission controllers waited for it to clear.

They also snapped pictures of the Martian surface features that suggested the presence of flowing water in the past. These pictures also revealed that Nix Olympica was the tallest mountain in the entire Solar System, which led to its reclassification as Olympus Mons.

Viking 1 and 2:

Following on the successes of the Mariner program, NASA dispatched two orbiter/lander missions to Mars in 1975 - Viking 1 and Viking 2. The objectives of these missions were to obtain data on Mars meteorological conditions, seismic environment, and magnetic properties. However, the main attraction of the mission was the search for biosignatures that would indicate the existence (past or present) of life on Mars.

The Viking orbiters confirmed previous findings of the Mariner 9 mission, revealing evidence of large floods that carved massive features on the surface, as well as the presence of rainfall in the southern hemisphere. The two landers also became the first robotic missions to successfully land and operate on the surface of Mars.

Unfortunately, the results of the biological experiments were inconclusive and have remained so until this day. While the Viking data has been reexamined several times (with one study in 2012 suggesting that it revealed signs of microbial life), no conclusive evidence has been found.

More Recent Missions:

With the conclusion of the Apollo Program, NASA and the Soviets began to refocus their exploration efforts to places closer to home and also farther away. For the remainder of the 1970s and through the 1980s, attention was largely focused on the deployment of space stations in Low Earth Orbit (LEO) and long-duration missions to the outer Solar System.

It was not until the 1990s that the exploration of Mars resumed. This time around, the stakes were raised with the introduction of robotic rovers and orbiters with more sophisticated suites of instruments. These missions would build on previous discoveries and reveal more about Mars' history and evolution.

Pathfinder and Sojourner:

In 1997, NASA successfully deployed the Mars Pathfinder lander (later renamed the Carl Sagan Memorial Station) to the surface of Mars. This lander carried the robotic wheeled rover known as Sojourner, which became the first rover to operate on the surface of Mars. The scientific objectives included analysis of the Martian atmosphere, climate, geology and the composition of its rocks and soil.

In addition, the Mars Pathfinder mission was also a "proof-of-concept" for various technologies that would play a vital role in future missions, especially those that were part of the Mars Exploration Program (MEP). These included an airbag landing system, automated obstacle avoidance, and the feasibility of sending remote-controlled mobile research laboratories to another planet.

Mars Global Surveyor:

In 1997, NASA's Mars Global Surveyor (MGS) successfully established orbit around the Red Planet. After trimming its orbit for about 18 months, the craft began its primary mapping mission of the surface by March of 1999. Until contact was lost in 2006 due to a technical glitch, the spacecraft remained in a nearly polar orbit over Mars and mapped the entire surface.

Combined with data on Mars' atmosphere and interior, the MGS returned more data about the red planet than all previous missions combined. The MGS was also the first mission to capture images that indicated that Mars might have sources of water close to its surface, which may periodically erupt and carve features on the surface. 

Other finds included magnetometer readings that showed that Mars' weak magnetic field is not generated in the planet's core but localized in particular areas of the crust. This suggested that it once had a global magnetic field which then disappeared. The spacecraft also provided scientists with the first 3-D views of Mars' northern polar ice cap and closeup images and temperature data of Phobos.

Mars Odyssey and Mars Express:

In 2001 and 2003, two orbiter missions arrived around Mars, both of which would prove vital to their respective space agency's research efforts. The first was NASA's 2001 Mars Odyssey orbiter, which was designed to hunt for evidence of past or present water on volcanic activity on Mars. In 2002, it succeeded in finding evidence of vast deposits of water ice in the upper three meters of soil around the south pole.

This mission was followed by the European Space Agency's (ESA) Mars Express orbiter, which carried a lander named Beagle 2. While the orbiter was similarly tasked with finding evidence of water ice on Mars' surface, the lander was designed to examine samples of Martian soil to look for biosignatures and biomolecules.

While contact was lost with the lander shortly after it entered the Martian atmosphere, it was later spotted by the orbiter and confirmed to be intact. This made the Beagle 2 the first British and European probe to achieve a soft landing on Mars. Meanwhile, the orbiter confirmed the presence of water ice and carbon dioxide at the planet's south pole.

Spirit and Opportunity:

NASA's second and third rovers would arrive on Mars by 2004 as part of the Mars Exploration Rover program. Named Spirit and Opportunity, these rovers were also the fourth and fifth installments in NASA's ongoing Mars Exploration Program (MEP). These two missions were tasked with exploring and characterizing Mars' surface geology to learn more about past water activity on Mars.

Among the rovers' many discoveries were multiple indications that Mars once had a warmer and wetter environment. This confirmed the theory that water once flowed on the planet and bolstered the case for there having been microbial life in the past. They also gathered data on the atmosphere, which helped scientists to characterize modern-day meteorological patterns on the planet.

Both missions were extended repeatedly, vastly exceeding their expected lifespans of just 90-days. Unfortunately, in May of 2009, Spirit became embedded in soft soil with only five working wheels. After months of trying to get the rover loose, NASA ended the mission on May 25th, 2011.

Opportunity continued to perform science operations until June of 2018 when a planet-wide dust storm caused it to lose power. By February 13th, 2019, NASA declared the mission complete but hopes to reestablish communications at a later date. Having stayed operational for 5498 Earth days, Opportunity is the longest-serving mission in history.

Pheonix Lander:

As part of the Mars Scout Program, the Pheonix Lander was dispatched to Mars to gather additional information on its surface and atmospheric conditions, primarily for the purpose of demonstrating that it was once a warmer and wetter planet.

The Phoenix touched down in the northern polar region in May of 2008. Once there, it began sampling the soil to assess the habitability of Mars at the ice-soil boundary. The lander found compelling evidence of water in Mars' past, which included an ocean covering much of the northern hemisphere, and clues as to how polar dynamics affected Martian weather.

Mars Reconnaissance Orbiter:

The Mars Reconnaissance Orbiter (MRO), a multipurpose spacecraft designed to survey and explore Mars, entered Mars orbit in March of 2006. With its advanced suite of instruments, the MRO was sent to study Martian landforms and surface conditions, detect water ice and minerals beneath the surface, monitor daily weather, and locate landing sites for future missions.

The orbiter is also testing a new telecommunications system that is able to transfer data to and from the spacecraft at a rate that is faster than all previous interplanetary missions combined and allows the MRO to serve as an important relay satellite for other missions.

Most Recent/Current Missions:

Today, there are eight functioning spacecraft and two functioning robotic missions exploring Mars. Of these, only a handful have been sent there in the past few years. And with the help of existing missions, what they have found has closed the book on some of the theories scientists have had about Mars.

These include the presence of a thicker atmosphere, water, and warmer temperatures in the past. However, questions relating to the past existence of life (and present) remain a mystery. As for what the future could hold for humanity and Mars, that too remains to be seen.

Curiosity:

As part of the Mars Exploration Program, the Mars Science Laboratory launched from Earth in 2011 and delivered the Curiosity rover to Mars by August of 2012. Unlike previous rovers, Curiosity is designed to operate for long periods of time on the Martian surface using a Multi-Mission Radioisotopic Thermoelectric Generator (MMRTG).

It's also much larger, heavier, and carries the most advanced suite of instruments of any rover, and takes advantage of entirely-new systems that will be used on next-generation missions (like the Mars 2020 rover). These include the "sky crane" landing system, which uses steerable rockets and a harness to slow the rover's descent and land it gently on the surface.

As a mobile laboratory, Curiosity's objectives include analyzing samples that were scooped from the soil or drilled from rocks. Along with analyzing local land formations and structures, the purpose of all of this is, to find clues to Mars' past and how and when it made the transition to the way it is today.

So far, Curiosity has made groundbreaking discoveries within Mars' Gale Crater. These include evidence that the crater was once a lakebed, that sedimentary flows gradually created Mount Sharp (in the center of the crater) over time and the discovery of methane and organic molecules emerging from the interior through cracks in the surface.

MAVEN:

The Mars Atmosphere and Volatile Evolution (MAVEN) reached Mars in September of 2014, where it began assessing the atmosphere to determine how, both, it and Mars' surface water were lost over time. Among other things, the data it gathered indicated that Mars' atmosphere was slowly stripped away over the course of hundreds of millions of years by solar wind.

Mangalyaan (Mars Orbiter Mission):

The Mangalyaan orbiter (aka. MOM), which reached Mars in September of 2014, is the Indian Space Research Organization's (ISRO) first robotic mission to another planet. Its primary objective is to act as a "technology-demonstrator" to help the ISRO develop the technologies necessary for the designing, planning, managing, and execution of interplanetary missions.

However, the orbiter is also equipped with scientific instruments to study the atmosphere and surface of Mars. The mission is noted for having achieved orbit around Mars on its first try, something that no previous mission has been able to do.

ExoMars Trace Gas Orbiter:

The result of collaborative efforts between the ESA and Roscosmos, the ExoMars TGO is an atmospheric research orbiter intended to gain a better understanding of Mars' atmosphere. Having reached Mars in October of 2016, the orbiter began studying the atmosphere for specific trace gases (such as methane) in the search for evidence of possible biological or geological activity.

A second part of the mission, the Schiaparelli EDM lander, was intended to deliver a small science package to the surface - the Dust characterization, Risk assessment, and Environment Analyser on the Martian Surface (DREAMS). This package contained a suite of sensors that would measure the wind, humidity, air pressure, temperature, transparency, radiation, and electrification of the atmosphere.

Schiaparelli's main purpose, however, was to serve as a technology demonstration vehicle that would test technology for performing a controlled landing on the surface of Mars. Due to a technical glitch, the EDM crashed on the surface and was lost, but not without providing plenty of information on its descent beforehand.

InSight:

The latest mission to arrive on Mars is NASA's Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) lander, which reached the Red Planet in November of 2018. This is the first mission to examine Mars with a heat flow probe and seismometer to learn more about its interior structure and geological history. In so doing, scientists hope to gain additional insight into what processes formed the rocky planets of the Solar System.

Future Missions:

In the very near future, multiple robotic missions are expected to reach Mars. Whereas NASA and the Russian space agency (Roscosmos) have sent the lion's share of missions in the past, emerging space powers will also be taking part. These include China and India, while the European Space Agency will also be expanding its presence.

These missions will be tasked with searching for more evidence of past and present life, learning more about what Mars' ancient environment was like and how it evolved, and paving the way for crewed missions (and maybe even human settlement) in the coming decades.

Mars 2020:

Following the path blazed by the Curiosity rover is the Mars 2020 rover, the latest mission to go to Mars as part of the MEP. The design is virtually identical to the Curiosity rover, except that the Mars 2020 mission also carries a Sample Caching System (SCS) that will allow it to prepare soil samples for an eventual return to Earth.

Another interesting feature is the Mars Oxygen In-Situ Resource Utilization Experiment (MOXIE), an onboard unit that can create breathable oxygen out of carbon dioxide gas. This instrument is designed to test technologies that could allow future missions of astronauts to provide for their own supplies of oxygen.

Rosalind Franklin:

Previously known as the ExoMars rover, the Rosalind Franklin is another collaborative effort between Roscosmos and the ESA. Once it reaches Mars in 2020, it will be supported by the TGO, which will act as a communications relay between the rover and Earth. The mission's primary objective is to find evidence of past life on Mars by examining a site that has a good chance of having preserved organic material from the very early history of the planet.

Similar to the Curiosity and the Mars 2020 rover, the rover will examine the chemical and physical properties of samples and look for biomarkers. Most of these will be drilled from the subsurface, at depths of up to 2 meters (~6.5 ft), which is deeper than any previous mission has probed. At these depths, organics are more likely to survive because they would be protected from radiation and photochemistry at the surface.

Hope Mars Mission:

Also called the Emirates Mars Mission, the Hope Mars probe will be launched by the United Arab Emirates in 2020, making it the first mission to Mars by any Arab or Muslim majority country. Once it reaches Mars, the probe will study the atmosphere on a daily basis to answer enduring mysteries: like why hydrogen and oxygen are being lost into space and the reasons for Mars' drastic climate change.

The probe will also study seasonal cycles, global weather events (like dust storms), and weather specific to certain geographic areas. This data will be shared internationally and will also help to model the Earth's atmosphere and study its evolution over millions of years.

2020 Chinese Mars Mission:

This Chinese spacecraft/rover is the first installment in the nation's Mars program. It will demonstrate the technology needed for a Mars sample-return mission, which China is hoping to mount by the 2030s. Once deployed, the rover will probe the ground with radar and examine soil samples to look for biomolecules and biosignatures.

Mangalyaan-2 (MOM-2):

As India's second interplanetary mission, the ISRO plans to launch the Mangalyaan-2 orbiter around 2022-2023 time frame. At present, it is not yet clear if the mission will consist of orbiter and lander/rover mission or to send another orbiter with more sophisticated instruments.

Conclusion:

Thanks to the many robotic spacecraft, landers, and rovers that we have sent to Mars, our understanding of the planet has grown and evolved considerably and in a relatively short time. The first missions to fly past Mars and land on its surface dispelled the notion that the planet had life or was home to a civilization.

For many decades to follow, Mars remained a cold planet that was effectively sterile in the public mind. But within the past few decades, new evidence has shown that Mars is actually a very dynamic place, a world that experiences temperature variations similar to those of Earth (and which is actually warmer sometimes than Earth in some regions).

On top of that, they also revealed that Mars was once a very different place - a world with oceans, lakes, and rivers that may have even supported life. Billions of years ago, that world began to change drastically, becoming the one we know today. This information is allowing us to construct a more complete picture of how our Solar System formed and evolved.

Someday, what we know about Mars (past and present) could allow us to build a permanent human presence there. Some even speculate that humanity will not survive in the long run unless planets like Mars are colonized.

If and when that happens, what we have learned about Mars may even allow us to turn it into a green planet - one that is once again warm and has oceans on its surface.

Further Reading:

• NASA - InSight Mission
• Wikipedia - Exploration of Mars
• NASA - Mars Science Laboratory
• NASA - Mars Exploration Rovers
• ESA - Robotic Exploration of Mars
• NASA - Mars Exploration Program
• Planetary Society - Missions to Mars
• National Geographic - Mars Exploration
• NASA Science - Solar System Exploration: Mars
• RussianSpaceWeb - Unmanned Missions to Mars