Artist's impression of the Milky Way Galaxy. Credit: ESO
In 1961, famed astronomer and astrophysicist Frank Drake formulated an equation for estimating the number of extraterrestrial civilizations in our galaxy at any given time. Known as the “Drake Equation“, this formula was a probabilistic argument meant to establish some context for the Search for Extraterrestrial Intelligence (SETI). Of course, the equation was theoretical in nature and most of its variables are still not well-constrained.
For instance, while astronomers today can speak with confidence about the rate at which new stars form, and the likely number of stars that have exoplanets, they can’t begin to say how many of these planets are likely to support life. Luckily, Professor David Kipping of Columbia University recently performed a statistical analysis that indicates that a Universe teeming with life is “the favored bet.”
Exoplanet Kepler 62f would need an atmosphere rich in carbon dioxide for water to be in liquid form. Artist's Illustration: NASA Ames/JPL-Caltech/T. Pyle
In the past few decades, astronomers have confirmed the existence of thousands of planets beyond our Solar System. Over time, the process has shifted from discovery to characterization in the hopes of finding which of these planets are capable of supporting life. For the time being, these methods are indirect in nature, which means that astronomers can only infer if a planet is inhabitable based on how closely it resembles Earth.
To aid in the hunt for “potentially habitable” exoplanets, a team of Cornell researchers recently created five models that represent key points in Earth’s evolution. These “snapshots” of what Earth looked like during various geological epochs could greatly enhance the search for extra-terrestrial life by providing a more complete picture of what a life-bearing planet could look like.
Scientists recently determined that a certain strain of Earth bacteria could thrive under conditions found on Enceladus. Credit: NASA/JPL/Space Science Institute
Saturn’s moon Enceladus has captivating scientists ever since the Voyager 2 mission passed through the system in 1981. The mystery has only deepened since the arrival of the Cassiniprobe in 2004, which included the discovery of four parallel, linear fissures around the southern polar region. These features were nicknamed “Tiger Stripes” because of their appearance and the way they stand out from the rest of the surface.
Since their discovery, scientists have attempted to answer what these are and what created them in the first place. Thankfully, new research led by the Carnegie Institute of Science has revealed the physics governing these fissures. This includes how they are related to the moon’s plume activity, why they appear around Enceladus’ south pole, and why other bodies don’t have similar features.
Hyperacid, hypersaline and hot ponds in the geothermal field of Dallol (Ethiopia). Despite the presence of liquid water, this multi-extreme system does not allow the development of life, according to a new study. The yellow-greenish colour is due to the presence of reduced iron. Image Credit: Puri López-García
In recent years research into extremophiles has captured the interest of astrobiologists. The discovery of lifeforms in some of Earth’s most extreme environments has helped shape our thinking about extraterrestrial life. Life on other worlds may not need the kind of temperate, balanced environment that most life on Earth is adapted to.
Reconstruction of what Earth's light curve would look like. Credit:
S. Fan et. al., arXiv (2019) arXiv:1908.04350
The study of exoplanets has matured considerably in the last ten years. During this time, the majority of the over 4000 exoplanets that are currently known to us were discovered. It was also during this time that the process has started to shift from the process of discovery to characterization. What’s more, next-generation instruments will allow for studies that will reveal a great deal about the surfaces and atmospheres of exoplanets.
This naturally raises the question: what would a sufficiently-advanced species see if they were studying our planet? Using multi-wavelength data of Earth, a team of Caltech scientists was able to construct a map of what Earth would look like to distant alien observers. Aside from addressing the itch of curiosity, this study could also help astronomers reconstruct the surface features of “Earth-like” exoplanets in the future.
Artist's concept of a Europa Clipper mission. Credit: NASA/JPL
In 2023, NASA plans to launch the Europa Clipper mission, a robotic explorer that will study Jupiter’s enigmatic moon Europa. The purpose of this mission is to explore Europa’s ice shell and interior to learn more about the moon’s composition, geology, and interactions between the surface and subsurface. Most of all, the purpose of this mission is to shed light on whether or not life could exist within Europa’s interior ocean.
This presents numerous challenges, many of which arise from the fact that the Europa Clipper will be very far from Earth when it conducts its science operations. To address this, a team of researchers from NASA’s Jet Propulsion Laboratory (JPL) and Arizona State University (ASU) designed a series of machine-learning algorithms that will allow the mission to explore Europa with a degree of autonom.
Artist’s impression of the protoplanetary disk around a young star V883 Ori. The outer part of the disk is cold and dust particles are covered with ice. ALMA detected various complex organic molecules around the snow line of water in the disk. Credit: National Astronomical Observatory of Japan
According to widely-accepted theories, the Solar System formed roughly 4.6 billion years ago from a massive cloud of dust and gas (aka. Nebular Theory). This process began when the nebula experienced a gravitational collapse in the center that became our Sun. The remaining dust and gas formed a protoplanetary disk that (over time) accreted to form the planets.
However, scientists remain unsure about when organic molecules first appeared in our Solar System. Luckily, a new study by an international team of astronomers may be able to help answer that question. Using the Atacama Large Millimeter-submillimeter Array (ALMA), the team detected complex organic molecules around the young star V883 Ori, which could someday lead to the emergence of life in that system.
A montage of some of the "ocean worlds" in our Solar System. From top to bottom, left to right, these include Europa, Enceladus, TItan and Ceres. Credit: NASA/JPL
For decades, scientists have believed that there could be life beneath the icy surface of Jupiter’s moon Europa. Since that time, multiple lines of evidence have emerged that suggest that it is not alone. Indeed, within the Solar System, there are many “ocean worlds” that could potentially host life, including Ceres, Ganymede, Enceladus, Titan, Dione, Triton, and maybe even Pluto.
But what if the elements for life as we know it are not abundant enough on these worlds? In a new study, two researchers from the Harvard Smithsonian Center of Astrophysics (CfA) sought to determine if there could in fact be a scarcity of bioessential elements on ocean worlds. Their conclusions could have wide-ranging implications for the existence of life in the Solar System and beyond, not to mention our ability to study it.
Artist’s depiction of a watery exoplanet orbiting a distant red dwarf star. Credit: CfA
In previous studies, questions on the habitability of moons and other planets have tended to focus on the existence of water. This has been true when it comes to the study of planets and moons within the Solar System, and especially true when it comes the study of extra-solar planets. When they have found new exoplanets, astronomers have paid close attention to whether or not the planet in question orbits within its star’s habitable zone.
This is key to determining whether or not the planet can support liquid water on its surface. In addition, astronomers have attempted to obtain spectra from around rocky exoplanets to determine if water loss is taking place from its atmosphere, as evidenced by the presence of hydrogen gas. Meanwhile, other studies have attempted to determine the presence of energy sources, since this is also essential to life as we know it.
In contrast, Dr. Lingam and Prof. Loeb considered how the existence of life on ocean planets could be dependent on the availability of limiting nutrients (LN). For some time, there has been considerable debate as to which nutrients would be essential to extra-terrestrial life, since these elements could vary from place to place and over timescales. As Lingam told Universe Today via email:
“The mostly commonly accepted list of elements necessary for life as we know it comprises of hydrogen, oxygen, carbon, nitrogen and sulphur. In addition, certain trace metals (e.g. iron and molybdenum) may also be valuable for life as we know it, but the list of bioessential trace metals is subject to a higher degree of uncertainty and variability.”
Artist rendering showing an interior cross-section of the crust of Enceladus, which shows how hydrothermal activity may be causing the plumes of water at the moon’s surface. Credits: NASA-GSFC/SVS, NASA/JPL-Caltech/Southwest Research Institute
For their purposes, Dr. Lingam and Prof. Loeb created a model using Earth’s oceans to determine how the sources and sinks – i.e. the factors that add or deplete LN elements into oceans, respectively – could be similar to those on ocean worlds. On Earth, the sources of these nutrients include fluvial (from rivers), atmospheric and glacial sources, with energy being provided by sunlight.
Of these nutrients, they determined that the most important would be phosphorus, and examined how abundant this and other elements could be on ocean worlds, where conditions as vastly different. As Dr. Lingam explained, it is reasonable to assume that on these worlds, the potential existence of life would also come down to a balance between the net inflow (sources) and net outflow (sinks).
“If the sinks are much more dominant than the sources, it could indicate that the elements would be depleted relatively quickly. In other to estimate the magnitudes of the sources and sinks, we drew upon our knowledge of the Earth and coupled it with other basic parameters of these ocean worlds such as the pH of the ocean, the size of the world, etc. known from observations/theoretical models.”
While atmospheric sources would not be available to interior oceans, Dr. Lingam and Prof. Loeb considered the contribution played by hydrothermal vents. Already, there is abundant evidence that these exist on Europa, Enceladus, and other ocean worlds. They also considered abiotic sources, which consist of minerals leached from rocks by rain on Earth, but would consist of the weathering of rocks by these moons’ interior oceans.
Artist’s rendering of possible hydrothermal activity that may be taking place on and under the seafloor of Enceladus. Credit: NASA/JPL
Ultimately, what they found was that, unlike water and energy, limiting nutrients might be in limited supply when it comes to ocean worlds in our Solar System:
“We found that, as per the assumptions in our model, phosphorus, which is one of the bioessential elements, is depleted over fast timescales (by geological standards) on ocean worlds whose oceans are neutral or alkaline in nature, and which possess hydrothermal activity (i.e. hydrothermal vent systems at the ocean floor). Hence, our work suggests that life may exist in low concentrations globally in these ocean worlds (or be present only in local patches), and may therefore not be easily detectable.”
This naturally has implications for missions destined for Europa and other moons in the outer Solar System. These include the NASA Europa Clipper mission, which is currently scheduled to launch between 2022 and 2025. Through a series of flybys of Europa, this probe will attempt to measure biomarkers in the plume activity coming from the moon’s surface.
Similar missions have been proposed for Enceladus, and NASA is also considering a “Dragonfly” mission to explore Titan’s atmosphere, surface and methane lakes. However, if Dr. Lingam and Prof. Loeb’s study is correct, then the chances of these missions finding any signs of life on an ocean world in the Solar System are rather slim. Nevertheless, as Lingam indicated, they still believe that such missions should be mounted.
Artist’s concept of a Europa Clipper mission. Credit: NASA/JPL
“Although our model predicts that future space missions to these worlds might have low chances of success in terms of detecting extraterrestrial life, we believe that such missions are still worthy of being pursued,” he said. “This is because they will offer an excellent opportunity to: (i) test and/or falsify the key predictions of our model, and (ii) collect more data and improve our understanding of ocean worlds and their biogeochemical cycles.”
In addition, as Prof. Loeb indicated via email, this study was focused on “life as we know it”. If a mission to these worlds did find sources of extra-terrestrial life, then it would indicate that life can arise from conditions and elements that we are not familiar with. As such, the exploration of Europa and other ocean worlds is not only advisable, but necessary.
“Our paper shows that elements that are essential for the ‘chemistry-of-life-as-we-know-it’, such as phosphorous, are depleted in subsurface oceans,” he said. “As a result, life would be challenging in the oceans suspected to exist under the surface ice of Europa or Enceladus. If future missions confirm the depleted level of phosphorous but nevertheless find life in these oceans, then we would know of a new chemical path for life other than the one on Earth.”
In the end, scientists are forced to take the “low-hanging fruit” approach when it comes to searching for life in the Universe . Until such time that we find life beyond Earth, all of our educated guesses will be based on life as it exists here. I can’t imagine a better reason to get out there and explore the Universe than this!
The Search for Extraterrestrial Intelligence (SETI) listens for radio signals from other civilizations. In this image, radio-telescopes in SETI's Allen Telescope Array (ATA) are hard at work with the Milky Way in the background. Image: SETI
The discovery of alien life is one of those things that everyone thinks about at some point. Hollywood has made their version of first contact very clear: huge alien vessels appear over Earth’s cities, panic ensues, and Will Smith saves the day with a Windows 3.1 virus. It’s lots of fun—and who knows?—it may end up being accurate. (Not the Windows 3.1 part.) But sci-fi books and movies aside, what do we really know about our attitude to the discovery of alien life?
We have an organization (SETI) dedicated to detecting the presence of alien civilizations, and we have a prominent scientist (Stephen Hawking) warning against advertising our own presence. Those represent the extremes—actively seeking out alien life vs. hiding from it—but what is the collective attitude towards the discovery of alien life? Scientists at Arizona State University (ASU) have studied that issue and detailed their results in a new study published in the journal Frontiers of Psychology.
The team of scientists tried to gauge people’s reactions to the discovery of alien life in three separate parts of their study. In the first case, they examined media reports of past announcements about the discovery of alien life, for example the announcement in 1996 that evidence of microbial life had been found in a Martian metorite.
Secondly, they asked a sample of over 500 people what their own reactions, and the reactions of the rest of humanity, would be to the hypothetical announcement of alien life.
Thirdly, the 500 people were split into two groups. Half were asked to read and respond to a real newspaper story announcing the discovery of fossilized Martian microbial life. The other half were asked to read and respond to a newspaper article announcing the creation of synthetic life by Craig Venter.
Martian meteorite ALH84001 was found in Antarctica in 1984 by a group of meteorite hunters from the US. Scientists who studied it suggested that it contained evidence of ancient Martian microbial life. Image: By Jstuby at English Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=47556214
In all three cases the life was microbial in nature. Microbial life is the simplest life form, so it should be what we expect to find. This is certainly true in our own Solar System, since the existence of any other intelligent life has been ruled out here, while microbial life has not.
Also, in all three cases, the language of the respondents and the language in the media reports was analyzed for positive and negative words. A specialized piece of software called Linguistic Inquiry and Word Count (LIWC) was used. It’s text-analysis software that scans written language and identifies instances of words that reflect positive affect, negative affect, reward, or risk. (You can try LIWC here for fun, if you like.)
Electron microscope images of the Martian meteorite ALH84001 showed chain-like structures that resembled living structures. Image: NASA
Analyzing Media Reports
The media reports used in the study were all from what the team considers reputable journalism outlets like The New York Times and Science Magazine. The reports were about things like unidentified signals from space that could have been alien in nature, fossilized microbial remains in meteorites, and the discovery of exoplanets in the habitable zones of other solar systems. There were 15 articles in total.
The authors of the study wanted to find out how people would react to the discovery of alien life, and to the discovery of potentially habitable exoplanets which might harbor life. In this artist’s illustration, exoplanets orbit a young, red dwarf star. Credit: NASA/JPL-Caltec
Overall, the study showed that language in media reports about alien life was more positive than negative, and emphasized reward rather than risk. So people generally find the potential of alien life to be a positive thing and something to be looked forward to. However, this part of the study showed something else: People were more positively disposed towards news of alien life that was microbial than they were towards alien life that could be present on exoplanets, where, presumably, it might be more than merely microbial. So, microbes we can handle, but something more advanced and a little doubt starts to creep in.
Reactions to Hypothetical Announcements of Alien Life
This part of the study aimed to assess people’s beliefs regarding how both they as individuals—and humanity as a whole—might react to the discovery of alien microbial life. The same LIWC software was used to analyze the written responses of the 500 people in the sample group.
The results were similar to the first part of the study, at least for the individuals themselves. Positive affect was more predominant than negative aspect, and words reflecting reward were more predominant than words reflecting risk. This probably isn’t surprising, but the study did show something more interesting.
When participants were asked about how the rest of humanity would respond to the announcement of alien life, the response was different. While positive language still outweighed negative language, and reward still outweighed risk, the differences weren’t as pronounced as they were for individuals. So people seem to think that others won’t be looking forward to the discovery of alien life as much as they themselves do.
Actual Reactions to the Discovery of Extraterrestrial Life
This is hard to measure since we haven’t actually discovered any yet. But there have been times when we thought we might have.
In this part of the study, the group of 500 respondents was split into two groups of 250. The first was asked to read an actual 1996 New York Times article announcing the discovery of fossilized microbes in the Martian meteorite. The second group was asked to read a New York Times article from 2010 announcing the creation of life by Craig Venter. The goal was to find out if the positive bias towards the discovery of microbial life was specific to microbial life, or to scientific advancements overall.
Saturn’s moon Enceladus could harbor microbial life in the warm salty water thought to exist under its frozen surface. Respondents in the study seemed to like that possibility. Credits: NASA/JPL-Caltech/Space Science Institute
This part of the study found the same emphasis on positive affect over negative affect, and reward over risk. This held true in both cases: the Martian microbial life article, and the artificially created life article. The type of article played a minor role in people’s responses. Results were slightly more positive towards the Martian life story than the artificial life story.
Overall, this study shows that people seem positively disposed towards the discovery of alien life. This is reflected in media coverage, people’s personal responses, and people’s expectations of how others would react.
This is really just the tip of the iceberg, though. As the authors say in their study, this is the first empirical attempt to understand any of this. And the study was only 500 people, all Americans.
How different the results might be in other countries and cultures is still an open question. Would populations whose attitudes are more strongly shaped by religion respond differently? Would the populations of countries that have been invaded and dominated by other countries be more nervous about alien life or habitable exoplanets? There’s only conjecture at this point.
Maybe we’re novelty-seekers and we thrive on new discoveries. Or maybe we’re truth-seekers, and that’s reflected in the study. Maybe some of the positivity reflects our fear of being alone. If Earth is the only life-supporting world, that’s a very lonely proposition. Not only that, but it’s an awesome responsibility: we better not screw it up!
Still, the results are encouraging for humanity. We seem, at least according to this first study, open to the discovery of alien life.
But that might change when the first alien ship casts its shadow over Los Angeles.
Did you know that it’s been almost 45 years since humans walked on the surface of the Moon? Of course you do. Anyone who loves space exploration obsesses about the last Apollo landings, and counts the passing years of sadness.
Sure, SpaceX, Blue Origins and the new NASA Space Launch Systems rocket offer a tantalizing future in space. But 45 years. Ouch, so much lost time.
What would happen if we could go back in time? What amazing and insane plans did NASA have to continue exploring the Solar System? What alternative future could we have now, 45 years later?
In order to answer this question, I’ve teamed up with my space historian friend, Amy Shira Teitel, who runs the Vintage Space blog and YouTube Channel. We’ve decided to look at two groups of missions that never happened.
In her part, Amy talks about the Apollo Applications Program; NASA’s original plans before the human exploration of the Moon was shut down. More Apollo missions, the beginnings of a lunar base, and even a human flyby of Venus.
In my half of the series, I look at Werner Von Braun’s insanely ambitious plans to send a human mission to Mars. Put it together with Amy’s episode and you can imagine a space exploration future with all the ambition of the Kerbal Space Program.
Keep mind here that we’re not going to constrain ourselves with the pesky laws of physics, and the reality of finances. These ideas were cool, and considered by NASA engineers, but they weren’t necessarily the best ideas, or even feasible.
So, 2 parts, tackle them in any order you like. My part begins right now.
Werner Von Braun, of course, was the architect for NASA’s human spaceflight efforts during the space race. It was under Von Braun’s guidance that NASA developed the various flight hardware for the Mercury, Gemini and Apollo missions including the massive Saturn V rocket, which eventually put a human crew of astronauts on the Moon and safely returned them back to Earth.
Wernher von Braun. Credit: NASA/Marshall Space Flight Center
Von Braun was originally a German rocket scientist, pivotal to the Nazi “rocket team”, which developed the ballistic V-2 rockets. These unmanned rockets could carry a 1-tonne payload 800 kilometers away. They were developed in 1942, and by 1944 they were being used in war against Allied targets.
By the end of the war, Von Braun coordinated his surrender to the Allies as well as 500 of his engineers, including their equipment and plans for future rockets. In “Operation Paperclip”, the German scientists were captured and transferred to the White Sands Proving Ground in New Mexico, where they would begin working on the US rocket efforts.
Von Braun and others standing in front a V-2 rocket engine at White Sands. Credit: U.S. Army/ Ordway Collection/Space Rocket Center
Before the work really took off, though, Von Braun had a couple of years of relative downtime, and in 1947 and 1948, he wrote a science fiction novel about the human exploration of Mars.
The novel itself was never published, because it was terrible, but it also contained a detailed appendix containing all the calculations, mission parameters, hardware designs to carry out this mission to Mars.
The Mars Project
In 1952, this appendix was published in Germany as “Das Marsproject”, or “The Mars Project”. And an English version was published a few years later. Collier’s Weekly Magazine did an 8-part special on the Mars Project in 1952, captivating the world’s imagination.
Here’s the plan: In the Mars Project, Von Braun envisioned a vast armada of spaceships that would make the journey from Earth to Mars. They would send a total of 10 giant spaceships, each of which would weigh about 4,000 tonnes.
Just for comparison, a fully loaded Saturn V rocket could carry about 140 tonnes of payload into Low Earth Orbit. In other words, they’d need a LOT of rockets. Von Braun estimated that 950 three-stage rockets should be enough to get everything into orbit.
Ships being assembled in orbit. Credit: Collier’s
All the ships would be assembled in orbit, and 70 crewmembers would take to their stations for an epic journey. They’d blast their rockets and carry out a Mars Hohmann transfer, which would take them 8 months to make the journey from Earth to Mars.
The flotilla consisted of 7 orbiters, huge spheres that would travel to Mars, go into orbit and then return back to Earth. It also consisted of 3 glider landers, which would enter the Martian atmosphere and stay on Mars.
Once they reached the Red Planet, they would use powerful telescopes to scan the Martian landscape and search for safe and scientifically interesting landing spots. The first landing would happen at one of the planet’s polar caps, which Von Braun figured was the only guaranteed flat surface for a landing.
A rocket-powered glider descending towards Mars. Credit: Collier’s
At this point, it’s important to note that Von Braun assumed that the Martian atmosphere was about as thick as Earth’s. He figured you could use huge winged gliders to aerobrake into the atmosphere and land safely on the surface.
He was wrong. The atmosphere on Mars is actually only 1% as thick as Earth’s, and these gliders would never work. Newer missions, like SpaceX’s Red Dragon and Interplanetary Transport Ship will use rockets to make a powered landing.
I think if Von Braun knew this, he could have modified his plans to still make the whole thing work.
Landed at the polar cap. Credit: Collier’s
Once the first expedition landed at one of the polar caps, they’d make a 6,400 kilometer journey across the harsh Martian landscape to the first base camp location, and build a landing strip. Then two more gliders would detach from the flotilla and bring the majority of the explorers to the base camp. A skeleton crew would remain in orbit.
Once again, I think it’s important to note that Von Braun didn’t truly understand how awful the surface of Mars really is. The almost non-existent atmosphere and extreme cold would require much more sophisticated gear than he had planned for. But still, you’ve got to admire his ambition.
Preparing the gliders for rocket-powered ascent. Credit: Collier’s
With the Mars explorer team on the ground, their first task was to turn their glider-landers into rockets again. They would stand them up and get them prepped to blast off from the surface of Mars when their mission was over.
The Martian explorers would set up an inflatable habitat, and then spend the next 400 days surveying the area. Geologists would investigate the landscape, studying the composition of the rocks. Botanists would study the hardy Martian plant life, and seeing what kinds of Earth plants would grow.
Zoologists would study the local animals, and help figure out what was dangerous and what was safe to eat. Archeologists would search the region for evidence of ancient Martian civilizations, and study the vast canal network seen from Earth by astronomers. Perhaps they’d even meet the hardy Martians that built those canals, struggling to survive to this day.
Once again, in the 1940s, we thought Mars would be like the Earth, just more of a desert. There’d be plants and animals, and maybe even people adapted to the hardy environment. With our modern knowledge, this sounds quaint today. The most brutal desert on Earth is a paradise compared to the nicest place on Mars. Von Braun did the best he could with the best science of the time.
Finally, at the end of their 400 days on Mars, the astronauts would blast off from the surface of Mars, meet up with the orbiting crew, and the entire flotilla would make the return journey to Earth using the minimum-fuel Mars-Earth transfer trajectory.
The planned trajectories to and from Mars. Credit: Collier’s
Although Von Braun got a lot of things wrong about his Martian mission plan, such as the thickness of the atmosphere and habitability of Mars, he got a lot of things right.
He anticipated a mission plan that required the least amount of fuel, by assembling pieces in orbit, using the Hohmann transfer trajectory, exploring Mars for 400 days to match up Earth and Mars orbits. He developed the concept of using orbiters, detachable landing craft and ascent vehicles, used by the Apollo Moon missions.
The missions never happened, obviously, but Von Braun’s ideas served as the backbone for all future human Mars mission plans.
I’d like to give a massive thanks to the space historian David S.F. Portree. He wrote an amazing book called Humans to Mars, which details 50 years of NASA plans to send humans to the Red Planet, including a fantastic synopsis of the Mars Project.
I asked David about how Von Braun’s ideas influenced human spaceflight, he said it was his…
“… reliance on a conjunction-class long-stay mission lasting 400 days. That was gutsy – in the 1960s, NASA and contractor planners generally stuck with opposition-class short-stay missions. In recent years we’ve seen more emphasis on the conjunction-class mission mode, sometimes with a relatively short period on Mars but lots of time in orbit, other times with almost the whole mission spent on the surface.”
