One by one they flickered to life. Venus, first, in 1962, and two and a half years later, Mars. Our spacecraft flew by those planets, orbited them, and became manmade meteors streaking toward the first soil we couldn’t generically call "earth." Later, when we grew ambitious and confident in our abilities, humanity reached for the outer planets, probing Jupiter and Saturn in 1973 and 1979. Each mission turned conjecture into fact, invalidated old assumptions, and brought us closer to one day answering the two fundamental questions of existence: where did all this come from, and where is it headed?

Mission successes don't happen in a void. For every newly lighted world there are crashed probes, lost spacecraft, and rockets destroyed on launch pads. The exploration of other worlds is a cumulative art, and with a steady cadence of missions comes an institutional knowledge for scientists and engineers. Every setback is its own library of insights. In 1964, when probe Mariner 3 missed Mars, its target, due to equipment failure, Mariner 4 was three weeks behind, and succeeded where its twin had failed.

The cadence cannot be interrupted, which is why many planetary scientists now eye warily their calendars. America's starvation budget for planetary exploration has stopped good missions from going forward, and keeps new missions from reaching the launch pad. One by one over the next three years, as missions end and spacecraft die, the outer planets will again go dark.

If NASA's New Horizons mission to Pluto is extended beyond 2017, the entire active human presence at the outer planets will consist of a single probe the size of a grand piano. If the mission is not extended, humanity's 43-year exploration of the outer planets will end, and humanity's horizon will shrink by about 2.5 billion miles. Worse, because of the time necessary to build a spacecraft and the harsh reality of orbital mechanics, the earliest a new mission could be sent beyond the asteroid belt is sometime in the 2020s.

The consequences of a diminished planetary science portfolio go beyond the loss of new wallpaper for desktop computers. Planetary exploration has changed the way we think about everything from the air we breathe to the oceans we sail. By exploring Venus, for example, scientists observed the full expression of the greenhouse effect, which in turn reshaped environmental priorities back on Earth. Meanwhile, the search for life on other planets inspired scientists to find life in unexpected places here at home.

"The more we learn about the other planets out there, the more we learn about Earth," said Dr. Curt Niebur, a program scientist for NASA.

The next three years of outer space exploration are going to produce spectacular scientific data. Very little is known about Pluto, for example, but that will change in July when New Horizons makes its approach. Once New Horizons completes its possible extended mission to an object in the Kuiper Belt, though, there is nothing budgeted in the pipeline to take its place. Yesterday invested in today. But we are not investing in tomorrow.

The value of planetary exploration

For all the scientific breakthroughs it produces, the space program in general — and planetary exploration in particular — is an inexpensive enterprise. "People grossly overestimate the budget that NASA gets," said Niebur. The president's fiscal year 2016 budget calls for $18.5 billion overall for NASA — 0.46 percent of the federal budget. "Most people think it's 10 times that much."

Of that, the allotment for planetary science has been cut to $1.36 billion — the fourth such proposed cut by the Obama administration, and far short of what is needed by the program. (The rest of NASA's budget goes to earth science, human space exploration, and operation of the International Space Station, among other things.) According to the Planetary Society, a nonprofit space research and advocacy organization, for the planetary science division to run well, the United States should spend at least $1.5 billion every year to explore other worlds — "less overall," they report, "than what Americans spent on dog toys in 2012."

Planetary exploration has changed the way we think about the air we breathe and the oceans we sail

Fiscal year 2013 saw the White House's Office of Management and Budget call for slashing planetary science funding by one-fifth. Though Congress restored much of the money, the program has yet to fully recover, and with the doleful figures in the 2016 budget, it is again up to Congress to find money to keep the program funded.

In that regard, planetary science is at a disadvantage compared to other federal programs. During the budget standoff in 2013, for example, national parks were closed, which prompted an immediate backlash from the public. But because it generally takes several years for spacecraft to reach the outer planets, they are already funded by the time they start returning data. In other words, the ticket is purchased before the flight arrives at its destination. As such, from the public's point of view, the planetary science program will seem stronger than ever, returning spectacular images of alien worlds, while in fact the program is hobbling along, ill-prepared for the future due to consecutive years of reduced budgets.

Missions can take decades to see through to completion. In 2014, the European Space Agency landed a robot on a comet. It was the culmination of a very long project. When the mission, called Rosetta, was first approved in 1994, new computers came installed with Microsoft Windows 3.1. It then took a decade to plan the mission and design and build the spacecraft and lander. Facebook was less than a month old when the spacecraft launched in 2004, and another decade would elapse before it arrived at comet 67P/Churyumov-Gerasimenko. When the Philae lander made contact with the comet, the mission had been in progress for 21 years, not including the years of research that preceded its approval.

Cassini-Huygens, NASA's ongoing flagship mission to Saturn, was launched in 1997. New Horizons was approved in 2001 and launched in 2006. It will arrive at Pluto in July 2015. Juno, which is set to orbit Jupiter for a year starting in 2016, was launched in 2011. Such lengthy timelines mean that planetary exploration is largely incompatible with jarring starts and stops. A steady launch/arrival tempo must be sustained; as one spacecraft is returning science, another should be en route to another celestial body. An interruption in the cadence means that the clock is reset.

Niebur said there are two major consequences to cutting the outer planet exploration budget. "First, we stop making new discoveries," he said. "The pace of the scientific research and scientific discoveries slows down." More importantly, perhaps, is that the scientists working on these missions only get older, and absent active missions they retire or find work in the private sector. Meanwhile, without ongoing missions, it gets harder to attract young scientists into the field. "The field slowly begins dying," said Niebur. "You start losing a lot of the knowledge that we've built up. And then when you finally do decide to begin missions again, you've got to spend the resources to rebuild that knowledge."

A new field, vulnerable to attack

The exploration of other worlds began in 1962 with the launch of the Mariner 2 space probe to Venus. Modern planetary science is a relatively new field, and resides at the intersection of multiple scientific disciplines to include astronomy, geology, oceanography, and atmospheric science, among others. Historically, it has lacked the political and cultural influence of astronomy or astrophysics. Because of this, it has remained particularly susceptible to cuts and even cancellation.

More on space exploration For NASA, sending a person to Mars is simple. Dealing with Congress is hard.

That almost happened in 1981, when the White House proposed slashing NASA's budget. The Reagan administration attempted to defund Galileo, the storied spacecraft that would eventually study the Jovian system. It also considered eliminating the Jet Propulsion Laboratory, the agency's research and development center. The White House stopped taking calls from James Beggs, NASA's administrator at the time. A position paper issued by the Office of Management and Budget noted, "OMB staff believe that lower priority programs such as planetary exploration must be curtailed — even if they have been successful in the past." George Keyworth, Reagan's science advisor, told the White House budget review board "the cut in planetary exploration represents an example of good management." Galileo was only saved at the last minute when Howard Baker, the Republican Senate Majority Leader, personally intervened, reaching out to the White House in support of the mission, eventually brokering a compromise to keep the planetary science alive.

The situation then was much more perilous than it is today. Planetary science is presently bolstered by its maturation over time as a field of study, and by its demonstrable successes. While NASA's human exploration program retools for the exploration of Mars (or the moon, or an asteroid, depending on the whims of whomever is elected president), the robotic program is garnering impressive headlines. The landing of Curiosity on Mars, for example, must surely rank as an engineering wonder of not one but two worlds. New Horizons's flyby of Pluto is likely to be one of the biggest stories of 2015, and part of science textbooks forever.

"It serves as reminder of what planetary exploration can do for the image of NASA and the public consciousness of NASA," said Casey Dreier, the advocacy director of the Planetary Society. "[The European Space Agency's] Rosetta was a great antidote for the dismal other news that was happening in the world at the end 2014. We had all this nasty stuff with ISIS and terrorists and international politics with an aggressive Russia, but here you have suddenly, oh yeah, look at this: here's a robot landing on a comet for the first time. This is what humanity can do as an expression of pure curiosity. It was an unambiguous reminder that we're not all bad."

Still, the Obama White House has been particularly uncompromising about cutting the budget for solar system exploration. In 2013, the Office of Management and Budget proposed cutting planetary science, specifically, by 21 percent, to $1.19 billion. The following year it proposed a budget of $1.22 billion, and in fiscal year 2015, it wanted $1.28 billion — each far below the $1.5 billion dog toy standard. The proposed cuts in 2015 went beyond belt-tightening, removing funding for NASA to operate the Mars rover Opportunity and the Lunar Reconnaissance Orbiter, which is currently circling the moon. (The president's proposed 2016 budget again attempts to kill Opportunity and the orbiter.) In each case, Congress found ways to reinsert much of the lost funding. Without the institutional support of the White House, however, NASA cannot count on the money materializing each year. The space agency cannot make five-year contracts and simply hope that Congress appropriates the money.

Our Magellan

In times of budgetary uncertainty, NASA is forced to proceed with only the most reliable mission proposals. This means a lot of thrilling plans to explore other worlds fall by the wayside. The most notable of these, perhaps, was the Titan Mare Explorer. TiME, as it was called, was a low-cost mission proposal in 2009 to send a spacecraft to Titan, one of Saturn's moons. The spacecraft was also a boat, and would have splashed down onto one of Titan's lakes. There, it would have sailed around, analyzing the chemistry of the sea and the makeup of the air above it. It would have taken photographs of the lake and its waves. It would have even had a microphone to hear Titan's waves lapping against its side. The very idea of such a mission outpaces the fever dreams of science fiction. Sadly, lacking funding, the mission never left PowerPoint, and the launch window is now closed. (A successor mission — this time using a submarine — has since been proposed.)

Another mission that didn't survive the proposal stage was the Europa Jupiter Science Mission-Laplace, a joint mission with the European Space Agency. NASA would send a probe to Europa, one of Jupiter's moons, and the European Space Agency would send a probe to Ganymede, another moon of Jupiter. Having two highly capable spacecraft in the same place at the same time would have greatly improved the quality of data produced because of the addition of interactive analysis systems. NASA pulled out of the mission in 2011 for budgetary reasons.

"The field slowly begins dying. You start losing a lot of the knowledge that we've built up."

The European Space Agency has vowed to carry on with its side of the deal, and has since reorganized its Ganymede mission as the Jupiter Icy Moon Explorer — the unfortunately abbreviated JUICE. Set to launch in 2022 and arrive at Jupiter in 2030, JUICE will examine Ganymede's magnetic field (it is the only moon in the solar system to have one) as well as its topography, oceans, and atmosphere.

Because of starvation budgets, it is nearly impossible to get a mission onto the launch pad and into space, though with seemingly superhuman perseverance it can be done. Consider the New Horizons mission to Pluto, humanity's last great hope to maintain an active presence in the outer planets from 2017 until a planned mission to Europa is underway. Dr. Alan Stern, the principal investigator of the New Horizons mission and former associate administrator for NASA's Science Mission Directorate, first conceived of a Pluto mission in the late 1980's. New Horizons was the sixth Pluto mission of which he was a part. The previous five were canceled before being realized.

"The timescale and the cost and the complexity all end up on the ‘hard' side of easy-to-hard to do outer planet missions," he said. Throughout the 1990s and early 2000s, each of the Pluto missions that NASA studied grew in cost to the point that the agency felt they were untenable. "There was only so much desire and so much budget, and when it got out of control on budget there wasn't enough desire to stomach the cost increases. So they put their pencils down. And then the scientific community would come back and say, ‘We really want this mission. Try it again. Let's think of a different approach.'"

The Pluto mission was thus opened up to any organization that wanted to make a proposal, with NASA choosing the most promising entry. Stern's team won the competition in 2001. "I was convinced as the project leader that if we ever got out of control on cost that we would be canceled as well. So I made sure we stayed in the [cost] box, which we did. And one of the breakthroughs of New Horizons is that it is a much lower-cost outer-planets mission than any in a long time. In fact, if you compare it to Voyager, its cost is about two dimes on the dollar. Twenty percent as much."

But even using the long timelines that characterize the exploration of the outer solar system, Stern and his team worked a long time — 14 years — to see New Horizons through from a concept to takeoff. "Persistence is something that we talk a lot about at New Horizons. We feel — and did from the beginning — that we were kind of the stewards of this. I felt a lot like this was probably the last chance."

As a result of the work and doggedness of the New Horizons team, the first probe to each planet in the solar system will have been launched by the United States. Such firsts transcend even the exciting research that results from a robust planetary exploration program, and will feature in classrooms for centuries to come. "In our own time it very much exemplifies best in our country to people of other countries," Stern said. "We do this with our dollars but we share the knowledge with all mankind. And even in foreign countries that don't get along with the United States, kids still learn about the exploration of planets and they know that the United States did it without having to be told. The names of programs like Apollo and Voyager are in textbooks in every language."

All these worlds are yours except Europa?

If humanity has a future in the outer planets, it is on Europa. For the second time running, the Decadal Survey, which represents a scientific consensus concerning the most pressing goals for planetary exploration, has recommended a Europa mission. (The most recent survey gave slightly higher priority to a Mars sample retrieval mission). In December's continuing resolution to fund the government, Congress specifically earmarked $100 million to study a possible Europa mission, and the proposed fiscal year 2016 budget likewise endorses a such a mission, meaning Congress and the White House might be in rare agreement on something of consequence.

Meanwhile, mounting evidence of the Jovian moon's habitability helps along the idea of such a mission. The conditions on Europa do not merely suggest that the moon contained microbial life 100 million years ago. The conditions suggest that Europa might have life today, and that life might be more complex than a microbe. Either way, there are staggering implications for our understanding of habitability and life in the universe. If life is found on Europa, it would mean that there are at least two habitable worlds in a single solar system, suggesting a galaxy teeming with life. Conversely, if Europa, with its ideal survival conditions, is found to be barren, it might mean a much lonelier universe. If the mission were in fact fully approved and funded, it wouldn't launch until sometime in the 2020s, before making the long journey to the Jovian system.

Dr. Louise Prockter, a planetary geologist and the assistant supervisor of the Science Branch at the Johns Hopkins University Applied Physics Laboratory, would serve as one of two deputy project scientists on the mission. She was the chair of Europa's science definition team, and much like Alan Stern has spent years working to turn a mission proposal into a spacecraft on the launch pad. She and her team have internalized the lessons of the collapse of the last Europa mission, the Europa Jupiter System Mission-Laplace.

"People have been slowly but surely buying into the fact that, yeah, maybe Europa is the place that we should be going as a community," she said. "That this is really a important target."

Her team's efforts are part of a larger endeavor that involves developing the science of Europa, finding ways to trim mission costs, and keeping the community of planetary scientists on board while attracting new supporters. The team's efforts seem to be paying off, helped along by the growing scientific evidence that favors Europa. "The other thing that's helped Europa is that astrobiology has become a much bigger aspect of science," Prockter said. "And Europa, we think, is probably the best place in the solar system to go and look for life outside of the earth. It's taken years and years and years of plugging away and showing up and presenting our studies and knocking down the issues every time they come up, every time there's a problem, just figuring out a way around it. ... We are finally getting close to the finish line."

Concerning the cost of what would be a flagship-class mission for NASA, she said the lessons learned from a previous Europa proposal have informed how this one is designed. "We were forced to go back to the drawing board and rethink our whole concept and it forced us to really get down to the basics about what is really important here, and how can we do that at a lower cost? The concept we have today — the Europa Clipper concept, as it's called — is the result of the last two or three years of really concentrated study, and that has allowed us to get to a really sophisticated level of detail."

Taking from the lessons of previous canceled missions to other worlds, her team is not anticipating technology that may not materialize. The Europa mission does not rely on instruments that should be smaller, or materials that might be lighter, which means the mission is ready to go, technologically. "One of the concepts we tried to keep in our minds while we were thinking about the science for Europa: we would think about no miracles," Prockter said. "No technology that didn't exist or that couldn't be adapted fairly readily from existing technology. ... So that we didn't need to wait another 10 years for anything new to be developed; we could start with what we have now. And that also helped us keep the cost fairly low."

If elected officials are waiting for a mission worth funding, short of discovering a field of alien-built oil wells on Pluto, the scientific consensus holds that there is nowhere in the outer planets more promising than Europa. There is some poetry in that moon being the future of planetary science; it was also part of the field's origin. In 1610, when Galileo discovered Europa and three other moons of Jupiter, he made humanity's tentative first step toward establishing planetary science as a field of study. Provided lawmakers write the check, however, the challenges only just begin. When asked what happens after a "yes" call from NASA, Prockter launched into an astonishing, off-the-cuff list of considerations.

"Every spacecraft has different parts, different subsystems, different elements. We've been studying this for a long time. We have already been investigating launch vehicles. We have investigated power. We are now going to solar power; we were originally going to be a nuclear powered spacecraft. We've spent years investigating what power would we need." Her team has worked with a science definition team to take scientific objectives and translate them into mission requirements. If, for example, someone wanted to resolve an image of Europa's surface at a certain resolution, a host of issues must first be addressed. "What kind of instrument do I need? What focal length of my camera do I need? Do I need a color filter? How close to the surface do I have to be? If I'm flying by, what speed do I have to fly by at to not smear that image out? So there are so many elements to every little decision that you make, every trade that you make."

"With the US doing fewer missions, you're having a shrinking of the human presence in the solar system"

The hardware considerations aren't limited to measurement instruments and imagery. "We have propulsion. We have thermal. We're out at Jupiter — it's pretty cold out there, but we have to survive for years. And we have to get enough power to power our solar panels. We have planetary protection. How do we not take bugs from Earth and contaminate the environment? How do we not crash into Europa? How do we make sure that that doesn't happen, or that if it does happen that we're prepared for that? Radiation: how do we shield all that radiation, all those particles? Do we know enough about them? What do we need to do while we're out there? Trajectory: we've tried several different trajectories to try and minimize the radiation."

There's also the basic question of building the spacecraft itself. "Where do you put things? How do you communicate with the ground? What sized antenna do you need? Can you get coverage from the ground stations on earth at the times you need them? There are a million different decisions to be made, but we've already made a lot of those trades, so we have this concept, and so when we get the go-ahead, when we're finally ready to go, we would actually start implementing that." Some decisions and trades must still be made. "Right now we don't have an actual payload. If NASA selects a payload from these instruments, they might not select the ones we've recommended. They might select other things because they think they're better, or their panels say they're better. So then we have to go back and if they gave us a different instrument, we'd have to figure out what science we can do with that instrument, and how do we accommodate that onto a spacecraft? It's pretty cool."

Beyond 2017

As NASA's exploration of other worlds contracts, foreign space agencies are beginning to stack triumph upon triumph. Two months before European Space Agency achieved the first soft landing on a comet, the Indian Space Research Agency put a probe in orbit around Mars. In December, the Japan Aerospace Exploration Agency launched Hayabusa 2, an asteroid sample return mission. In 2013, China set a lander and rover on the moon as part of an aggressive plan to put Chinese footprints on the lunar surface. There are some things, however, that only NASA can do.

"Nobody can do deep space like NASA can," said Emily Lakdawalla, the senior editor of the Planetary Society. "Other nations can go to the moon, Mars, and to the inner solar system like Venus and Mercury. But they don't have nuclear power sources. They don't have radioisotope thermoelectric generators — only the United States and Russia have those. Right now, nobody but the United States can go beyond Jupiter. With the US doing fewer planetary missions, you're having a shrinking of the human presence in the solar system and fewer missions out into the deepest part of the solar system. But there will be a lot more stuff going on at the moon and Mars and asteroids."

These robots will likely run much longer than their expected end-of-mission dates. "The fact that we have so many active missions at the same time — it's great but it's also a headache for NASA bookkeepers because it doesn't cost nothing to keep these missions going." Going forward, she said, NASA should consider a new way to plan for success so that extended missions of spacecraft don't take money from other planned missions. "You kind of wish that when a government agency were super successful that they might throw a little bit more money at that government agency."

In the meantime, the lights in the outer solar system will continue to switch off, one probe and planet at a time. NASA will continue to absorb broadsides from the Office of Management and Budget and do its best with such halfhearted executive mandates as the asteroid redirect mission. "If we're not inspired by that, it's not NASA's fault — it's our leadership's fault," said Lakdawalla. "And we need our Congress and our president and the people of the United States to stand up and say, ‘This isn't good enough. I want my moon base. I want my Mars base, and I'm willing to put the money forward to make that happen.' And if you're me, I want my outer planets missions. I want a Uranus orbiter. I want go back to Jupiter. I want to fly to the plumes of Enceladus. I want a boat on Titan. Those are what I want. I understand that not all of the American public agrees with all of those goals, so I'm not going to get them all. But I would like at least one of them."