2014 was a year of history making and foundation laying in spaceflight. The set piece, without question, was the European Space Agency (ESA) spacecraft Rosetta, which rendezvoused with comet 67P/Churyumov–Gerasimenko in August, and months later tasked its plucky lander Philae with becoming the first human-built craft to achieve a soft landing on a comet. The saga of Philae's descent, its bouncing trajectory over 67P's surface, and the frantic (and successful) effort by its mission controllers to achieve its science objectives before loss of power drew a worldwide audience, and delivered a mix of anxiety, uncertainty, and gratification reminiscent of Curiosity's descent and landing in 2012.

Beyond that main event was a stacked undercard. Two missions entered Mars orbit in 2014. One of these, the Mars Orbiter Mission, represents India's first attempt to orbit a planet besides Earth, and it was a resounding success. A crowdfunded team of unaffiliated engineers and scientists were able to regain control of a decades-old spacecraft, ISEE-3. Commercial resupply of the International Space Station continued apace, and key milestones for the commercial crew program were reached, with SpaceX and Boeing tapped for the job. The putative beyond-Earth-orbit spacecraft of America's future, the Orion capsule, made its first orbital flight and reentry without a hitch. Kepler, the most prodigious exoplanet hunter to date, was resurrected via a brilliant end-around solution to its reaction wheel woes, and went right on making discoveries.

There's more -- far too much to catalog here. As eventful as 2014 was, it also set up a packed slate of momentous spaceflight developments for the coming year. Here is a (non-comprehensive) inventory of what we're in for in 2015.

6 January 2015 -- CRS-5 & Barge Landing Attempt of Falcon 9 First Stage

With the Space Shuttle now 3 years into retirement and NASA thinking bigger than Low Earth Orbit (LEO), the task for keeping the International Space Station supplied, and eventually crewed, has been entrusted to private entities. The principle behind commercial resupply is to significantly reduce the cost of frequent ferrying trips to the ISS by enlisting companies to develop the infrastructure and deliver the goods. SpaceX, which now counts four successful resupply missions to its name, has been exploring the potential of vehicle reusability to achieve cost reduction.

Getting into orbit from the surface of the Earth is difficult business. You start out traveling at about 465 meters per second (if you're launching due east, your initial velocity is the speed of the Earth's rotation). When all is said and done, you have to be at least 100 kilometers high and traveling around 7.5 kilometers per second (about 17,000 miles per hour) to avoid falling back to Earth. To get there, you have to climb out of a gravity well trying to drag you back to the bottom, and slog through a thick atmosphere that resists your acceleration. This is why mass is so important for a launch vehicle. The more you have, the harder it is to get the vehicle up to the desired altitude and velocity before the engines burn out.

This is why rockets typically use multiple stages. A stage can burn and produce thrust until it is out of propellant, and then its entire dry mass -- the mass of the empty tanks, the engines, and any other assemblies -- can be discarded, leaving you with the next, lower-mass stage to continue adding velocity. The first stage is usually the largest. It has to travel through the thickest parts of the atmosphere and contribute the most thrust, and therefore needs the most propellant and largest (or most) rocket engines.

This is where SpaceX is trying to reduce cost. For the most part, once the first stage of a rocket is discarded, it's gone forever -- it falls back to the Earth, perhaps into the ocean, and all of the assembly cost, valuable engine hardware, and tankage is gone forever. SpaceX intends to enable the first stage of their launch vehicle, the Falcon 9, to perform a controlled, powered descent back to the Earth's surface, and land upright on legs, allowing for its intact recovery and reuse. The potential savings are obvious; there would be no need to build a new launch vehicle for every flight, dramatically reducing the cost of each mission.

Thus far into its launch manifest, SpaceX has tested this concept by bringing the first stage back to Earth and landing it in the Atlantic Ocean, with and without the deployable legs, to determine feasibility. It's mostly been successful, though these first stages have not been recoverable since they would subsequently tip over in the water.

CRS-5, scheduled for launch on 6 January, marks the first time that SpaceX will give the first stage something to land on. And what a something.

CEO Elon Musk revealed in November the acquisition of a 300 foot by 175 foot barge, equipped with powerful station-keeping thrusters, that will serve as an oceangoing landing platform for the first stage of the Falcon vehicle. It's a sizeable vessel, but consider the context: during the 3rd resupply mission, according to the press kit, the first stage separated at 80 kilometers in altitude, traveling at about 3.4 kilometers per second. Hitting a 300 foot wide target from that altitude is roughly equivalent to successfully dropping a basketball into a regulation hoop from the top of 432 Park Avenue, the second tallest building in New York City. SpaceX must achieve this with a vehicle that is traveling at 10 times the speed of sound at the time of separation.

The eventual hope for a typical mission is to fly the first stage directly back to the launch site in Cape Canaveral, but the barge isn't just a practice tool. For certain missions the Falcon 9 must deliver a payload to Geosynchronous Transfer Orbit (GTO), a preliminary orbit that eventually allows the payload to enter Geosynchronous Earth Orbit (GEO), an orbit in which it circles the Earth at the same speed with which the Earth rotates, remaining visible in the sky over the entire day for a given observer.

The apogee (furthest distance from Earth) of this orbit is much higher than that which is required to reach the ISS -- 35,786 kilometers, compared to around 350 to 400 kilometers for the ISS. More propellant is needed to accomplish this, so the first stage would not have enough remaining to return to the launch site. The barge, theoretically, would allow the first stage to land in the Atlantic ocean, refuel itself, and fly back to the launch site for reuse. It's extremely ambitious, but so far SpaceX has managed to deliver on a host of ambitious goals.

The company itself forecasts a low probability of success on the first attempt, but it's worth your effort to watch them try. SpaceX launches have great webcast coverage, with cameras on the vehicle that capture stage separation and vehicle burnout (check out this replay of CRS-4 to see what I mean). News of the first stage's fate will likely emerge shortly after the launch, so keep an eye on Twitter. Any measure of success would mark a huge step towards easier and cheaper access to space.

Early 2015 -- Hotfire Testing of RS-25 Engine for Use on SLS

While private companies provide access to Earth orbit, NASA plans to concentrate on manned missions beyond Earth orbit (BEO), which makes for heavy payloads. To launch these payloads, they are building the Space Launch System (SLS), a massive rocket build on Shuttle-derived technology. In particular, the core stage of SLS will be powered by four RS-25 engines, also known as Space Shuttle Main Engines (SSMEs) -- three of these lifted the Shuttle to orbit during ascent.

The first flight of the SLS is slated for 2018, and NASA is preparing one of the SSMEs for this flight for hotfire testing in early 2015. So who cares about a simple test?

On the one hand, use of the SSME sort of emblemizes the concerns about the viability of SLS as a launch vehicle. The SSME was extensively engineered for extreme reliability and efficiency, the former because they were to constantly be reused, and the latter because after the separation of the solid rocket boosters the SSMEs alone had to propel the Shuttle to orbit. As a result of these priorities it's a complex and expensive engine, and yet, for each launch of the disposable SLS rocket, four will be dumped into the ocean, never to be used again. It doesn't bode well for launch costs or Congressional friendliness.

On the other hand, the markedly more nostalgic hand, the SSME is a very cool rocket engine -- one of my favorites. It's the most efficient rocket engine built to date, using the most efficient propellant combination -- liquid hydrogen for fuel and liquid oxygen as the oxidizer. The result is a small but brilliant blue cone of flame, and simple water vapor as the exhaust. Oh, and a brain-shaking roar. Here is a great video of the SSME on the test stand at Stennis, including some gimbaling towards the end. And this is a great view of the SSMEs igniting during the launch of STS-134.

So I was sad to see the SSME headed for retirement, and I'm glad it's been repurposed. Even if SLS doesn't reach its full potential, or wants for funding, watching an SSME light up on the test stand will be a sight for sore nerd eyes in 2015.

6 March 2015 -- Dawn Arrives at Ceres

The same convention of the International Astronomical Union that demoted Pluto to dwarf planet in 2006 also elevated Ceres to the same classification. And with good reason -- Ceres is the largest object in the asteroid belt between the orbits of Mars and Jupiter, and makes up around 30% of the total mass of the belt. It's suspected that Ceres has a significant layer of water ice, and a potential thin atmosphere. Emissions of water vapor were detected at the beginning of 2014 via spectroscopic observation. It's a juicy science target, yet this is the clearest observation we've made of it, taken by the Hubble Space Telescope in 2004:

Dawn, launched by NASA in 2007, will be able to resolve Ceres more clearly than this as early as late January, and will spiral into its first science orbit in early March. At that time, it will be the first spacecraft to orbit a dwarf planet, and the first spacecraft to orbit two deep space bodies, having also explored the asteroid 4 Vesta in 2011 and 2012. It carries a camera for imaging and mapping the surface of Ceres, a spectrometer covering visual and infrared wavelengths, and a gamma ray and neutron detector, the latter two instruments for investigating the chemical composition of Ceres and its potential atmosphere.

Any object harboring serious indications of the presence of water is of extreme interest to scientists for what it can potentially tell us about the formation of the solar system, as well as the presence of water on our own planet. The data will be a nice complement to Rosetta's exploration of 67P, combining to paint us a clearer picture of the smaller bodies in our solar system than we've ever had before.

14 July 2015 -- New Horizons Flyby of Pluto, Charon, et. al.

New Horizons, launched in 2006, will see the culmination of nearly a decade of hard work and patience this year. New Horizons will be making observations for many weeks preceding and following its encounter with Pluto on 14 July, but the actual flyby will take less than 30 minutes, during which its suite of scientific instruments will investigate not only Pluto but also its known satellites -- Charon, Hydra, Nix, Kerberos, and Styx.

Until now, all of our investigations of what was once considered our solar system's ninth planet have been conducted at a distance. Voyager I had the opportunity to visit Pluto, but mission planners decided instead to conduct a flyby of the more scientifically interesting Titan, Saturn's smoggy moon. This maneuver flung Voyager I out of the plane of the ecliptic, eliminating the opportunity for a Pluto flyby.

Observations of Pluto have been made by a host of Earth-based observatories, gathering evidence that makes it a very promising target for planetary science: a likely differentiated composition of rock and ice, a thin atmosphere, and interesting variations in pressure and temperature. Hubble has attempted imaging Pluto, but it's so small that this image is the best that can be resolved, only after extensive processing:

PR concerns obviously take a back seat to the enormous amount of science New Horizons will conduct in its flyby, but I do think, from the public's perspective, photography is the embodiment of true discovery. In 1965, Mariner 4 sent back a host of scientific data following its first-ever flyby of Mars, but what fascinated the public were the images it collected. The Voyager probes sent back captivating photographs of Jupiter, Saturn, Uranus, and Neptune in the 70s and 80s.

An image is much more accessible than a paper in a scientific journal, even if the latter might tell you more. We've known about Pluto for 84 years, and we've never yet had a clear image of it; unlike Saturn, or Jupiter, or Mars, there is no picture to call to mind when someone mentions Pluto. Finally having that, I think, will raise a substantial swell of public interest, at least for a time.

Besides that, New Horizons will be able to map at least some of Pluto's surface, and may provide some answers as to the apparent variations in color and albedo (light reflectivity) that previous Earth-based observations have hinted at. Additional science goals include determining atmospheric and surface chemical composition, measuring surface temperature and mapping variations, and searching for signs of an atmosphere on Charon, Pluto's largest satellite. Imaging of the smaller, more recently discovered moons -- Hydra, Nix, Kerberos, and Styx -- will also be undertaken. After closest approach, New Horizons can observe some of the nighttime sides of Pluto and Charon, and investigate how light and radio signals interact with their atmospheres (if Charon does indeed have one).

The full slate of data will take weeks to transmit back to Earth, and many months more to interpret, but a few decent photographs should be available not too long after the encounter. Keep an eye on the New Horizons twitter account on the days preceding and following, as well as that of the excellent Emily Lakdawalla, and, while you're at it, mine.

13 August 2015 -- Rosetta/Philae Approach and Perihelion

When Rosetta first encountered 67P last August, the comet was 3.6 AU (astronomical units; 1 AU is the distance between the Earth and Sun, about 150 million kilometers) from the Sun. The closest that its orbit takes it to the Sun -- its perihelion -- is 1.2 AU. This is the point it will reach on 13 August, but throughout the year the comet's increasing proximity to the Sun will impart changes that Rosetta will have the opportunity to observe up close.

Comets are made up mostly of ice and dust, and not typically very dense. As they approach the sun, the increasing temperatures cause sublimating ice and various gases to be ejected from the comet, forming the trademark tail that can be observed before and after perihelion. Rosetta has already seen outgassing from 67P, but the phenomenon will become orders of magnitude more intense in the coming weeks and months, with crescendo at perihelion. Mission scientists will have a first hand look at how a comet changes throughout its orbit, and will gather more data pertaining to 67P's composition and the history of its formation.

Philae may also return for an encore performance. After completing its science objectives on the surface in November, the lander succumbed to a loss of power and entered a hibernation safe mode. Having bounced twice, touching down far from its intended landing site, it's possible that Philae is wedged against some kind of rock formation that is preventing its solar panels from getting enough sunlight. Whether or not that's the case, it's hoped that the steadily increasing intensity of sunlight reaching the surface of the comet as it approaches perihelion will result in Philae having enough power to reboot. If that happens, Philae could observe potential effects of outgassing and temperature fluctuations from the surface, while Rosetta conducts orbital science.

The ESA has a lot of Twitter accounts related to the mission and the spacecraft, but the main Rosetta account always seems to deliver the goods. Be on the look out for some spectacular imaging, as always.

Late 2015 -- First Launch of a Falcon Heavy

Right now, the heaviest lifter the United States has access to is the Delta IV Heavy, which can put up to 29 metric tons in low Earth orbit. For several years now, while the Falcon 9 has been delivering on a crowded launch manifest, SpaceX has been developing the Falcon Heavy, a rocket intended to deliver payloads as large as 53 tons to low Earth orbit.

The Falcon Heavy uses a Falcon 9 first stage as the core stage, with two additional Falcon 9 first stages strapped on as boosters to the sides, to be jettisoned when they exhaust their propellant, with the center core continuing on.

The Falcon Heavy is also planned to use a technique called propellant crossfeed, where the strap-on boosters feed propellant both to their own engines and to the center core's engines. This increases the efficiency and consequently the possible payload of the rocket.

Though it won't be implemented for the first flight, the Falcon Heavy will take SpaceX's reusability plans a step further, with the company intending to land both the strap-on boosters and the center core safely for future reuse. Here is where the barge comes in again. While the boosters should have the capability to return to the launch site, the center core will be too far away and traveling too fast at the time of burnout. It could, however, be landed on a properly positioned barge and refueled for a return flight.

It's no guarantee to fly in 2015, but right now the first flight of the Falcon Heavy, a demonstration flight only, is intended to take place before the year is out. Following that, there are a few customers, including the Department of Defense, lined up for Falcon Heavy launches. If successful, the rocket will be the heaviest lifter available until the SLS flies, if it ever does.

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Here are a few other quick things to watch for:

• BEAM Launch to ISS -- Bigelow Aerospace, which has been developing lightweight inflatable modules to be used as habitats in space, plans to deliver its first operational module, the Bigelow Expandable Activity Module (BEAM), to the ISS. This first module will be used only for testing, and astronauts will only venture into it a few times, but it will be a big step towards the use of inflatable habitats for long-term human occupation in space. BEAM is schedule to fly with CRS-8, scheduled for launch on 2 September.

• Akatsuki Second Orbital Insertion Attempt -- Akatsuki, a Japanese probe intended to make observe the climate dynamics of Venus, will try for a second time to enter orbit around Venus in November. Its previous attempt in 2010 was unsuccessful, due to a failure of its main engine. Mission planners have opted to try and use the reaction control system (RCS), normally intended only to change the spacecraft's orientation, to perform orbital insertion when Akatsuki encounters Venus again. While the resulting higher orbit will likely limit the probe from achieving its science objectives, it would be an impressive recovery of a seemingly doomed mission.

• Progress on the Europa Clipper Mission -- Europa is an enticing target for science, and perhaps the most likely body other than Earth in our solar system to harbor life, with a massive liquid water ocean lying under several kilometers of an icy shell. Europa Clipper is a planned mission to investigate the Jovian moon more thoroughly than ever, conducting up to 50 flybys from a high Jupiter orbit with a full suite of scientific instruments. There's no scheduled launch date yet, but 2015 could see several milestones in the development and planning of the mission.

• Continued Exploration by the Curiosity Rover -- Curiosity has continually refined our understanding of the geological history of Mars, the most recent feather in its cap being the discovery of methane with possible seasonal spikes. Having now established that the Gale Crater was once a shallow lake of liquid water, Curiosity will carry on investigating Mount Sharp, and evidence of the flow of water that once occurred there.

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Image credits: Cover - Alex Parker; SpaceX barge - Elon Musk via Twitter; Shuttle main engine ignition - Wikipedia; Ceres - Hubblesite; Pluto - Hubblesite; Falcon Heavy conceptual image - Nellillustrazione Artistica Di Copertina.