Last year offered a mixed bag for spaceflight aficionados. The highs were very high, with SpaceX flying, landing, and reflying rockets at an unprecedented rate while finally beginning to deliver on its considerable promise. But the lows were pronounced, too, with the loss of the Cassini spacecraft in the outer Solar System and NASA's continued lack (for nearly a full year) of an administrator.

There were also delays upon delays. The ultra-expensive James Webb Space Telescope saw its launch date slip from 2018 into some time in 2019. NASA's Space Launch System rocket saw its maiden launch slip from late 2018 into 2019 and then again into 2020. The Falcon Heavy also moved to the right on the calendar, from November, then December, and finally into early 2018.

But all of those delays mean that the last couple of years of the 2010s should feature a lot of spaceflight action, and a good chunk of that will occur in the next 12 months. Looking ahead at what is to come, here are the key spaceflight milestones we're most eager to see in 2018, grouped by the approximate quarter of the year in which they might happen.

1Q: Falcon Heavy flies

After years of waiting, the Falcon Heavy is finally ready to fly. The first projections for the Falcon Heavy pegged its maiden launch date about half a decade ago, and for the longest time it was something always talked about but never seen. But now, we know the rocket is real, as SpaceX's heavy lift booster has been spotted towering alongside Launch Complex-39A at Kennedy Space Center in Florida. Already, SpaceX has tested the rocket's transporter, and the company should perform a static fire of the Falcon Heavy's 27 engines within the next week or so. A launch could come some time this month.

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Although this is just a test flight—with a whimsical payload of Elon Musk's cherry-red Tesla roadster bound for an elongated orbit around Mars—the flight of the Falcon Heavy nonetheless represents a landmark moment in the US spaceflight industry. It will mark the first time a US company has developed a heavy lift rocket with largely its own financing. True, SpaceX leveraged some NASA funding to build the Falcon 9 rocket. But it has developed the core stage and integrated two Falcon 9 boosters to create the heavy rocket with its own resources. This is a truly private rocket.

In as few as two or three years, Blue Origin should complete development of its own heavy lift rocket, New Glenn, as well. Successful flights of both would signal that the US rocket industry can develop large, reusable rockets without large infusions of government money. This will likely raise further questions about why NASA, at the cost of about $2 billion annually to taxpayers, is independently developing its own heavy lift rocket, the Space Launch System, to compete with these private efforts.

1Q-4Q: Small satellite launchers

This could be the year of small satellite launchers. The New Zealand-based Rocket Lab has already flown its Electron booster into space once, and the company plans a second test flight, "Still Testing," early in 2018. If this mission is successful, the company could begin flying small satellites weighing about 150kg into space during the first half of this year.

Other companies are getting closer to spaceflight, too. Virgin Orbit says it will be ready to launch rockets carrying small satellites from its Cosmic Girl aircraft during the first half of this year. This means that after years of waiting, small satellite customers could have not one, but two choices for dedicated rides into space in 2018.

Then there's Vector Space Systems, which spent 2017 launching prototypes of its Vector-R launch vehicle. The company has also said it is on track for orbital launches in 2018, but Vector appears to be behind Rocket Lab and Virgin at this point. Other small satellite launchers—such as Stratolaunch Systems, Interorbital Systems, and PLD Space—are also working toward launches by the end of this decade.

1Q-2Q: Block 5 booster

The Falcon Heavy launch will grab the headlines this year for SpaceX, but in terms of its long-term future, the successful debut of the company's upgraded Falcon 9 rocket will probably be more consequential. This "final" version of the Falcon 9 rocket is called the Block 5 version.

SpaceX has begun to redefine the aerospace industry with its reusable Falcon 9 rocket, which has now returned safely to Earth 20 times. Nevertheless, it is not entirely revolutionary to return a booster, spend several months checking and re-checking every component, and then flying the same rocket a second time. The real revolution in spaceflight will come through rapid, low-cost reusability that truly cuts the cost of access to space.

That is what the Block 5 variant of the Falcon 9 rocket will bring, SpaceX founder Elon Musk has said. Each of these rockets, optimized for landing with a minimum of impact on the booster, could be able to fly 10 or (potentially many) more times while being turned around for reuse within weeks, if not days. If the Block 5 version is a success, there can be little question that SpaceX will conquer much of the global launch market. But first, it must fly.

The Crew Dragon spacecraft will also launch on the Block 5 booster, and NASA has said it wants multiple flights of the rocket before astronauts are cleared to fly on top of it. Therefore, the sooner the Block 5 variant flies, the sooner crew flights can begin.

2Q: LightSail 2

One of the most intriguing missions of this year will come with the launch of the Planetary Society's LightSail 2 mission. The spacecraft is booked as a secondary payload on what will probably be the third flight of the Falcon Heavy rocket, currently scheduled for no earlier than June 2018.

The crowdfunded project will attempt to make solar sail-powered flight in Earth orbit. Once in space, the 61cm-long spacecraft will separate from the upper stage of the Falcon Heavy rocket and undergo a series of maneuvers to deploy a light sail made of Mylar that will spread out to encompass an area of 32 square meters.

After the light sail has deployed, the spacecraft will attempt to use the constant pressure from solar particles to raise itself to higher orbits around the Earth, perhaps by as much as 1km per day. If this test is successful, solar sail technology could be deployed on future missions as low-cost, propellant-free propulsion to explore planets and moons in our Solar System.