While many in the industry were at first skeptical of NASA’s goal to put resupply flights to the International Space Station in the hands of commercial operators, the results speak for themselves. Since 2012, the SpaceX Dragon family of spacecraft has been transporting crew and cargo from American soil to the orbiting laboratory, a capability that the space agency had lost with the retirement of the Space Shuttle. Putting these relatively routine missions in the hands of a commercial provider like SpaceX takes some of the logistical and financial burden off of NASA, allowing them to focus on more forward-looking projects.SpaceX Dragon arriving at the ISS for the first time in 2012.But as the saying goes, you should never put all of your eggs in one basket. As successful as SpaceX has been, there’s always a chance that some issue could temporarily ground either the Falcon 9 or the Dragon.While Russia’s Progress and Soyuz vehicles would still be available in an emergency situation, it’s in everyone’s best interest that there be multiple backup vehicles that can bring critical supplies to the Station.Which is precisely why several new or upgraded spacecraft, designed specifically for performing resupply missions to the ISS and any potential commercial successor, are coming online over the next few years.In fact, one of them is already flying its first mission, and will likely have arrived at the International Space Station by the time you read this article.Cygnus XLThe Cygnus was the second commercial spacecraft to deliver cargo to the ISS back in 2013, and like the Dragon, has gone through several upgrades and revisions over the years. Rather than starting from a clean slate, the Orbital Sciences Corporation based the vehicle’s pressurized module on the Multi-Purpose Logistics Module which was originally designed to fly inside the Space Shuttle’s cargo bay to provide onboard laboratory space before the construction of the ISS. This was paired with a service module that was derived from their line of communication satellites.Orbital Sciences Corporation was eventually acquired by Northrop Grumman, which now operates the latest version of the spacecraft, the Cygnus XL. This latest version of the cargo craft lifted off for the first time on September 14th, and is currently en route to the ISS.It retains the same 3.07 m (10.1 ft) diameter of the original Cygnus, but the length of the vehicle has been increased from 5.14 m (16.9 ft) to 8 m (26 ft). This has nearly doubled the internal pressurized volume of the craft, and the payload capacity has been increased from 2,000 kg (4,400 lb) to 5,000 kg (11,000 lb).While the Dragon can autonomously dock with the ISS, the Cygnus XL needs to be captured by an astronaut using the Station’s robotic arm, and manually moved into position where it’s eventually bolted into place — a process known as berthing. This is a more labor intensive method of connecting a visiting spacecraft, but it does have at least one advantage, as the diameter of the berthing ports is larger than that of the docking ports. At least in theory, this means Cygnus XL would be able to deliver bulkier objects to the Station than the Dragon or any other spacecraft that makes use of the standard docking ports.Like the earlier versions of the craft, Cygnus XL is an expendable vehicle, and lacks the heat shield that would be necessary to reenter Earth’s atmosphere safely. Once the vehicle delivers its cargo and is detached from the Station, it’s commanded to perform a deorbit maneuver which will cause it to burn up in the atmosphere. But even this serves an important function, as the astronauts will load the vehicle with trash before it departs, ensuring that refuse from the Station is destroyed in a safe and predictable manner.HTV-XLike the Cygnus XL, the HTV-X is an upgraded version of a spacecraft which has already visited the ISS, namely the H-II Transfer Vehicle (HTV). Designed and built by the Japan Aerospace Exploration Agency (JAXA), the first flight of this upgraded cargo vehicle is tentatively scheduled for late October.The HTV-X reuses the pressurized module from the HTV, though it has been slightly enlarged and is now located at the rear of the spacecraft instead of the front. The cargo module is in turn attached to a service module that’s responsible for power generation, communications, and propulsion. For all intents and purposes, this service module is its own independent spacecraft, and JAXA is currently investigating future applications which would see this module mated with other payloads for various low Earth orbit missions.Attached to the opposite side of the service module is an unpressurized cargo module. This is similar to the “trunk” of the Dragon spacecraft, in that it’s essentially just a hollow cylinder with shelves and mounting points inside. This module could potentially be used to bring up components that are intended to be attached to the outside of the ISS, or it could hold experiments and modules that are designed to be exposed to the space environment.Like the Cgynus XL, the HTV-X will berth to the ISS rather than dock, and it will also burn up after its mission is complete. However the HTV-X is designed to fly freely on its own for up to 18 months after it delivers its cargo to the Station, which JAXA calls the “Technology Demonstration Phase” of the mission. This will essentially allow the agency to perform a second mission after the vehicle has completed its supply run, greatly improving the overall cost effectiveness of the program.Dream ChaserFar and away the most ambitious of these new spacecraft is the Dream Chaser, developed by Sierra Space. Reminiscent of a miniature version of the Space Shuttle, this winged vehicle is designed to land like an airplane at the end of its mission. This not only means it can bring material back down to Earth at the end of its mission, but that it can do so in a much less jarring manner than a capsule that ends up splashing down into the ocean under parachutes. This is a huge benefit when dealing with fragile cargo or scientific experiments, and is a capability not offered by any other currently operational spacecraft.The Dream Chaser has been in active development for over 20 years, but its origins date back even farther than that, as it’s based on HL-20 Personnel Launch System concept from the 1980s. While it was initially designed for crew transport, it lost out to SpaceX and Boeing during NASA’s Commercial Crew Program selection in 2014. It did however secure a contract from the space agency in 2016 for six cargo missions to the ISS. To qualify for these missions, several changes were made to the original design, such as the addition of an expendable module that will attach to the rear of the vehicle to increase its relatively limited internal cargo capacity of 910 kg (2,000 lb) by 4,500 kg (10,000 lb).The first orbital test flight of the Dream Chaser is currently scheduled to take place before the end of the year, but that date has already slipped several times. Being a reusable vehicle like the Dragon, the first Dream Chaser spaceplane is expected to fly multiple operational missions while a second craft is being assembled.After completing their contractually obligated missions to the ISS, there are currently plans for the Dream Chaser to fly at least one mission for the United Nations Office for Outer Space Affairs, which will carry an array of scientific experiments provided by member nations that do not have their own domestic space programs. The company also says they remain committed to bringing the crewed version of Dream Chaser to fruition, likely as part of their partnership with Blue Origin to develop the Orbital Reef — a “mixed-use business park” in space.Time is Running OutIt might seem strange that three different spacecraft are scheduled to enter service before the end of the year, but of course, the clock is ticking. Although the date has been pushed out a number of times over the years, the current 2030 timeline for the decommissioning of the International Space Station seems to be holding so far. With as little as five years left to go before the ISS joins us Earthlings back here on the surface, it’s now or never for any vehicles designed for service missions. This is doubly true for companies such as Sierra Space, who have already agreed to perform a set number of missions.At the same time, any of these vehicles could support a future commercial space station, should one actually materialize. We’ve covered some of the post-ISS plans previously, but given how volatile the aerospace world is, nothing is a given until it’s actually in orbit.