After more than a month in orbit, SpaceX’s CRS-21 Dragon cargo ship is due to return to Earth early Monday morning, laden with over 5,200 pounds (2,350 kg) of payloads and experiment results, destined for the eager hands of researchers back home. Undocking of the vehicle—the first Cargo Dragon to dock and undock, rather than berth and unberth—from the International Space Station (ISS) is targeted for 9:25 a.m. EST.
After departing International Docking Adapter (IDA)-3 on the space-facing (or “zenith”) port of the station’s Harmony node, closely monitored by Expedition 64 astronaut Kate Rubins, CRS-21 will maneuver itself smoothly away from the complex and execute Dragon’s first-ever parachute-aided splashdown in the Atlantic Ocean about 12 hours later.
Launched on 6 December atop the veteran B1058 Falcon 9 first-stage core—the selfsame booster as was previously used last summer to lift Dragon Endeavour crewmen Doug Hurley and Bob Behnken to orbit for their historic Demo-2 mission—CRS-21 marked the first outing of a Crew Dragon-class spacecraft in its uncrewed cargo-carrying configuration.
It flew under the second-phase Commercial Resupply Services (CRS2) contract between SpaceX and NASA; a contract definitized back in January 2016, which is expected to see the Hawthorne, Calif.-headquartered launch services provider fly at least six Cargo Dragons to the ISS by 2024. Current plans are for two more CRS2-contracted missions (CRS-22 and CRS-23) to launch as soon as May and September of this year.
This next round of missions rides on the highly successful coattails of the earlier CRS1 batch of flights, whose initial $1.6 billion contract was signed between SpaceX and NASA in December 2008 and saw the first-generation Cargo Dragon fly 20 missions between May 2012 and March of last year. They trucked a total of 95,000 pounds (43,000 kg) of cargo uphill to the ISS and brought 75,000 pounds (34,000 kg) into the eager hands of waiting scientists on the ground. In doing so, Cargo Dragon represented the United States’ only means to bring large quantities of research results back to Earth following the 2011 retirement of the Space Shuttle fleet.
And in a remarkable demonstration of the ship’s reusability capabilities, the 20 missions flown under the CRS1 banner did so using only 12 Cargo Dragons: three of which flew twice whilst three others—including the vehicle used for last March’s CRS-20 flight—visited the space station on as many as three occasions. It is a level of reusability that SpaceX aspires to replicate with its CRS2-class vehicles.
A key characteristic of the newer ships is their capacity to dock autonomously at the ISS, with correspondingly reduced demands upon crew labor and time. CRS1-class Dragons—as well as Japan’s H-II Transfer Vehicle (HTV) and Northrop Grumman Corp.’s Cygnus—were and are captured by means of the station’s 57.7-foot-long (17.6-meter) Canadarm2, then robotically “berthed” at a Common Berthing Mechanism (CBM) on either the nadir port of either the Unity or Harmony nodes.
But when CRS-21 docked smoothly at IDA-3 on 7 December, only 26 hours after liftoff, it marked the shortest interval between launch and arrival of any U.S. unpiloted visiting vehicle. Its month aboard the station has certainly been a busy one, supporting the needs of the seven members of Expedition 64: Commander Sergei Ryzhikov of Russia, his U.S. crewmates Kate Rubins, Mike Hopkins, Victor Glover and Shannon Walker, Japan’s Soichi Noguchi and Russian cosmonaut Sergei Kud-Sverchkov.
Following its launch with more than 6,400 pounds (2,900 kg) of payloads and supplies, CRS-21’s docking at IDA-3 was closely monitored by Rubins and Glover from the multi-windowed cupola. Counting the presence of Dragon Resilience at Harmony’s forward-facing IDA-2 port, this was the first time that two Dragons—one crewed, one uncrewed—had resided simultaneously at the ISS.
The U.S. Operational Segment (USOS) crew members wasted little time removing, installing and activating Dragon’s haul of science, as Rubins worked to install new science freezers in the station, Walker and Hopkins offloaded time-critical research samples and Glover set up newly-delivered rodent habitats. Noguchi set almost immediately to work on the Space Organogenesis human stem cell investigation, collecting biological samples and research hardware from the voluminous interior of Dragon and setting it up for a protracted series of observations in Japan’s Kibo lab.
Over the next several weeks, research associated with CRS-21 entered high gear, as Rubins set work investigating cardiovascular cells in the station’s Life Science Glovebox as part of Stanford University’s Cardinal Heart study. This experiment uses Engineered Heart Tissues (EHTs) as part of efforts to understand the aging and weakening of heart muscles to provide better treatments for humans both on Earth and in space. Hopkins joined Rubins as part of the study inside Kibo.
Elsewhere, a group of mice ferried uphill aboard Dragon were settled into their specialized habitats aboard the station’s U.S. Destiny lab by Walker, Hopkins and Glover, as part of extensive observations of their behavior in the peculiar microgravity environment. Specific focuses of the investigation emphasized the function of arteries, veins and lymphatic structures in their eyes and changes in their retinas both before and after missions in space.
This is expected to reveal new insights into changes of visual acuity and, in turn, may help to address the phenomenon of vision impairment—known as Spaceflight-Associated Neuro-Ocular Syndrome (SANS)—which afflicts about 40 percent of astronauts on long-duration voyages aboard the ISS.
With a new U.S. Extravehicular Mobility Unit (EMU) space suit (Serial Number 3015) having been brought to the station aboard CRS-21, Hopkins and Glover worked to swap it with another suit (Serial Number 3008) which is returning to Earth for maintenance. The outgoing suit hardware has supported dozens of shuttle and ISS spacewalks since its maiden use during the second Hubble Space Telescope (HST) servicing mission in February 1997.
After being used during eight shuttle missions—including three HST flights in 1997, 1999 and 2009—and by members of three ISS expeditions, it was returned to Earth for servicing and brought back to the station in December 2015 aboard the OA-4 Cygnus cargo ship. Since then, it has kept ten astronauts alive (including Rubins herself) during a total of 26 spacewalks between January 2016 and January 2020.
Notably, on 19 December, NanoRacks’ Bishop commercial airlock was robotically extracted from CRS-21’s unpressurized “trunk” by ground controllers and—using Canadarm2—was maneuvered into position and attached to the port side of the station’s Tranquility node.
Last week, as the Dragon presence on the ISS entered its homestretch, Walker and Noguchi devoted a portion of their workdays to packing cargo and experiment samples aboard the ship for departure. And just last Thursday, Rubins stowed microbial cultures in science freezers for transfer to the upgraded Dragon, which has double the quantity of powered lockers as its CRS1-class predecessor.
Following tomorrow’s scheduled undocking of CRS-21, the spacecraft will utilize its thrusters to maneuver to a safe separation distance, before executing a deorbit “burn” and parachute-aided splashdown just off the Florida coast in the Atlantic Ocean.
Splashdown is presently targeted for 9 p.m. EST Monday. Returning to an oceanic landing just off the Space Coast will enable its science samples to be quickly transported to the Space Station Processing Facility (SSPF) at the Kennedy Space Center (KSC) within hours, enabling research teams to collect data with minimal losses of microgravity-induced effects.
“I am excited to finally see science returning here again because we can get these time-sensitive experiments into the lab faster than ever,” said KSC utilization project manager Jennifer Wahlberg. “Sending science up to space and then receiving it again on the runway was definitely something in the shuttle days that we really took pride in, and being able to rejoin that process is great.”
Previous CRS1-class Dragons landed in the Pacific Ocean and their science cargoes were processed at SpaceX’s facility in McGregor, Texas, before return to NASA’s Johnson Space Center (JSC) in Houston. This produced a correspondingly longer return timeframe.
“Using the previous Dragon spacecraft, it could take up to 48 hours from the time the capsule hits the water in the Pacific Ocean for it to be back in Long Beach, California,” said KSC’s Research Integration Office utilization flight lead Mary Walsh. “We then started distributing those samples about four or five hours after that. Now we are going to have early return science in-hand and turn it over to researchers at just four to nine hours after splashdown.”
“This allows us to do different types of science,” said Jennifer Buchli, deputy chief scientist for the ISS Program at JSC. “In the past, if you wanted to watch an organism readapt to gravity, the best case was by the tme you got it back to the lab from splashdown, you were getting data within 18 hours. However, you start to see gravity readaptation responses within organisms within 13 hours. This quicker return of just a few hours opens up a whole new area of science.”