After a long and launchless six weeks, United Launch Alliance (ULA) has moved a step nearer to sending Europe’s Solar Orbiter on its multi-year voyage to study the Sun at closer range, in greater detail and from higher heliographic latitudes than ever before. The giant Atlas V booster assigned to the mission has wrapped up a smooth fueled (or “wet”) dress rehearsal (WDR) at Space Launch Complex (SLC)-41 at Cape Canaveral Air Force Station, Fla.
“NASA requires a WDR for missions with limited launch opportunities such as those to planets and the Sun,” says NASA. “WDRs allow us to test the rocket early in an effort to mitigate issues that could result in a missed opportunity for launch.”
The Wet Dress Rehearsal (WDR) was conducted primarily on account of the criticality of Solar Orbiter’s launch window. The spacecraft—which has been on the drawing boards at the European Space Agency (ESA) for more than two decades—will fly within 0.3 Astronomical Units (AU) of the Sun, a mere 26 million miles (42 million km), deep inside the orbit of the planet Mercury. Moreover, it will gradually adjust its trajectory to achieve heliographic latitudes as high as 34 degrees, allowing its fields and particles sensors and imaging instruments to directly observe the poles of the Sun as never before and examine fast-moving solar wind ions and coronal plasmas almost from their point of origin.
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But to get so blisteringly close to the Sun, Solar Orbiter will undertake a hazardous journey that is expected to involve multiple flybys of Venus and one of Earth.
Following launch, the spacecraft will hurtle past Venus in December 2020 and again in August 2021, before returning to Earth and passing within 270 miles (430 km) of our home planet in November 2021. These three initial Gravity Assist Maneuvers (GAMs) are part of an intricate celestial “dance” that Solar Orbiter will take to reduce its orbital energy and increase its heliographic inclination to bring it deeper towards the Sun and into ever-higher latitudes. Departing Earth at the end of 2021, the “cruise” phase of its mission will end and the “science phase” will begin. Multiple Venus GAMs will be performed throughout the coming decade to crank Solar Orbiter’s latitude up as high as 34 degrees. And it is precisely because of the spacecraft’s date with Venus that a tight, three-week “launch window” from 5-23 February is needed.
“WDRs are milestones meant to mitigate any issues before the actual launch day arrives,” ULA noted. “Typically, they are performed during the launch campaign for all planetary missions that have very short timeframes to launch. Missing that launch period would mean lengthy delays to wait for the next opportunity.”
Processing of the myriad components for the mission entered high gear in Florida last November, when Solar Orbiter itself arrived aboard an Antonov An-124 aircraft from Germany’s IABG engineering and test center, near Munich, and the components of the Atlas V rocket—its Common Core Booster (CCB) and single-engine Centaur upper stage—were delivered from ULA’s facility in Decatur, Ala. Early in January, the critical Launch Vehicle On Stand (LVOS) milestone was passed when the 107-foot-long (32-meter) CCB was erected on the Mobile Launch Platform (MLP) in the Vertical Integration Facility (VIF) at SLC-41. Shortly thereafter, a single solid-fueled booster was affixed to the CCB’s base and the 41-foot-long (12.6-meter) Centaur was hoisted atop the stack.
Bearing a strangely “incomplete” look—incomplete, of course, because there was no sign yet of Solar Orbiter and its payload shroud—the Atlas V was rolled from the VIF to the pad surface on Tuesday, 21 January, for the WDR. The CCB was loaded with its flight supply of 25,000 gallons (113,650 liters) of highly refined rocket-grade kerosene (RP-1), with the intention that this fuel would remain aboard through launch. Early Wednesday, preparations for the WDR got underway when the rocket stages were powered up, their guidance systems underwent tests and final preparations of ground systems. Supervised by ULA Launch Conductor Scott Barney and ULA Launch Director Lou Mangieri, all seemed ready for cryogenic tanking of the Centaur with liquid oxygen and hydrogen.
However, Wednesday was not to be ULA’s day. Shortly before cryogenic tanking was due to start, a cold air duct belonging to the environmental control system somehow became detached from the Centaur. Out of a sense of prudency, the Atlas V was rolled back to the VIF and the air duct replaced. Following a swift turnaround, the rocket was returned to the pad on Thursday for another attempt at the WDR on Friday. This time, and without incident, 66,000 gallons (300,000 liters) of liquid oxygen and hydrogen were pumped into the Centaur’s tanks, just as they will be on the real launch day. The mock “countdown” proceeded through each of its scheduled steps, including the final planned hold at T-4 minutes, before the test ended shortly prior to T-0.
Following the WDR, the Atlas V was safed and its cryogenic tanks drained to wrap up the test. Yesterday (Saturday), the booster was returned to the VIF for the integration of its primary payload. Solar Orbiter was encapsulated in its two-piece payload fairing on 20 January and is expected to be hoisted atop the rocket early next week. With the payload in place, the Atlas V—which is flying for only the sixth time in its “411” configuration, with a 13-foot-diameter (4-meter) fairing, a single strap-on booster and a single-engine Centaur—will stand some 189 feet (57.6 meters) tall.
Ironically, the delay caused by the cold air duct replacement and the corresponding slippage of the WDR has directly impacted Solar Orbiter’s launch window. Rather than aiming for the opening day, 5 February, ULA is presently targeting the third day of the window, at 11:15 p.m. EST on Friday 7th.
The spacecraft meanwhile was recently encapsulated for launch inside an aerodynamic bullet-like cover on Jan 20, which will protects it during ground operations, launch and ascent. Solar Orbiter’s instruments are sensitive to contamination, and the fairing has special provisions to make sure those instruments are not affected by particles or humidity.