CAPSTONE Achieves Lunar Orbit

CAPSTONE, the first cubesat launched toward the moon and on a nearly five month mission to explore the Near Rectilinear Halo Orbit (NRHO) has reached the intended orbit, NASA reported Monday.   

"We received confirmation that CAPSTONE arrived in near-rectilinear halo orbit, and that is a huge, huge step for the agency," said NASA's chief of exploration systems development, Jim Free, on Sunday evening. "It just completed its first insertion burn a few minutes ago. And over the next few days they'll continue to refine its orbit, and be the first cubesat to fly and operate at the Moon."

This is an important orbit for NASA, and a special one, because it is really stable, requiring just a tiny amount of propellant to hold position. At its closest point to the Moon, this roughly week-long orbit passes within 3,000 km of the lunar surface, and at other points it is 70,000 km away. NASA plans to build a small space station, called the Lunar Gateway, here later this decade.

The Lunar Gateway that they mention is an important part of the Artemis moon missions and going back to the moon "to stay," as the program says.  Lunar Gateway is essentially a miniature version of the Space Station in the NRHO lunar orbit that will host astronauts for a day or two on the way to and from the moon.  That makes CAPSTONE a part of the Artemis program and achieving orbit before tonight's launch is some sort of poetic thing.  It was essentially the first launch of the Artemis program.  

Borrowing some content from my post on the launch (middle story of three here) back on June 28th,

Rocket Lab successfully launched NASA's CAPSTONE mission (Cislunar Autonomous Positioning System Technology Operations and Navigation Experiment) this morning (EDT) (video of the launch here).  This mission uses a low power upper stage that isn't a part of the regularly-launched Rocket Lab Electron that launched the satellite, and which will raise the orbit slowly over the next few months until it achieves the desired orbit around the start of November. This is the first attempt to fly an NRHO, which NASA intends to use for the Artemis Lunar Gateway, arguably making this mission the first of the Artemis program.

The orbit, formally known as a near rectilinear halo orbit (NRHO), is significantly elongated. Its location at a precise balance point in the gravities of Earth and the Moon, offers stability for long-term missions like Gateway and requires minimal energy to maintain. CAPSTONE’s orbit also establishes a location that is an ideal staging area for missions to the Moon and beyond. The orbit will bring CAPSTONE within 1,000 miles of one lunar pole on its near pass and 43,500 miles from the other pole at its peak every seven days, requiring less propulsion capability for spacecraft flying to and from the Moon’s surface than other circular orbits.

This graphic, from NASA's Ames Research Center, shows the CAPSTONE satellite approaching the moon (top left), the spacecraft's elongated polar orbit around the moon (top right) and the crown-shaped path that lunar polar orbit looks like as seen from the Earth (bottom). 

You might recall that CAPSTONE had some scary moments on its journey to the moon, the first a week after launch, and the second in September (second half of this post).  Thankfully, ground teams were able to save the mission both times.  CAPSTONE is small as satellites go, but large for a cubesat.  Ars Technica reports its size is referred to as a 12U cubesat, with a mass of around 50 lbs. It could fit comfortably inside a mini-refrigerator. 

The reason it took nearly five months to reach the moon is the tiny propulsion unit called Photon that lifted it to the far orbit.  

Electron is the smallest rocket to launch a payload to the Moon, and its manufacturer, Rocket Lab, stressed the capabilities of the booster and its Photon upper stage to the maximum to send CAPSTONE on its long journey to the Moon. This was Rocket Lab's first deep space mission.

To get inserted into its desired orbit required a tiny amount of thrust. 

For example, the burn executed by CAPSTONE on Sunday evening to transition into a near-rectilinear halo orbit was extremely tiny. According to Advanced Space, the vehicle burned its thruster for 16 minutes at about 0.44 Newtons, which is equivalent to the weight of about nine pieces of standard printer paper.

It might be more helpful for us Americans to say the engine had a thrust of 1.58 ounces.