NASA's CAPSTONE is a small cubesat with big goals, and now it has gone where no cubesat has gone before, becoming the first small satellite ever to orbit the Moon. After months of a slow, precise approach – and a few days of nailbiting lost contact – reaching this orbit is key as it is testing out something very important for the future of lunar (and even Martian) exploration.
CAPSTONE, which weighs just 25 kilograms (55 pounds), is a pathfinder mission for the eccentric halo-shaped orbit planned for NASA's Lunar Gateway, a space station set to orbit the Moon and act as an outpost to support the Artemis missions – and perhaps, one day, be a stepping stone to Mars.
“The CAPSTONE mission operations team confirmed that NASA’s CAPSTONE spacecraft arrived at its orbit at the Moon Sunday evening. The CubeSat completed an initial orbit insertion maneuver, firing its thrusters to put the spacecraft into orbit, at 7:39 p.m. EST,” NASA said.
CAPSTONE has been placed in a near-rectilinear halo orbit, or NRHO. The NRHO is a special solution to the three-body problem that should theoretically guarantee a stable orbit with minimal adjustment, and it’s a big deal.
It is easy to predict the motion of two bodies moving under each other's gravity, no matter the initial conditions. Newton’s laws of gravity are, in most cases, enough and you don’t have to bring in Einstein’s theory of general relativity. But add a third object to the system and chaos ensues, severely reducing the ability to predict where the objects will be in relation to each other.
Even when the two other bodies are as big as the Earth and the Moon, a space station’s mass cannot be considered inexistent. With the exclusion of a few special cases, the motion of three bodies doesn’t repeat. This particular NRHO is also special because it is the orbit chosen for the Lunar Gateway. CAPSTONE is going to conduct a couple of clean-up maneuvers before further measurement to confirm that it is staying in the NRHO.
CAPSTONE, and eventually the Lunar Gateway, will be in a seven-day elongated path around the Moon that is almost face-on when seen from Earth. The NHRO will take an object placed there to as near as 3,000 kilometers (1,865 miles) from the surface of the Moon and then as far as 70,000 kilometers (43,500 miles).
Drawbacks from this orbit are the need to occasionally spend fuel and carry out corrective maneuvers, but it does allow the least number of eclipses on board from either the Earth’s or the Moon’s shadow as well as inexpensive (in terms of energy requirements) transfers from both the Earth and Moon’s surfaces.
CAPSTONE will spend six months orbiting the Moon providing valuable insights into this unusual orbital configuration.