Why does the Orion capsule of Artemis I slow down as it approaches the Moon?

Orion approaches the Moon. Or is it not rather the Moon approaching the capsule? Artemis I’s ship does slow down as it approaches the Moon, but that’s not abnormal.

Orion has never been so close to the Moon. The capsule of the Artemis program is preparing to fly over our natural satellite this Monday, November 21, 2022. It is at 1:57 p.m. very precise that the unmanned spacecraft must be closest to the Moon during this flyby, only 128 km from altitude. Orion’s arrival around the Moon can be followed online and live.

Looking at the images, one may be surprised to see that the capsule is advancing very slowly towards its objective. One could even say that it is more the Moon that seems to be “advancing” towards the capsule, rather than the reverse. It is enough to compare the speed of their displacements in space. According to instructions from NASA, Orion is moving at a speed of 1,140 km/h (as indicated around 12:15 p.m.). The Moon moves at the speed of 3,683 km/h in its orbit around the Earth.

Orion approaching the Moon, artist's impression.  // Source: Flickr/CC/NASA/Liam Yanulis
Orion approaching the Moon, artist’s impression. // Source : Flickr/CC/NASA/Liam Yanulis

We realize that Orion slows down as it approaches the Moon, which is moving much faster than the capsule, by looking at this animation from NASA, broadcast on November 19 on Twitter. Orion’s trajectory is shown in red, relative to Earth (in blue) and the lunar orbit (in white).

Earth’s gravity attracts Orion, even as the capsule approaches the Moon

The Artemis I spacecraft slows down as it heads towards the Moon. Indeed, the Earth’s gravity attracts it: the European service module will trigger its thrusters on Monday to reach lunar orbit! sums up the ESA (European Space Agency) in this publication.

On its way to the Moon, Orion follows an elliptical (oval) orbit around our planet. ” As the spacecraft approaches the apogee of orbit (the furthest point from Earth), it will slow down, as Earth’s gravity pulls it back towards itself. It’s a bit like throwing a stone upwards: the closer it gets to the highest point, the more it slows down, and the more it falls back to Earth. », details the ESA on his site. The use of thrusters from the European module will make it possible to change Orion’s speed and perform the gravitational assistance maneuver. The capsule will thus be able to fit (without slowing down) into an orbit around the Moon, thanks to its gravity.

Once in this orbit, Orion will pass very close to the lunar surface, at 100 km, or much further, at 70,000 km. You can follow the route of the Artemis I mission in real time, to see where the capsule is.

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