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At some point this year, the MESSENGER mission to Mercury will come to an end, as the spacecraft has exhausted its supply of maneuvering propellant necessary to keep it in orbit. It is currently using pressurized Helium from its fuel tanks as reaction mass in the interim.

What I don't understand is why its orbit is decaying so rapidly. Mercury has no appreciable atmosphere:

Mariner 10's ultraviolet observations have established an upper bound on the exospheric surface density at about 105 particles per cubic centimeter. This corresponds to a surface pressure of less than 10−14 bar (1 nPa).

So if it isn't the atmosphere causing the decay, what is it? Solar pressure? A lumpy gravitational field? (similar to the Moon), or a combination of both?

Since the craft is in a highly elliptical orbit with a periapsis close to the surface between Mercury and the Sun, and its apoapsis is far away from the planet, I can only assume with no atmospheric decay process, its orbit is 'translating' relative to the planet.

Do we additionally have any information on the specifics of this translation, and how fast it is happening?

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Mercury's orbit isn't "decaying" so much as it is changing shape. The primary cause is perturbations from the Sun's gravity field. That MESSENGER is subject to a good deal of solar radiation pressure and that Mercury's gravity field is a bit lumpy add to this.

The primary mechanism is the Kozai effect. This effect applies to an object in a highly eccentric and highly inclined orbit about a secondary that in turn is orbiting a primary; the effect makes the orbit of the object undergo changes in eccentricity and inclination. The perturbative effects of the Sun's gravity are acting to make MESSENGER's orbit a bit less inclined. This makes MESSENGER's orbit more eccentric, and eventually, the periapsis will become equal to Mercury's radius.

CRASH!!.

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