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I have heard a bit about why interplanetary probes get deorbited, but that was about Jupiter and Saturn that have potentially life-friendly moons. But regarding Mercury, no life could exist there: the atmosphere, temperature, etc. would not support it.

Why was MESSENGER deorbited?

Russell Borogove
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Wheatley Aquario
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    A quick glance at Wikipedia and a Times article indicate that it simply ran out of fuel, and it was orbiting close enough that the very tenuous atmosphere of Mercury (and I hypothesize the effects of the solar wind played a role) simply meant that there was no way to keep it from spiralling down to the surface. -- Not deorbited to dispose of the craft; just deorbited because there was no way to keep it in orbit. – Ghedipunk Aug 22 '18 at 22:42
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    @Ghedipunk I think you might convert it to an answer, too. – peterh Aug 22 '18 at 23:06
  • @Ghedipunk Thank you! You learn new things every day :) – Wheatley Aquario Aug 22 '18 at 23:11
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    Related: Through what process does MESSENGER undergo orbital decay? – Russell Borogove Aug 22 '18 at 23:42
  • @WheatleyAquario have a look at the answer to the question linked in the previous comment. Do you think that answers your question? If not, it might be good to edit your question and clarify what's different here. If it does, then your question can be marked as a duplicate (which is a good, not a bad thing) and then your question will help direct future readers to that answer. – uhoh Aug 23 '18 at 01:47
  • @WheatleyAquario you could then ask a new question, for example "I have read here that through the Kozai effect, the Sun's gravity slowly distorted MESSENGER's orbit around Mercury until it crashed into the planet's surface. Could someone help me better understand how that works and how to calculate how long it took?" or something similar, with – uhoh Aug 23 '18 at 01:51
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    @uhoh I am not gonna change the question, it can be marked. Thanks for the support, I am just new over here.. – Wheatley Aquario Aug 23 '18 at 05:07
  • Everyone is being so nice now that nobody want's to be the first one to click "duplicate" ;-) – uhoh Aug 23 '18 at 11:00
  • I don't think it's a duplicate. This question asks why, which means it asks for a human motivation. The linked question asks how, which means the physical causes leading to deorbiting. – gerrit Aug 23 '18 at 11:11
  • @gerrit how's that? – uhoh Aug 23 '18 at 14:29
  • I also don't think this is a duplicate. (I would have voted to close rather than providing an answer if I did think this was the case.) As @gerrit noted, this question is about intent: Why was an orbit with the inevitable outcome as described in the related question chosen? – David Hammen Aug 23 '18 at 14:52

2 Answers2

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Why was MESSENGER deorbited?

It wasn't. (The wikipedia page on MESSENGER is wrong in this regard.)

MESSENGER was inserted into an orbit about Mercury with a high inclination, a high eccentricity, and a rather low periapsis. The Kozai mechanism would have naturally resulted in this orbit evolving toward an even greater eccentricity, with the inevitable outcome of the vehicle eventually colliding with the planet. The spacecraft performed a number of orbital maneuvers to delay this inevitability. The outcome was still inevitable given the choice of orbits; the vehicle had a limited amount of fuel for its thrusters.

The question thus becomes "Why was MESSENGER placed in such an orbit to begin with?" The answer to this question involves the science the mission was intended to perform and the thermal characteristics of the vehicle and of Mercury.

A polar orbit was chosen because this let the vehicle observe, close up, Mercury's north pole. This was also why the periapsis was made so low. The eccentricity had to be made rather high because thermal radiation from Mercury's daytime side (up to 427 degrees Celsius) would have prematurely cooked the vehicle had the orbit been anywhere close to circular. The periapsis passes had to be very fleeting. The science returns would not have been nearly as good as they were had the orbit been less inclined or higher up.

David Hammen
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While the question could be marked as duplicate of Through what process does MESSENGER undergo orbital decay?, there's some hesitation to do that so I'll post an answer and also link to @DavidHammen's answer.

edit: and now also link to @DavidHammen's other answer!

MESSENGER was not specifically sent crashing into Mercury by direction of NASA, in fact they delayed this by boosting it to a higher orbit. But NASA knew that by putting it into a low orbit around Mercury in the first place (low in order to observe Mercury, the whole point of the mission), MESSENGER was destined to crash.

This is because the spacecraft was orbiting around a very low mass body, very close to a very large mass body (the Sun), and the Sun's tugging on MESSENGER's orbit increased its eccentricity until it intersected the planet's surface.

From the April 2015 SpaceRef article MESSENGER's Operations at Mercury Extended:

MESSENGER mission controllers conducted a maneuver yesterday to raise the spacecraft's minimum altitude sufficiently to extend orbital operations and further delay the probe's inevitable impact onto Mercury's surface.

The previous maneuver, completed on March 18, raised MESSENGER to an altitude at closest approach from 11.6 kilometers (7.2 miles) to 34.4 kilometers (21.4 miles) above the planet's surface. Because of progressive changes to the orbit over time in response to the gravitational pull of the Sun, the spacecraft's minimum altitude (periapsis) continued to decrease.

At the time of yesterday's maneuver, MESSENGER was in an orbit with a closest approach of 5.5 kilometers (3.4 miles) above the surface of Mercury. With a velocity change of 2.96 meters per second (6.63 miles per hour), four of the spacecraft's 12 smallest monopropellant thrusters nudged the spacecraft to an orbit with a closest approach altitude of 27.5 kilometers (17.1 miles). This maneuver also increased the spacecraft's speed relative to Mercury at the maximum distance from Mercury, adding about 1.2 minutes to the spacecraft's eight-hour, 17.6-minute orbit period.

Yes, that's that's only 5.5 kilometers above the surface of Mercury!!

So even a very weak tug by the Sun via the Kozai mechanism will result in a quick demise, and the three mission extensions only served to delay the inevitable.

Notice the nice timing by the way, Messenger crashed soon after the end of the 2nd mission extension.

Dates from Wikipedia (mostly)

Flyby of Earth (gravity assist)  02-Aug-2005  2,347 km
Flyby of Venus (gravity assist)  24-Oct-2006  2,990 km  
Flyby of Venus (gravity assist)  05-Jun-2007    337 km
Flyby of Mercury                 14-jan-2008    200 km  
Flyby of Mercury                 06-Oct-2008    200 km
Flyby of Mercury                 29-Sep-2009    228 km

Mercury Orbital insertion 18-Mar-2011, 1 year mission


mission extension 1 year         17-Mar-2012


mission extension 2 years        17-Mar-2013


Deorbited/Destroyed              30-Apri-2015

below: from here

enter image description here

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uhoh
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    The spaceref.com article to which you linked describes the last two orbital correction maneuvers made with the use of hydrazine (OCM-14 and OCM-15). The spacecraft made four more correction maneuvers after that: OCM-15a, OCM-16, OCM-17, and OCM-18. So what did the spacecraft use if it had no fuel left? The answer is that the spacecraft still had some helium left, nominally used only as a pressurant for the hydrazine. Using the helium as a cold gas propellant extended the vehicle's life for another three weeks after that article was written. – David Hammen Aug 23 '18 at 15:07
  • @DavidHammen that's incredible, I love it! – uhoh Aug 23 '18 at 15:09