Editing MESSENGER (section)

==Mission background==

===Previous missions===
In 1973, [[Mariner 10]] was launched by NASA to make multiple flyby encounters of Venus and Mercury. Mariner 10 provided the first detailed data of Mercury, mapping 40–45% of the surface.<ref name="USATMessenger">{{cite news|url=https://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm |title=MESSENGER to test theory of shrinking Mercury|newspaper=USA Today| first=Tariq | last= Malik|date=August 16, 2004 |access-date=May 23, 2012}}</ref> Mariner 10's final flyby of Mercury occurred on March 16, 1975. No subsequent close-range observations of the planet would take place for more than 30 years.

===Proposals for the mission===
In 1998, a study detailed a proposed mission to send an orbiting spacecraft to Mercury, as the planet was at that point the least-explored of the inner planets. In the years following the Mariner 10 mission, subsequent mission proposals to revisit Mercury had appeared too costly, requiring large quantities of propellant and a [[heavy lift launch vehicle]]. Moreover, inserting a spacecraft into orbit around Mercury is difficult, because a probe approaching on a direct path from Earth would be accelerated by the [[Sun]]'s gravity and pass Mercury far too quickly to orbit it. However, using a trajectory designed by Chen-wan Yen<ref>{{cite web|title=Finally! NASA Prepares to Orbit Mercury|url=https://science.nasa.gov/science-news/science-at-nasa/2011/15mar_messenger|website=Science NASA|publisher=NASA|access-date=March 26, 2018|archive-date=March 25, 2023|archive-url=https://web.archive.org/web/20230325052032/https://science.nasa.gov/science-news/science-at-nasa/2011/15mar_messenger/|url-status=dead}}</ref> in 1985, the study showed it was possible to execute a [[Discovery Program|Discovery-class]] mission by using multiple, consecutive gravity assist, 'swingby' maneuvers around Venus and Mercury, in combination with minor propulsive trajectory corrections, to gradually slow the spacecraft and thereby minimize propellant needs.<ref name="1998Design">{{Cite journal | first = J. V. | last = McAdams | author2 = J. L. Horsewood | author3 = C. L. Yen | title = Discovery-class Mercury orbiter trajectory design for the 2005 launch opportunity | journal = 1998 Astrodynamics Specialist Conference | publisher = American Institute of Aeronautics and Astronautics/American Astronautical Society | place = Boston, MA | pages = 109–115 | date = August 10–12, 1998 | id = AIAA-98-4283 | url = http://messenger.jhuapl.edu/the_mission/publications/McAdams.et.al.1998.pdf | url-status = dead | archive-url = https://web.archive.org/web/20130513075815/http://messenger.jhuapl.edu/the_mission/publications/McAdams.et.al.1998.pdf | archive-date = May 13, 2013 }}</ref>

===Objectives===
The ''MESSENGER'' mission was designed to study the characteristics and environment of Mercury from orbit. The scientific objectives of the mission were:<ref name='Objectives1'>{{cite web | url = https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2004-030A | title = MESSENGER – Mission description | access-date = July 8, 2013 | publisher = NASA}}</ref><ref>{{cite web | url = http://discovery.nasa.gov/messenger.cfml | title = Discovery Program: MESSENGER | access-date = July 8, 2013 | publisher = NASA | url-status = dead | archive-url = https://web.archive.org/web/20130603184332/http://discovery.nasa.gov/messenger.cfml | archive-date = June 3, 2013 }}</ref>
* to characterize the chemical composition of Mercury's surface.
* to study the planet's geologic history.
* to elucidate the nature of the global magnetic field ([[magnetosphere]]). 
* to determine the size and state of the [[Planetary core|core]].
* to determine the volatile inventory at the poles.
* to study the nature of Mercury's [[exosphere]].