Bobby Williams

The MESSENGER mission to Mercury: spacecraft and mission design

Abstract A Mercury orbiter mission is challenging from thermal and mass perspectives. The Mercury Surface, Space Environment, Geochemistry, and Ranging (MESSENGER) mission overcomes these challenges while avoiding esoteric technologies by using an innovative approach with commonly available materials, minimal moving parts, and maximum heritage. This approach yields a spacecraft with good margins in all categories and low technical risk. The key concepts are a ceramic-cloth sunshade, an integrated lightweight structure and high- performance propulsion system, and a solar array incorporating optical solar reflectors (OSRs). The sunshade maintains the spacecraft at room temperature. The integrated structure and propulsion system provides ample mass margin. The solar array with OSRs, which has already undergone significant testing, provides thermal margin even if the panels are inadvertently pointed directly at the Sun at 0.3 AU. 0.3 AU .

Venus gravity field: Pioneer Venus Orbiter navigation results

Abstract The gravity field of Venus has been modeled by a spherical harmonic expansion of the potential to degree and order seven. The estimates of these coefficients were obtained by combining information from 43 short arcs (4 hr) of line-of-sight Doppler data centered at periapsis. The data arcs were distributed in longitude and time over more than two circulations of Venus by the Pioneer Venus Orbiter subperiapsis point which was confined to the band of latitudes from 14°N to 17°N. Convergence of the solution has been assured by iterating upon the initial estimate. All estimates were performed with zero a priori information on the gravity coefficients. Since the altitude of periapsis for most of the orbits was within the sensible Venusian atmosphere, drag effects on the estimated harmonics have been removed using an exponential atmosphere density model. Estimates of the mass parameter (GM) of Venus using this dataset are also evaluated.

Trajectory Design and Maneuver Strategy for the MESSENGER Mission to Mercury

Destined to become the first spacecraft to orbit the planet Mercury, the MESSENGER spacecraft was launched on 3 August 2004. The 6.6-year ballistic trajectory to Mercury will utilize six gravity-assist flybys of Earth (one), Venus (two), and Mercury (three). With three trajectory correction maneuvers completed by mid-December 2005, many more maneuvers will be necessary during the journey to Mercury and the subsequent 1-year duration Mercury orbit phase. The spacecrafts design and operational capability will enable real-time monitoring of every course-correction maneuver. A complex mission plan will provide multiple opportunities to obtain observational data that will help fulfill the missions scientific objectives. Soon after entering Mercury orbit in mid-March 2011, the initial primary science orbit will have an 80-deg orbit inclination relative to Mercurys equator, 200-km periapsis altitude, 60°N subspacecraft periapsis latitude, and a 12-h orbit period. With science goals requiring infrequent orbit-phase trajectory adjustments, pairs of orbit-correction maneuvers occur at about the same time every Mercury year, or every 88 days. For the first time, the spacecrafts orbit design at Mercury accounts for the best available Mercury gravity model, small solar pressure perturbations due to changes in the solar array tilt angle, and an improved strategy for performing orbit correction maneuvers.

Recovery of near's mission to eros

Abstract On December 20, 1998, the main bipropellant thruster on the Near Earth Asteroid Rendezvous (NEAR) spacecraft was commanded to start a 15-minute burn. It was the first and largest of four planned maneuvers that would cancel NEARs velocity relative to (433) Eros to allow capture into orbit about the asteroid in January 1999. But the burn aborted and the spacecraft tumbled, causing loss of communication. The spacecraft corrected the problem, but used 29 kg of fuel in the process. A day later, NEAR was reacquired with just enough time to upload a sequence of commands to image Eros extensively as the spacecraft hurtled past the asteroid on December 23. The NEAR team designed a large bipropellant maneuver that successfully cancelled most of NEARs velocity relative to Eros on January 3, 1999. But then NEAR was almost 1 million kilometers from Eros, so a long “U-turn” journey was begun to return to the asteroid on February 14, 2000. The spacecraft remains healthy and enough fuel remains to enter orbit about Eros to complete all of the mission goals. This will be the first time that an interplanetary spacecraft has failed an orbital capture burn and returned to accomplish its objectives. This is possible because NEAR had a generous fuel supply and a robust contingency plan.

Navigation for low-cost missions to small solar-system bodies

Abstract A variety of low-cost space missions planned by NASA for flight in the late 1990s and early 2000s will involve rendezvous with, and orbits about, small solar-system bodies such as asteroids and comets. Rendezvous missions of this nature have never been performed, all previous small-body encounters having been flybys. Thus in navigating these missions there are a number of issues and challenges which are new. This paper will identify the different mission phases for small body encounters and the navigation requirements, objectives and goals involved with each phase. In addition, certain practical limitations with respect to mission design will be identified and the scientific information obtained by navigation during the mission discussed. The phases and issues addressed in the paper are: preencounter characterization, encounter and rendezvous with the body, post-encounter characterization, initial orbit strategy and the mission phase itself. Each of these phases have integral importance and are critical to the success of the entire mission. Inherent in each of these phases are all the traditional navigation concerns, such as a priori knowledge, maneuver design and execution, data acquisition, orbit determination, orbit reconstruction and control. The paper explains how these traditional roles will be implemented for future small-body missions, including the use of autonomous navigation where practical. Navigation of spacecraft to and about small solar-system bodies is challenging and raises many issues of fundamental importance which should be understood by the mission navigators, designers and sponsors. This paper will identify the most important issues and discuss ways in which they may be dealt with. It also provides a methodology with which to approach navigation for small-body missions.

Tuning the MESSENGER State Estimation Filter for Controlled Descent to Mercury Impact

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission is the seventh in NASA’s Discovery Program. The spacecraft has been orbiting Mercury since March 2011 and after propellant reserves are depleted will impact the planetary surface at the end of March 2015. In preparation for the controlled descent, the MESSENGER navigation operations team has begun the process of updating the filter inputs for the state estimation parameters through a statistical analysis of the orbit determination solutions accumulated to date.

Accommodating Navigation Uncertainties in the Pluto Encounter Sequence Design

The New Horizons encounter with the Pluto system was a historic achievement in planetary exploration. Launched on January 19, 2006, the spacecraft executed its close encounter with Pluto on July 14, 2015, acquiring the first-ever close up data of Pluto, its five known satellites, and the surrounding plasma and particle environment. During its 9½ year cruise, the spacecraft also conducted a flyby of an asteroid in 2006 and a Jupiter gravity assist in 2007 during which over 700 observations of Jupiter, the Galilean satellites, and the plasma and particle environment near Jupiter were acquired. Led by Principal Investigator Alan Stern, New Horizons was the first launch of NASA’s New Frontiers Program and the first mission to Pluto and the Kuiper Belt.