Beyond_Earth-_A_Chronicle_of_Deep_Space_Exploration_1958-2016.pdf

213

MESSENGER

Nation: USA (84)

Objective(s): Mercury orbit

Spacecraft: MESSENGER

Spacecraft Mass: 1,107.9 kg

Mission Design and Management: NASA / APL

Launch Vehicle: Delta 7925H (no. D307)

Launch Date and Time: 3 August 2004 / 06:15:57 UT

Launch Site: Cape Canaveral Air Force Station / SLC-17B

Scientific Instruments:

• 1. Mercury dual imaging system (MDIS)
• 2. gamma-ray spectrometer (GRS)
• 3. neutron spectrometer (NS)
• 4. x-ray spectrometer (XRS)
• 5. magnetometer (MAG)
• 6. Mercury laser altimeter (MLA)
• 7. Mercury atmospheric and surface composition spectrometer (MASCS)
• 8. energetic particle and plasma spectrometer (EPPS)
• 9. radio science experiment (RS)

Results: MESSENGER (Mercury Surface, Space Environment, Geochemistry, and Ranging) was the seventh Discovery-class mission, and the first spacecraft to orbit Mercury. Its primary goal was to study the geology, magnetic field, and chemical composition of the planet. It was the first mission to Mercury after Mariner 10, more than 30 years before. MESSENGER was launched into an initial parking orbit around Earth after which its PAM-D solid motor fired to put the spacecraft on an escape trajectory into heliocentric orbit at 0.92 × 1.08 AU and 6.4° inclination to the ecliptic.

<!-- image -->

The six-and-a-half-year road to Mercury was punctuated by several gravity-assist maneuvers through the inner solar system, including one flyby of Earth (on 2 August 2005), two flybys of Venus (on 24 October 2006 and 5 June 2007), and three flybys of Mercury (on 14 January 2008, 6 October 2008, and 29 September 2009). The gravity-assist maneuvers allowed the spacecraft to overcome the problem of massive acceleration that accompanies flight toward the Sun; instead, the flybys helped to decelerate MESSENGER's velocity relative to Mercury and also conserve propellant for its orbital mission (although it prolonged the length of the trip). The Earth flyby allowed mission controllers to properly calibrate all of the spacecraft's instruments while also returning spectacular images of the Earth–Moon system. During the second Venusian flyby (at a range of only 338 kilometers), MESSENGER relayed back a vast amount of data, including visible and near-infrared imaging data on the upper atmosphere. Some of the investigations, especially its study of the particle-and-fields characteristics of the planet, were coordinated with ESA's Venus Express mission. The three Mercury flybys further slowed down the spacecraft, although during the last encounter in September 2009, MESSENGER entered a "safe mode" and, as a result, collected no data on Mercury. Fortunately, the spacecraft revived 7 hours later. MESSENGER finally entered orbit around Mercury at 00:45 UT 18 March 2011, nearly seven years after launch and began formal data collection on 4 April. The vehicle's orbit was highly elliptical, approximately 9,300 × 200 kilometers with a 12-hour orbital period. One of MESSENGER's most remarkable images was its mosaic of the Solar System, obtained on 18 February 2011 with all the planets visible except Uranus and Neptune, a visual counterpart to the image of the solar system taken by Voyager 1 on 14 February 1990. The spacecraft completed its primary year-long mission on 17 March 2012, having taken nearly 100,000 images of the surface of Mercury. Among its initial discoveries was finding high concentrations of magnesium and calcium on Mercury's nightside, identifying a significant northward offset of Mercury's magnetic field from the planet's center, finding large amounts of water in Mercury's exosphere, and revealing evidence of past volcanic activity on the surface. In November 2011, NASA announced that MESSENGER's mission would be extended by a year, thus allowing the spacecraft to monitor the solar maximum in 2012. The extended mission lasted from 18 March 2012 to 17 March 2013. During this phase, by 20 April, with the help of three engine firings, the orbital period was reduced to 8 hours. It was also during this period, in early May 2012, that MESSENGER took its 100,000th photograph from orbit. By this time, the imaging instrument had globally mapped in both high-resolution monochrome and color, the entire surface of the planet. It was during this first extended mission that the spacecraft found evidence of water ice at Mercury's poles, frozen at locations that never see sunlight (made possible by the fact that the tilt of Mercury's rotational axis is almost zero.) A second extension was soon granted that extended the mission to March 2015, and on 6 February 2014, NASA reported that MESSENGER had taken its 200,000th orbital image, far exceeding the original expectation of at least 1,000 photographs. During the second extension, MESSENGER photographed two comets: Comet 2P/Encke and Comet C/2012 S1 (also known as Comet ISON). Beginning the summer of 2014, controllers began moving MESSENGER gradually, burn by burn, to a very low orbit for a new research program. By 12 September 2014, just after the 10th anniversary of its launch, the spacecraft's orbit was down to a mere 25 kilometers. Since then, mission controllers implemented at least two orbital maneuvers (on 12 September and 24 October) to raise its orbit and continue its latest extended mission. By Christmas Day 2014, it was clear that the spacecraft's propellants were running out and that MESSENGER would impact the planet in late March 2015. On 21 January 2015, mission controllers carried out one last maneuver to raise the spacecraft's orbit sufficient to continue more science activities to early in the spring. On 16 April 2015, NASA announced that the spacecraft would impact the surface of Mercury by 30 April after it ran out of propellant. As scheduled, on that day, at 19:26 UT, MESSENGER slammed into the planet's surface at about 14,080 kilometers/hour, creating a new crater on Mercury. Impact coordinates were probably close to 54.4° N / 149.9° W, near the Janácek crater in Suisei Planitia.

---

207

Deep Impact

Nation: USA (85)

Objective(s): comet impact, comet flyby

Spacecraft: DIF + DI Impactor

Spacecraft Mass: 650 kg

Mission Design and Management: NASA / JPL

Launch Vehicle: Delta 7925-9.5 (no. D311)

Launch Date and Time: 12 January 2005 / 18:47:08 UT

Launch Site: Cape Canaveral Air Force Station / SLC-17B

Scientific Instruments:

Flyby Spacecraft:

• 1. high resolution instrument (HRI)
• 2. medium resolution instrument (MRI)

Impactor:

• 1. impact or targeting sensor (ITS)

Results: Unlike previous cometary flyby missions, such as Vega, Giotto, and Stardust, the Deep Impact spacecraft, the eighth mission in NASA's Discovery program, was intended to study the interior composition of a comet by deploying an impact probe that would collide with its target. The spacecraft comprised two distinct parts, a flyby bus and an impactor. The former, weighing 601 kilograms, was solar powered and carried two primary instruments. The HRI, the main science camera for Deep Impact, was one of the largest space-based instruments ever built for planetary science. It combined a visible-light multi-spectral CCD camera (with a filter wheel) and an imaging infrared spectrometer called the Spectral Imaging Module (SIM). The MRI was the functional backup for the HRI, and like the HRI, it also served as a navigation aid for Deep Impact. The 372-kilogram Impactor carried

<!-- image -->

the ITS, nearly identical to the MRI, but without the filter wheel, which was designed to measure the Impactor's trajectory and to image the comet from close range before impact. One of the more unusual payloads on board was a mini-CD with the names of 625,000 people collected as part of a campaign to "Send Your Name to a Comet!" After launch, Deep Impact was put into low Earth orbit, then an elliptical orbit (163 × 4,170 kilometers), and after a third stage burn, the spacecraft and its PAM-D upper stage departed on an Earth escape trajectory. There were some initial moments of anxiety when it was discovered that the spacecraft had automatically entered "safe mode" shortly after entering heliocentric orbit, but by 13 January, Deep Impact returned to full operational mode following a program to "tumble" the vehicle using its thrusters. The spacecraft traveled 429 million kilometers for nearly six months (including course corrections on 11 February and 4 May 2005) on an encounter with Comet 9P/Tempel. As the spacecraft approached its target, it spotted two outbursts of activity from the comet, on 14 June and 22 June 2005. At 06:00 UT (or 06:07 UT Earth-receive time) on 3 July 2005, Deep Impact released the Impactor probe, which, using small thrusters, moved into the path of the comet, where it hit the following day, 4 July at 05:44:58 UT at a relative velocity of 37,000 kilometers/hour. The impact generated an explosion the equivalent of 4.7 tons of TNT and a crater estimated to be about 150 meters in diameter. Minutes after the impact, the Flyby probe passed the nucleus at a range of about 500 kilometers at 05:59 UT on 3 July and took images of the resultant crater (although it was obscured by the dust cloud), ejecta plume, and the entire nucleus. Simultaneous observations of the impact were coordinated with ground-based observatories as well as space-based ones, such as the European Rosetta (which was about 80 million kilometers from the comet), Hubble, Spitzer, the Swift x-ray telescope, and XMM-Newton. The Impactor itself took images as little as 3 seconds before impact, which were transmitted via the flyby vehicle back to Earth. Controllers registered about 4,500 images from the three cameras over the next few days. Based on the results of Deep Impact's investigations, scientists concluded that Comet Tempel 1 had probably originated in the Oort Cloud. The data also showed that the comet was about 75% empty space. Although Deep Impact's primary mission was over, because the vehicle still had plenty of propellant left, on 3 July 2007, NASA approved a new supplemental mission for Deep Impact, known as EPOXI, derived from the combination of the two components of this extended flight: Extrasolar Planet Observations (EPOCh) and Deep Impact Extended Investigation (DIXI). This so-called "Mission of Opportunity" was originally focused on Comet 85P/Boethin; on 21 July 2005, Deep Impact was set on a trajectory to conduct a flyby of Earth in anticipation of the intercept of Boethin. Unfortunately, scientists lost track of Comet Boethin (possibly because the comet had broken up) and Deep Impact was instead directed towards Comet 103P/Hartley (or Hartley 2) beginning with a burn on 1 November 2007. EPOXI's new plan set Deep Impact on three consecutive Earth flybys, spread over two years (in December 2007, December 2008, and June 2010) before the final trek to meet Comet Hartley 2. These flybys essentially "stole some energy" from the spacecraft, thus dropping Deep Impact into a smaller orbit around the Sun. Before the second Earth flyby, Deep Impact performed its EPOCh mission using the HRI instrument to perform photometric investigations of extrasolar planets around eight distant stars, returning nearly 200,000 images. In the fall of 2010, Deep Impact began its investigations of Comet Hartley 2, conducting its flyby of the target at a range of 694 kilometers at 15:00 UT on 4 November 2010. As with the encounter with Comet Tempel 1, Deep Impact used its three instruments to study Hartley 2 for three weeks. Some of the images were so clear that scientists were able to identify jets of dust with particular features on the comet's nucleus. The data showed that the two lobes of Hartley 2 were different in composition. Once past this second cometary encounter, Deep Impact had little propellant for further cometary investigations, but there was a possibility that the spacecraft, if still in working condition, could be used for a flyby of Near Earth Asteroid 2002 GT in 2020. With that goal in mind, thrusters were fired in December 2011 and October 2012 for targeting purposes. In the meantime, the spacecraft was used for remote study of faraway comets such as C/200P1 (Garradd) in early 2012 and C/2012 S1 (ISON) in early 2013. Communications with Deep Impact were lost sometime between 11 August and 14 August 2013, and after "considerable effort" to contact the spacecraft, NASA announced on 20 September that it had officially abandoned efforts to contact Deep Impact.