Results: Wind was part of the International Solar Terrestrial Physics (ISTP) program, a joint project between (principally) the United States, Japan, and the European Space Agency (although a few other countries such as Russia, the Czech Republic, and France made significant contributions) to study the behavior of the solar-terrestrial system. Participating spacecraft included Geotail, Wind, Polar, SOHO, and Cluster. The first of two NASA-sponsored Global Geospace Science (CGS) vehicles, the Wind spacecraft carried eight instruments (including the French WAVES and the Russian Konus) to investigate the solar wind's encounters with Earth's magnetosphere and ionosphere in order to determine the origins and three-dimensional characteristics of the solar wind. The spacecraft's original mission was to directly move to the L1 Lagrange point but because the SOHO and ACE spacecraft were directed to that location, the WIND mission was reformulated to operate for some time in a unique figure-eight-shaped elliptical orbit around Earth at 28,000 × 1.6 million kilometers, partially maintained by periodic "double flybys" of the Moon.

In this orbit, Wind was positioned so as to make use of lunar gravity assists to maintain its apogee over the day hemisphere of Earth and conduct magnetospheric observations. The closest of its 19 flybys of the Moon between 1 December 1994 and 17 November 1998 took place on 27 December 1994 at a range of 11,834 kilometers. Finally, by November 1996, Wind was in a "halo orbit" around the Sun–Earth L1 libration point where solar and terrestrial gravity are approximately equal. Parameters varied between 235 and 265 Earth radii. In this orbit, Wind measured the incoming solar wind, and magnetic fields and particles on a continuous basis, providing about an hour warning to the other ISTP-related spacecraft on changes in the solar wind. On 17 November 1998, Wind began to move into a series of "petal" orbits, designed to take it out of the ecliptic plane. Wind's trips above and below the ecliptic (up to 60°) allowed the spacecraft to sample regions of interplanetary space and the magnetosphere that had not been previously studied.

By 2004, it was back at L1 where it has remained. The original projected lifetime of the vehicle was anticipated to be three to five years but WIND continues to be largely operational in 2017, 22 years after its launch, with one instrument, the TGRS gamma-ray spectrometer turned off, and a couple of detectors on the EPACT instrument, non-functional. Current projections suggest that it will have enough fuel to remain at L1 for at least another 60 years. Despite the formal conclusion of the ISTP in December 2001, Wind continues to play a supporting role for a variety of other spacecraft supporting solar research, including Polar, Cluster, Geotail, Image, SOHO, and ACE.

SOHO

Nation: ESA and USA (2) Objective(s): Sun–Earth L1 Lagrange Point Spacecraft: SOHO Spacecraft Mass: 1,864 kg Mission Design and Management: ESA / NASA Launch Vehicle: Atlas Centaur IIAS (AC-121 / Atlas IIAS no. 8206 / Centaur II) Launch Date and Time: 2 December 1995 / 08:08:01 UT Launch Site: Cape Canaveral Air Force Station / Launch Complex 36B

Scientific Instruments:

solar-ultraviolet measurements of emitted radiation experiment (SUMER)
coronal diagnostic spectrometer (CDS)
extreme ultraviolet imaging telescope (EIT)
ultraviolet coronograph spectrometer (UVCS)
large angle and spectrometric coronograph (LASCO)
solar wind anisotropies experiment (SWAN)
charge, element, and isotope analysis experiment (CELIAS)
comprehensive suprathermal and energetic particle analyzer (COSTEP)
energetic and relativistic nuclei and electron experiment (ERNE)
global oscillations at low frequencies experiment (GOLF)
variability of solar irradiance and gravity oscillations experiment (VIRGO)
Michelson Doppler imager/solar oscillations investigation (MDI/SOI)

Results: The ESA-sponsored Solar and Heliospheric Observatory (SOHO) carries 12 scientific instruments to study the solar atmosphere, helioseismology, and the solar wind. Information from the mission has allowed scientists to learn more about the Sun's internal structure and dynamics, the chromosphere, the corona, and solar particles. The SOHO and Cluster missions, part of ESA's Solar Terrestrial Science Programme (STSP), are ESA's contributions to the International Solar Terrestrial Physics (ISTP) program, which has involved the work of other spacecraft such as Wind and ACE, which, like SOHO, operate in the vicinity of the Sun–Earth L1 point. NASA contributed three instruments on SOHO as well as launch and flight operations support. About two months after launch, on 14 February 1996, SOHO was placed at a distance of 1.5 million kilometers from Earth in an elliptical Lissajous orbit around the L1 libration point where it takes approximately six months to orbit L1 (while the L1 itself orbits the Sun every 12 months).

The spacecraft returned its first image on 19 December 1995 and was fully commissioned for operations by 16 April 1996. SOHO finished its planned two-year study of the Sun's atmosphere, surface, and interior in April 1998. Communications with the spacecraft were interrupted for four months beginning 24 June 1998, after which the spacecraft was apparently spinning, losing electrical power, and not pointing at the Sun. After intensive search efforts, by 25 September, controllers managed to regain control and return SOHO to "normal mode." Because of the failure of onboard gyros, ESA developed a special gyroless method of orientation (which used reaction wheels) that was successfully implemented beginning 1 February 1999. Barring three instruments, the spacecraft was functional and was declared operational once again by mid-October 1998.

SOHO's original lifetime was three years (to 1998), but in 1997, ESA and NASA jointly decided to prolong the mission to 2003, thus enabling the spacecraft to compare the Sun's behavior during low dark sunspot activity (1996) to the peak (around 2000). One of SOHO's most important discoveries has been locating the origin of the fast solar wind at the corners of honeycomb-shaped magnetic fields surrounding the edges of large bubbling cells located near the Sun's poles. Another has been its discovery, as of September 2015, of over 3,000 comets (more than one-half of all known comets), by over 70 people representing 18 different nations. These discoveries were made possible because of the LASCO instrument that while observing the Sun, blocked out the Sun's glare, rendering comets visible. SOHO's mission at L1 has now been extended six times, most recently in June 2013, to at least December 2016. In December 2015, SOHO marked 20 years of continuous operation, having fundamentally changed our conception of the Sun "from a picture of a static, unchanging object in the sky to the dynamic beast it is," in the words of Bernhard Fleck, the ESA project scientist for SOHO. The longevity of the mission has allowed SOHO to cover an entire 11-year solar cycle and the beginning of a new one. One of the recent highpoints of the mission was SOHO's observation of a bright comet plunging toward the Sun on 3–4 August 2016 at a velocity of nearly 2.1 million kilometers/hour.

189 Lunar Prospector

Nation: USA (72)

Objective(s): lunar orbit

Spacecraft: Lunar Prospector

Spacecraft Mass: 300 kg

Mission Design and Management: NASA / ARC

Launch Vehicle: Athena-2 (LM-004)

Launch Date and Time: 7 January 1998 / 02:28:44 UT

Launch Site: Cape Canaveral Air Force Station / SLC-46

Scientific Instruments:

1. electron reflectometer and magnetometer (MAG/ER)
1. gamma-ray spectrometer (GRS)
1. neutron spectrometer (NS)
1. alpha particle spectrometer (APS)
1. Doppler gravity experiment (using S-band antenna) (DGE)

Results: Lunar Prospector was designed to collect data to compile the first complete compositional and gravity maps of the Moon during its planned one-year mission in lunar polar orbit. It was the third mission of NASA's Discovery Program of low-cost, highly-focused, and relatively frequent missions that were competitively selected. After two mid-course corrections, Lunar Prospector entered orbit around the Moon 105 hours after launch. Initial parameters were 92 × 153 kilometers. After two further corrections on 13 and 15 January, the spacecraft entered its operational 100 × 100-kilometer orbit at 90° inclination with a period of 118 minutes. Perhaps of most interest to scientists was to continue investigations into the signs of water ice on the Moon as found by the Clementine probe.

In March 1998, NASA announced that data from Lunar Prospector suggested the presence of water ice at both the lunar poles; the neutron spectrometer instrument detected hydrogen, assumed to be in the form of water. The information indicated that a large amount of water ice, possibly as much as 300 million (metric) tons was mixed into the regolith at each pole, the first direct evidence of water ice at the lunar poles. The spacecraft also detected strong localized magnetic fields, mapped the global distribution of major rock types, and discovered signs of a 600-kilometer diameter, iron-rich core. On 10 December 1998, Lunar Prospector's orbit was lowered to 40 kilometers to perform high-resolution studies. A subsequent maneuver on 28 January 1999 changed the orbit to 15 × 45 kilometers and ended the space probe's primary mission but began an extended mission for an additional seven months. Lunar Prospector was deliberately impacted onto the shadowed Shoemaker crater on the lunar surface at 09:52:02 UT on 31 July 1999. Observations of the resulting dust cloud by Earth-based telescopes showed no spectral signature characteristic of water vapor. The vehicle carried part of the cremated remains of geologist Eugene Shoemaker to the lunar surface.

190 NEAR Shoemaker

Nation: USA (68)

Objective(s): asteroid (Eros) orbit and landing

Spacecraft: NEAR

Spacecraft Mass: 805 kg

Mission Design and Management: NASA / GSFC / APL

Launch Vehicle: Delta 7925-8 (no. D232)

Launch Date and Time: 17 February 1996 / 20:43:27 UT

Launch Site: Cape Canaveral Air Force Station / Launch Complex 17B

Scientific Instruments:

1. multi-spectral imager (MSI)
1. magnetometer (MAG)
1. near infrared spectrometer (NIS)
1. x-ray/gamma ray spectrometer (XGRS)
1. laser rangefinder (NLR)
1. radio science and gravimetry experiment

Results: Near Earth Asteroid Rendezvous (NEAR) was the first mission flown under NASA's Discovery program, a series of low-cost (less than c. $150 million in mid-nineties dollar amounts) planetary science projects that were selected competitively and led by a Principal Investigator rather than a NASA manager. NEAR's primary goal was to rendezvous with the minor planet 433 Eros (an S-class asteroid), approximately 355 million kilometers from Earth, and gather data on its physical properties, mineral components, morphology, internal mass distribution, and magnetic field. The spacecraft was the first to rely on solar cells for power during operations beyond Mars orbit. On the way to its primary mission, NEAR performed a 25-minute flyby of the asteroid 253 Mathilde on 27 June 1997. Closest approach to 1,200 kilometers was at 12:56 UT.

During the encounter, the spacecraft photographed 60% of the minor planet from a range of 1,200 kilometers. The collected information indicated that the 4.5 billion-year-old asteroid is covered with craters and less dense than previously believed. After a mid-course correction on 3 July 1997, NEAR flew by Earth on 23 January 1998 at 07:23 UT for a gravity assist on its way to Eros. Closest approach was 540 kilometers. After the Earth flyby encounter, NEAR's previously planned mission profile had to be revised in the light of an aborted engine burn on 20 December 1998 that prevented a critical trajectory correction to meet up with Eros a month later. Instead, NEAR was put on a backup trajectory that afforded a different flyby than that originally planned. As part of this new plan, the spacecraft first flew past Eros on 23 December 1998 at 18:41:23 UT at a range of 3,827 kilometers (distance measured from the center of mass) during which it observed about 60% of the asteroid, and discovered that the minor planet was smaller than expected. NEAR also found that the asteroid has two medium-sized craters, a long surface ridge, and a density similar to Earth's crust. After several more trajectory adjustments, NEAR finally moved into orbit around Eros at 15:33 UT on 14 February 2000, roughly a year later than intended, becoming the first human-made object to orbit a minor planet. Orbital parameters were 321 × 366 kilometers. Through 2000, NEAR's orbit was shifted in stages to permit specific research programs. There were a few problems in the lead up to the landing on the asteroid. For example, on 13 May 2000, controllers had to turn off the Near Infrared Spectrometer due to an excessive power surge. By 30 April the spacecraft was in its operational orbit at an altitude of about 50 kilometers from Eros' center. Later, on 13 July, NEAR entered an even lower orbit at 35 kilometers that brought the vehicle as close as 19 kilometers from the surface. After about 10 days, it moved back into a higher orbit. On 26 October, NEAR performed another close flyby of the surface, this time to just 5.3 kilometers. By the end of the year, the spacecraft had entered a circular 35-kilometer low orbit around the asteroid, and began to make a series of very close passes—on the order of 5 to 6 kilometers—to its surface. Following a slow controlled descent, during which it took 69 high-resolution photos of Eros, NEAR touched down on Eros at a gentle 6.4 kilometers/hour, just south of a saddle-shaped feature named Himeros, on 12 February 2001 at 19:44 UT. This was the first time a U.S. spacecraft was the first to land on a celestial body, having been beaten by the Soviets in landing on the Moon, Mars, and Venus. Remarkably, the orbiter survived contact and returned valuable data, especially from its gamma-ray spectrometer, for about two weeks. Last contact with NEAR was on 28 February 2001, the spacecraft having succumbed to the extreme cold (–173°C). NASA's attempt to contact the probe nearly two years later on 10 December 2002 was unsuccessful. NEAR data showed that Eros had no magnetic field. It mapped more than 70% of the surface using the near infrared spectrometer and provided important data about the asteroid's interior. The spacecraft returned about 10 times more data than originally planned, including 160,000 images. Earlier, on 14 March 2000, a month after entering asteroid orbit, NASA renamed the NEAR spacecraft NEAR Shoemaker in honor of Eugene M. Shoemaker (1928–1997), the renowned geologist.

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