Beyond Earth - A Chronicle of Deep Space Exploration 1958-2016

194

ACE

Nation: USA (71)

Objective(s): Sun–Earth L1 Lagrange Point

Spacecraft: ACE

Spacecraft Mass: 752 kg

Mission Design and Management: NASA / GSFC

Launch Vehicle: Delta 7920-8 (no. D247)

Launch Date and Time: 25 August 1997 / 14:39 UT

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

Scientific Instruments:

• 1. solar wind ion mass spectrometer (SWIMS) and solar wind ion composition spectrometer (SWICS)
• 2. ultra-low energy isotope spectrometer (ULEIS)
• 3. solar energetic particle ionic charge analyzer (SEPICA)
• 4. solar isotope spectrometer (SIS)
• 5. cosmic ray isotope spectrometer (CRIS)
• 6. solar wind electron, proton, and alpha monitor (SWEPAM)
• 7. electron, proton, and alpha-particle monitor (EPAM)
• 8. magnetometer (MAG)
• 9. real time solar wind experiment (RTSW)

Results: The Advanced Composition Explorer (ACE) spacecraft was designed to study spaceborne energetic particles from the Sun–Earth L1 libration point, about 1.4 million kilometers from Earth. Specifically, the spacecraft was launched to investigate the matter ejected from the Sun to establish the commonality and interaction between the Sun, Earth, and the Milky Way galaxy. In addition, ACE also provides real-time space weather data and advanced warning of geomagnetic storms. ACE's nine instruments have a collecting power that is 10 to 10,000 times greater than anything previously flown. After launch, the spacecraft's Delta 2 launch vehicle's second stage reignited (after 4 hours) to insert the satellite into a 177 × 1.37 million-kilometer orbit. After reaching apogee a month after launch, ACE inserted itself into its Lissajous orbit around the L1 point. The spacecraft was declared operational on 21 January 1998 with a projected two- to five-year lifetime. As of early 2015, it continues to provide near-real-time 24/7 coverage of solar wind parameters and measure solar energetic particle intensities. With the exception of the SEPICA instrument (data from which was no longer received after 4 February 2005), all instruments on ACE remain operational as of mid-2017, and the propellant on board could theoretically allow a mission until about 2024.

195

Cassini-Huygens

Nation: US and ESA (3)

Objective(s): Saturn orbit, Titan landing

Spacecraft: Cassini and Huygens

Spacecraft Mass: 5,655 kg

Mission Design and Management: NASA / JPL / ESA

Launch Vehicle: Titan 401B-Centaur (TC-21 / Titan 401 no. 4B-33)

Launch Date and Time: 15 October 1997 / 08:43 UT

Launch Site: Cape Canaveral Air Force Station / Launch Complex 40

Scientific Instruments:

Cassini:

• 1. cosmic dust analyzer (CDA)
• 2. visible and infrared mapping spectrometer (VIMS)
• 3. imaging science system (ISS)
• 4. radar
• 5. ion and neutral mass spectrometer (INMS)
• 6. radio and plasma wave spectrometer (RPWS)
• 7. plasma spectrometer (CAPS)
• 8. ultraviolet imaging spectrograph (UVIS)
• 9. magnetospheric imaging instrument (MIMI)
• 10. dual technique magnetometer (MAG)
• 11. composite infrared spectrometer (CIRS)
• 12. radio science subsystem (RSS)

Huygens:

• 1. atmospheric structure instrument (HASI)
• 2. gas chromatograph neutral mass spectrometer (GC/MS)
• 3. aerosol collector and pyrolyzer (ACP)
• 4. descent imager/spectral radiometer (DISR)
• 5. surface science package (SSP)
• 6. Doppler wind experiment (DWE)

Results: The Cassini-Huygens project was the result of plans at NASA dating from the early 1980s and formally approved in 1989 as a joint NASA-ESA mission. Having survived several attempts by Congress to cancel the mission, the mission that emerged was a cooperative project with ESA (as well as the Italian Space Agency, ASI) involving a NASA-supplied spacecraft, Cassini, that orbits Saturn, and an ESA-supplied lander, Huygens, which descended into the atmosphere of Titan, Saturn's largest moon, in 2005. ASI provided the high-gain and low-gain antenna assembly and a major portion of the radio system. The primary scientific goals of the mission included a diverse set of investigations of Saturn, its moons, and its near environment. The 3,132-kilogram orbiter with an original design life of 11 years was powered by three radioisotope thermoelectric generators (RTGs), of the same design as the RTGs carried aboard Ulysses, Galileo, and New Horizons.

The 335-kilogram Huygens, named after the Dutch physicist Christiaan Huygens (1629–1695), was designed to investigate Titan's atmosphere's chemical properties and measure wind, temperature, and pressure profiles from 170 kilometers down to the Moon's surface. The probe was not designed to survive past landing although scientists did not rule out the possibility entirely. Cassini's trip to Saturn included four gravity assists. Seven months after launch, the spacecraft passed Venus on April 26, 1998 at a range of 284 kilometers, gaining 26,280 kilometers/hour. Cassini performed a second flyby of Venus on June 24, 1999 at a range of 623 kilometers and one of Earth at 03:28 UT on 18 August 1999 at a range of 1,171 kilometers, before heading to Jupiter. During this portion of the traverse, Cassini passed by the asteroid 2685 Masursky on 23 January 2000, flying as close as 1.5 million kilometers at 09:58 UT.

During the encounter, Cassini used its remote sensing instruments to investigate the asteroid's size and dimensions and albedo. Nearly a year later, on 30 December 2000, the spacecraft passed by Jupiter at a distance of about 9.7 million kilometers. Among the data it returned was a detailed global color image of Jupiter, probably the most complete of the whole planet ever produced. In 2001–2002, controllers noticed a "haze" in images returned by the narrow-angle camera but these were eliminated following phases of heating the spacecraft. Its investigations en route to Saturn included an experiment in October 2003 in which scientists observed a frequency shift in radio waves to and from the probe as those signals traveled close to the Sun. These results confirmed theoretical predictions based on Einstein's general theory of relativity. In May 2004, Cassini-Huygens entered the Saturn system, i.e., the gravitational pull of Saturn became higher than the pull from the Sun. After over five years of inactivity, Cassini's main engine was fired on 27 May as a test prior to orbital insertion. After a flyby of the Moon Phoebe on June 11 (at a distance of just 2,068 kilometers), Cassini performed one more correction five days later. Finally, on 1 July 2004, the spacecraft engine fired for 96 minutes, thus inserting Cassini-Huygens into a 0.02 × 9 million-kilometer orbit around Saturn. It was the first human-made object to enter orbit around Saturn. In its initial months, Cassini provided detailed data on Titan during three flybys (on 2 July, 27 October, and 13 December) and discovered two small new moons (Methone and Pallene).

On Christmas Day 2004 at 02:00 UT, the Huygens lander, which had remained dormant for more than six years, separated from Cassini and began its 22-day coast to Titan. It entered Titan's atmosphere at 09:05:56 UT on 14 January 2005 and within 4 minutes had deployed its 8.5-meter diameter main parachute. A minute later, Huygens began transmitting a wealth of information back to Cassini for over 2 hours before impacting on the surface of Titan at 11:38:11 UT at a velocity of 4.54 meters/second. Landing coordinates were 192.32° W / 10.25° S, about 7 kilometers from its target point. A problem in the communications program limited the number of images that Huygens transmitted to Cassini, from about 700 to 376. Yet, to the excitement of planetary scientists back on Earth, it continued its transmissions for another 3 hours and 10 minutes during which it transmitted a view of its surroundings (224 images of the same view). Huygens appears to have landed in surface resembling "sand" made of ice grains; surface pictures showed a flat plain littered with pebbles as well as evidence of liquid acting on the terrain in the recent past. Subsequent data confirmed the existence of liquid hydrocarbon lakes in the polar regions of Titan. In April 2016, ESA announced that one of Titan's three large seas close to the north pole, known as Ligeia Mare, is filled with pure liquid methane, with a seabed covered by a "sludge" of organic-rich material.

The Cassini orbiter meanwhile continued its main orbital mission investigating the Saturn system, its voyage punctuated by repeated "targeted" flybys—flybys actively implemented by trajectory corrections—of various moons, particularly Titan, Enceladus, Tethys, Hyperion, Dione, Rhea, and Iapetus. Cassini ended its primary mission on 27 May 2008 with its 43rd flyby of Titan. During this period, the spacecraft discovered two new moons (Daphnis and Anthe), uncovered much valuable data about Titan, including the first radar images of the moon's surface taken during its 27 October 2004 flyby and clear evidence of existing large lakes of liquid hydrocarbon in the northern latitudes of Titan, and performed a number of radio occultation experiments to study the size-distribution of particles in Saturn's rings and atmosphere. Perhaps the most exciting flyby was one on 12 March 2008 when Cassini flew within 50 kilometers of the surface of Enceladus, passing through the plumes from its southern geysers. The spacecraft detected water and carbon dioxide and also mapped surface features. In April 2008, NASA approved a two-year extension of its mission (i.e., 60 more Saturn orbits), which officially began on 1 July 2008 and was called the Cassini Equinox Mission, named as such because the mission coincided with Saturn's equinox. Further moons of Saturn were identified (Aegaeon and S/2009 S 1, the latter a "propeller moonlet" perhaps only 400 meters across) while additional encounters with Enceladus allowed Cassini to acquire very high-resolution images of its surface and directly sample its cryo-volcanic plumes that appear to contain complex organic chemicals. During the two-year Equinox Mission, which ended in September 2010, Cassini performed 26 targeted flybys of Titan, 7 of Enceladus, and 1 each of Dione, Rhea, and Helene.

On 3 February 2010, NASA announced that Cassini's mission would continue beyond the original two-year extension into the new Cassini Solstice Mission that would last until September 2017, a few months past Saturn's summer solstice. The new mission was named after the Saturnian summer solstice occurring in May 2017, which marked the beginning of summer in the northern hemisphere and winter in the southern hemisphere. (The spacecraft had arrived at Saturn just after the planet's northern winter solstice. The extension thus allowed scientists to study a complete seasonal period of the planet.) The Cassini Solstice Mission was guided principally by its ability to continue close studies of Titan (particularly seasonal climate change such as storms, flooding, and changes in lakes) and Enceladus (particularly, its "astrobiological potential") but also some of the other icy moons (such as Dione and Rhea), the planet itself (and its magnetosphere), and its rings. The extension allowed 155 orbits around Saturn, plus 54 flybys of Titan, 11 of Enceladus, 2 of Rhea, and 3 of Dione. At the beginning of the Solstice Mission, on 2 November 2010, Cassini ran into a problem when a malfunction in the spacecraft's computer shut down all nonessential systems. Slowly, over a period of about three weeks, controllers were able to restore all of Cassini's instruments to working order. Only one targeted flyby of Titan was affected during the interim. On 6 March 2014, the spacecraft conducted its 100th flyby of Titan (at a range of 1,500 kilometers), conducting gravity measurements in order to explore the existence of a global subsurface ocean. By July 2014, Cassini had identified at least 101 distinct geysers erupting on the south polar region of Enceladus. Researchers concluded that it is possible for liquid water to reach all the way to the surface from the moon's underground sea (whose existence had been announced in April 2014). The presence of this salty underground ocean, about 30–40 kilometers thick, under a 10-kilometer ice shell, raises the possibility that microbial life might exist there.

Significant events in 2015–2016 included a close flyby (at 47,000 kilometers) of Rhea in February 2015, allowing very high-resolution images of the natural satellite, a flyby (at 34,000 kilometers) of the irregularly-shaped Hyperion in May 2015 showing a deeply impact-scarred surface, and two last flybys of Dione in June and August 2015, at just 516 and 474 kilometers range, respectively. Perhaps the most spectacular mission event of the year was Cassini's "deep-dive" to just 49 kilometers above the south polar region of the geologically active Enceladus in October 2015. During the encounter, the spacecraft's gas analyzer and dust detector instruments sampled the moon's plume of gas and dust-sized icy particles. A final flyby of Enceladus was carried out in December at a range of 4,999 kilometers, concluding a chapter in the Cassini mission's encounters with Saturn's moons. In December 2015, Cassini initiated a number of delicate orbital maneuvers designed to tilt the spacecraft's orbit out of Saturn's ring-plane. Each maneuver was followed by a gravity-assist from Titan ("Titan does all the heavy lifting," noted Earl Maize, Cassini project manager at JPL), thus sending the vehicle to an increasingly higher inclination (relative to Saturn's equator). These maneuvers set up Titan for its final dramatic year in 2016–2017, involving two distinct phases of the mission. On 30 November 2016, Cassini set off on a path that carried it high above and under Saturn's poles, diving every seven days through the hitherto unexplored region at the outer edge of the main rings. This phase of the mission, called "Cassini's Ring-Grazing Orbits" involved 20 "dives" through this region, that ended on 22 April 2017. During some of these passes, the spacecraft directly sampled ring particles and molecules of faint gases close to the rings. In March and April 2017, ring crossings had the spacecraft fly through the dusty outer regions of the F ring. After the last ring-grazing orbit concluded on 22 April 2017, a flyby of Titan reshaped Cassini's trajectory to send it on a new phase of the mission, the "Grand Finale," involving 22 plunges, the first on 26 April through the 2,400-kilometer gap between Saturn and its innermost ring. The mission concluded on 15 September 2017 on its 293rd orbit of Saturn when Cassini plunged into the Saturnian atmosphere ending one of the most ambitious and spectacular missions in the history of planetary exploration. By most estimates, the spacecraft burned up in the atmosphere and was destroyed about 45 seconds after the last transmission. During the final moments of the descent, data from eight of Cassini's science instruments beamed important data back to Earth, giving insight into the planet's formation and evolution. Cassini was named after Italian astronomer Giovanni Cassini (1625–1712).