Over the next decade spacecraft will explore every corner of our Solar System as a prelude to the biggest mission of all - taking man to Mars
From Sun-blasted Mercury to light-starved Pluto, an armada of explorers will spread out across the Solar System over the next decade, searching for clues to the formation of planets and the origins of life. While attention and resources will increasingly focus on man's return to the Moon, and preparations for missions to Mars, these robotic probes are certain to bring discoveries, and disappointments.
Starting with Mariner 2's visit to Venus in 1962 and ending with Voyager 2's flyby of Neptune in 1989, it took just 27 years for the planets, except distant Pluto, to be explored by spacecraft. The next decade or so will see several of those planets, and their moons, revisited in more detail - beginning next month when NASA's Cassini Saturn orbiter releases the European Space Agency's Huygens probe to land on the moon Titan.
Messenger to Mercury
With only one previous visitor, NASA's Mariner 10 in 1974-5, Mercury is the target for two missions. Launched in August, NASA's Messenger is en route to the innermost planet via gravity-assist flybys of the Earth in August 2005, two of Venus in October 2006 and June 2007 and three of Mercury itself in January and October 2008 and September 2009. The spacecraft is scheduled to enter a highly elliptical orbit around Mercury in March 2011, on a 12-month mission to map the rocky planet in colour and analyse its Moon-like surface, faint atmosphere and magnetosphere.
Magnetospheric activity discovered by Mariner 10 will be studied in detail by the joint ESA/Japan Aerospace Exploration Agency (JAXA) Bepi Colombo mission planned for launch in 2011-12. The first mission to the planet by either agency, this will involve two spacecraft: Europe's Mercury Planetary Orbiter observing the surface and interior; and Japan's Mercury Magnetospheric Orbiter studying the magnetic field. On arrival in 2016 - via flybys of the Moon, Venus and Mercury itself - the spacecraft will separate for a year-long mission in different polar orbits.
Venus visitors
The first planet to be explored, by NASA's Mariner 2 in 1962, Venus has been undisturbed since NASA's Magellan orbiter ended its four-year mapping mission in 1994. These and other missions, including the Soviet Venera landers, revealed an uninviting planet with surface temperatures hotter than Mercury, pressures 90 times those on Earth, a carbon dioxide atmosphere, sulphuric acid rain and hurricane-force winds.
Despite this, two new missions are planned: ESA's Venus Express and JAXA's Planet-C. Europe's first mission to Earth's nearest planetary neighbour is scheduled for launch in November 2005, entering highly elliptical polar orbit in April 2006 for a 500-day mission to map surface temperatures and study the Venusian atmosphere in depth. Venus Express uses the same spacecraft bus design as ESA's successful Mars Express.
Japan's Planet-C is planned to reach Venus in 2009 after launch in 2008. Equipped with infrared and ultraviolet cameras, the spacecraft will study the puzzle of the 100km/s (60 miles/s) storm winds - 60 times the planet's speed of rotation - and look for active volcanoes. Planet-C uses the same spacecraft design as JAXA's Nozomi Mars orbiter, which failed to reach its destination after launch in 1998.
Ambitions for Jupiter
Most massive of the planets, and a mini-solar system in its own right, Jupiter is the target for NASA's most ambitious space probe yet. Prometheus 1, otherwise known as the Jupiter Icy Moons Orbiter (JIMO), is proposed as the first mission under the US space agency's Prometheus initiative to develop nuclear electric propulsion and power technology. Set for launch in 2015 at the earliest, JIMO would orbit three planet-sized Jovian moons - Calisto, Europa and Ganymede - that may harbour life-sustaining oceans beneath their icy surfaces.
Jupiter was last explored by NASA's Galileo, which was launched in 1989 and ended its mission in 2003 by deliberately plunging into the gas giant's atmosphere. Now in preliminary design, JIMO will forgo the usual gravity-assisted flybys and make a five- to eight-year direct journey to Jupiter under fission-powered ion propulsion. The ample power available on orbit from the nuclear reactor will allow the craft to carry a substantial payload of instruments to explore each moon for at least 30 days, including a radar to measure ice thickness and a laser to map surface contours.
A Jupiter polar orbiter, Juno, is under concept study by NASA for launch in 2010. The mission's goal is to understand the origin and evolution of the planet by using a spin-stabilised, solar-powered spacecraft in elliptical polar orbit to look for the ice-rock core, determine the atmospheric composition, study the weather, investigate the magnetic field and explore the polar magnetosphere of Jupiter.
Saturn, Uranus, Neptune
Another gas giant is currently under observation by NASA's Cassini, which began a four-year orbit of Saturn and its moons in June and which is scheduled to release the ESA-supplied Huygens lander on 24 December. Saturn was last visited by NASA's Voyager 2 deep-space probe during its flyby in 1981.
The disc-shaped Huygens probe is expected to enter the atmosphere of Titan, the largest moon in the Solar System, on 14 January, sampling the atmosphere as it descends by parachute to a "landfall" that could involve a hard landing or a splash-down into a cryogenic ocean of liquid methane and ethane.
Voyager 2 made the only visit so far to Uranus when it flew past the bright blue-green ice giant in 1986 en route to deep space. No missions to the seventh planet have yet progressed beyond the conceptual stage, but NASA is looking into a mission to Neptune to collect data on the internal structure and composition of an ice giant to compare with information yielded by missions to gas giants Jupiter and Saturn.
The only visit to Neptune was Voyager 2's 1990 flyby. Now a Neptune Orbiter with Probes mission is under study. A nuclear-electric-powered mission launched between 2016 and 2018 would arrive at the distant eighth planet around 2035. An alternative mission using solar-electric propulsion is under study that would involve lengthy gravity-assist flybys. The probe would explore Neptune's atmosphere, as well as its enigmatic ring system and icy moons - especially Triton, which scientists believe is a captured Kuiper Belt Object (KBO).
To Pluto and beyond
Although Pluto's highly elliptical orbit periodically brings it closer to the Sun than Neptune, the smallest, coldest and most distant planet has yet to be visited by a spacecraft. But NASA's planned New Horizons mission to Pluto and the Kuiper Belt is scheduled for launch in January 2006, the probe making a gravity-assist flyby of Jupiter in February 2007 and expected to arrive at the "double planet" system of Pluto and its moon Charon in July 2015. After 150 days of observations, the spacecraft will head deeper into the Kuiper Belt to study one or more of the icy mini-worlds, or KBOs, in the region beyond Neptune's orbit. The mission is planned to continue until 2020, New Horizons drawing power from its radioisotope thermal generator.
Primordial fragments
In addition to planetary missions, flights to explore asteroids and comets are either under way or planned. Mostly orbiting the Sun in a belt between Mars and Jupiter, asteroids are rocky fragments left over from the formation of the Solar System. Composed of dirt and ice, comets are similar leftovers, most coming from a region beyond the orbit of Neptune, and are among the least-changed objects in the Solar System.
NASA's Stardust probe, which collected particles during its close encounter with comet 81P/Wild 2 in January, is scheduled to return the samples to Earth in January 2006 - aiming to avoid the crash-landing suffered by the agency's Genesis solar wind sample-return probe earlier this year.
The first-ever asteroid sample return mission is now under way, with Japan's Muses-C spacecraft, christened Hayabusa, en route to rendezvous with the asteroid Itokawa in October 2005. Launched in May 2003, Hayabusa uses ion propulsion. The spacecraft will attempt to bring a horn-shaped sample collector into contact with the asteroid's surface then fire a projectile into the surface, funnelling the impact fragments into a container that will be sealed for return to Earth in June 2007.
NASA's Deep Impact mission to send a 350kg (770lb) copper projectile into comet 9P/Tempel 1 is now scheduled for launch on 8 January 2005. The probe is planned to reach Tempel 1 in July next year, where a battery-powered impactor will be released into the comet's path to collide with at a speed of 10.2km/s, creating a crater at least 25m (80ft) deep and 100m in diameter. Flying past at 500km, the mother ship will use cameras and spectrometer to study the crater's formation and comet's interior.
Scheduled for launch in June 2006, NASA's Dawn spacecraft is intended to orbit two of the largest asteroids in the Solar System, Vesta and Ceres, using ion propulsion to reach its targets then enter and depart orbit. The probe will reach Vesta in July 2010, departing a year later for a rendezvous with Ceres in August 2014 and a second year-long period of observations.
Finally launched in March, ESA's Rosetta probe is en route via Earth and Mars flybys to enter orbit around the comet 67P/Churyumov-Gerasimenko in August 2014. The main spacecraft will deploy a small, 100kg instrumented lander on to the 4km-wide icy nucleus in November 2014, then orbit the comet for two years to observe changes as it heads around the Sun. The mission is scheduled to end in December 2015.
Looking further ahead, a sample return mission to the near-Earth asteroid Hera has been proposed to NASA. This would use touch-and-go impregnated pad collectors to gather samples from three different regions of the asteroid's surface. Each 12cm (4.7in) -diameter disc, coated with a 1cm-deep layer of silicone grease, would collect around 100g (3.5oz) of material with each brief touchdown.
Another mission proposed to NASA is Comet Odyssey, which would be launched in October 2009 to orbit the nucleus of comet 46P/Wirtanen - Rosetta's original target - for an extended period. As the comet heads outward from the Sun, the ion-propulsion spacecraft would make repeated slow flybys through the active cometary coma before entering orbit, initially 100km from the nucleus but lowering to 40km or less over the 4.5-month mission.
Back to the Moon
Under US President George Bush's space exploration vision, resources are expected to shift towards human and robotic missions to the Moon and Mars over the coming decades. Although the vision was intended to give NASA a new sense of direction, other space agencies are keen to align their exploration priorities with those of the USA. This is leading to renewed attention on the long-neglected Moon, now viewed as a vital stepping stone to Mars and beyond.
NASA's last mission to the Moon was Lunar Prospector, which ended an 18-month orbital mission in July 1999 by intentionally smashing into the south pole to further the search for water. Europe is now mounting its first mission to the Moon, with ESA's Smart-1 probe entering lunar orbit in November. A technology demonstrator rather than an explorer, Smart-1's primary mission is to test an ion propulsion system, hence its leisurely journey to the Moon after launch in September 2003. Now in orbit, the craft is looking for water ice at the lunar poles as well as mapping the Moon's topography and surface distribution of minerals.
As if to underline the international interest in the Moon, China, India and Japan all plan lunar missions. Japan's two projects, Lunar-A and Selene, have been delayed several times by budget and technical difficulties. JAXA's first mission to the Moon, Lunar-A, was due for launch this year and is designed to deploy two penetrators, one on the nearside and one on the farside, able to drive 2m deep into the surface to measure temperatures and study seismic activity.
The Selene mission is more complex, involving an orbiter that will deploy a separate data-relay satellite on arrival then conduct a landing demonstration at the end of its year-long lunar mapping mission. Currently planned for launch in August 2005, the main spacecraft will be placed in a 100km-high polar circular orbit, while the relay satellite will enter an elliptical orbit. At the end of the mission the orbiter's propulsion module will separate and land on the Moon.
India plans to launch its first mission to the Moon, the Chanrayaan-1 lunar orbiter, in 2008 on a two-year mission to conduct high-resolution three-dimensional mapping of the surface from 100km orbit in the visible, near-infrared, X-ray and low-energy gamma ray spectra. China's Chang'e lunar orbiter is scheduled for launch in 2006 as a precursor of a lunar rover in 2010 and a sample return mission in 2020.
NASA's plans to return to the Moon with an ambitious lunar robotic exploration programme to pave the way for manned missions received a setback when US Congress slashed the funding allocated to the first of the robotic probes - the Lunar Reconnaissance Orbiter (LRO). In a move that could presage future conflicts over the direction of NASA's exploration programme, Congress cited concerns that the probe was focused too much on engineering in support of human missions and not enough on basic science.
Proposed for launch in September 2008, funds permitting, the LRO would orbit at 30-50km for one to five years to study the radiation environment, map the topography and scan for resources to identify future landing sites for robotic and human missions. This would be followed in 2009-10 by a lunar robotic lander. NASA is studying Moonrise, a 2010 mission to land two identical craft in the Aiken Basin, at the lunar south pole, and return more than 2kg of materials from the Moon's mantle.
The US space agency's draft "strawman" strategy for lunar exploration calls for a robotic infrastructure to be put in place at a single site - probably at the south pole - between 2011 and 2015 to support human missions of extending duration and increasing size beginning by 2015 at the earliest and 2020 at the latest.
Search for life on Mars
A manned mission to Mars is unlikely before 2025-30, but the robotic onslaught on the Red Planet is well under way. Five craft are active on or above the planet. NASA's Mars Exploration Rovers Spirit and Opportunity are still roaming the Gusev Crater and Meridiani Planum, on opposite sides of Mars, in a search for signs of past surface water that has continued months beyond the vehicles' expected lives. In orbit are NASA's Mars Global Surveyor and Mars Odyssey and ESA's Mars Express.
The next in a series of robotic explorers, the Mars Reconnaissance Orbiter, is scheduled to arrive in March 2006 after launch in August 2005. Operating for a full Martian year, the craft will image the planet with a resolution of 20-30cm looking for signs of water and potential landing sites. A subsurface sounding radar supplied by the Italian space agency will search for underground water.
Next in sequence is the former Mars Surveyor lander, originally planned for launch in 2001 but cancelled after the humiliating loss of NASA's Mars Polar Lander and Mars Climate Orbiter in 1999. The renamed Phoenix, or Mars Scout 1, will be launched in late 2007 to land at the icy north pole where, using a robotic arm, it will dig trenches in the arctic terrain and conduct in-situ analyses to look for evidence of past life.
NASA's next project is planned to be the Mars Science Laboratory, a long-range, long-duration rover to be launched in late 2009. The mobile laboratory will collect and analyse soil and rock samples, looking for organic compounds. A Mars sample return mission is in the plans for launch in 2014, or 2011 if the pace of exploration can be accelerated, with a follow-up mission planned for 2016.
A Mars sample return, meanwhile, is one of two flagship missions being studied by ESA under its Aurora exploration programme. The first mission to be assessed is ExoMars, comprising an orbiter, descent module and a rover with drilling and sampling equipment to search for signs of life. This is to be followed by a sample return mission involving five spacecraft: Earth-Mars transfer stage, orbiter, descent module, ascent module and Earth re-entry vehicle. Plans call for a launch as early as 2011.
TIM FURNISS / LONDON & GRAHAM WARWICK / WASHINGTON DC
Source: Flight International
