The Solar System in Close-Up

by John Wilkinson

  Fig. 2.2The Hubble Space Telescope (Credit: NASA/HST).

  During its lifetime, the HST has been studying the universe at wavelengths from the infrared through to the ultraviolet. Hubble has been used to view over 30,000 objects throughout the universe and recorded over 44 terabytes of data. Astronomers using the HST data have published over 8700 scientific papers. Its high-resolution images of Mars, Jupiter, Saturn and Neptune are providing surprising detail about these planets. The world was amazed in July 1994 when Hubble produced images showing the impact of the comet Shoemaker-Levy 9 on Jupiter. Hubble has been used to study the Great Nebula in Orion and it has detected dusty discs around protostars that are thought to be new solar systems forming. Discs of matter have also been seen swirling around super massive black holes at the centre of galaxies and quasars, as well as structure in the spiral arms of nearby galaxies.

  The Chandra Space Telescope

  The Chandra space telescope was launched by the shuttle Columbia in July 1999, and is designed to observe X-rays from high-energy regions of the universe, such as the remains of exploded stars. X-rays provide scientists with a different perspective when exploring space. Chandra discovered that Jupiter’s X-rays are coming from its poles and not the auroral rings on the planet.

  Chandra has 8 times greater resolution and is able to detect sources more than 20 times fainter than any other previous X-ray telescope. Chandra’s highly elliptical orbit allows it to observe continuously for up to 55 h of its 65 h orbital period. At its furthest orbital point from Earth, Chandra is one of the most distant Earth-orbiting satellites. This orbit takes it beyond the geostationary satellites and beyond the outer Van Allen belt.

  Although Chandra was initially given an expected lifetime of 5 years, on 4 September 2001 NASA extended its lifetime to 10 years based on the observatory’s outstanding results. Physically Chandra could last much longer. A study performed at the Chandra X-ray Centre indicated that the observatory could last at least 15 years.

  The XXM-Newton Space Telescope

  The XMM-Newton telescope was launched by rocket in December 1999, and is designed to investigate the origins of the universe by probing cosmic matter from black holes. It has detected X-ray emission from solar system objects and is also used to study star forming regions in the universe. XMM-Newton orbits Earth in an elliptical orbit between 7000 and 114,000 km above Earth. The satellite weighs 3800 kg and is 10 m long and is 16 m in span with its solar arrays deployed. It holds three X-ray telescopes, developed by Media Lario of Italy, each of which contains 58 Wolter-type concentric mirrors. The combined collecting area is 4300 cm2. The three European Photon Imaging Cameras are sensitive over the energy range 0.2–12 keV. Other instruments onboard are two reflection-grating spectrometers that are sensitive to below 2 keV, and a 30 cm diameter Ritchey-Chretien optical/UV telescope.

  The Spitzer Space Telescope

  The Spitzer Space Telescope (also known as the Space Infrared Telescope Facility or SIRTF) was launched in August 2003 and observes mainly in the infrared region. Infrared rays are blocked by the Earth’s atmosphere, so observation is only possible from space at this wavelength. This telescope orbits Earth at a height of 568 km and is used to study asteroids in the solar system, gas giant planets, dusty stars and distant galaxies. In 2005 Spitzer was used to discover the dwarf planet Makemake in the outer solar system.

  Instead of orbiting Earth like other space telescopes, Spitzer tags along behind Earth as it orbits the Sun. This keeps it clear of Earth’s heat and makes for better pictures. Spitzer will be able to see every part of the sky at least every 6 months during its life.

  The Kepler Space Telescope

  Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft has a mass of 1039 kg and contains a 1.4-m primary mirror feeding an aperture of 0.95-m—at the time of its launch this was the largest mirror on any telescope outside of Earth orbit. The focal plane of the spacecraft’s camera is made up of 42 CCDs at 2200 × 1024 pixels, which made it at the time the largest camera yet launched into space, possessing a total resolution of 95 megapixels. Heat pipes connected to an external radiator cool the array.

  The spacecraft is named after the Renaissance astronomer Johannes Kepler and was launched on 7 March 2009. Kepler’s sole instrument is a photometer that continually monitors the brightness of over 145,000 stars in our region of the Milky Way galaxy. This data is transmitted to Earth, then analysed to detect periodic dimming caused by exoplanets that cross in front of their host star. As of February 2014, Kepler and its follow-up observations had found over 900 confirmed exoplanets in more than 76 stellar systems, along with a further 2903 unconfirmed planet candidates. In November 2013, astronomers reported that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarf stars within the Milky Way Galaxy.

  Kepler orbits the Sun, which avoids Earth occultations, stray light, and gravitational perturbations and torques inherent in an Earth orbit. Kepler’s photometer points to a field in the northern constellations of Cygnus, Lyra and Draco, which is out of the ecliptic plane, so that sunlight never enters the photometer as the spacecraft orbits the Sun.

  In April 2012, an independent panel of senior NASA scientists recommended that the Kepler mission be continued through 2016. According to the senior review, Kepler observations needed to continue until at least 2015 to achieve all the stated scientific goals (see Fig. 2.3).

  Fig. 2.3Structure of the Kepler Space Telescope (Credit: NASA).

  Future Space Telescopes

  There are a number of space telescopes panned to be launched in the near future. These include LISA Pathfinder (2015), James Webb ST (2018), Hard X-ray Modulation Telescope (2014–2016) and Dark Matter Particle Explorer (2015–2016).

  The James Webb Space Telescope (JWST), previously known as Next Generation Space Telescope, is a planned space telescope designed for observations mainly in the infrared region of the spectrum. It is to be the scientific successor to the Hubble Space Telescope and the Spitzer Space Telescope. The main technical features are a large and very cold 6.5-m diameter mirror and four specialised instruments at an observing position far from Earth, orbiting the Earth–Sun L2 point. The combination of these features will give the telescope unprecedented resolution and sensitivity from long-wavelength visible to the mid-infrared, enabling its two main scientific goals—studying the birth and evolution of galaxies, and the formation of stars and planets. JWST’s instruments will not measure visible or ultraviolet light like the Hubble Telescope, but will have a much greater capacity to collect infrared light.

  The JWST telescope is a project of the National Aeronautics and Space Administration (NASA, the United States space agency), with international collaboration from the European Space Agency (ESA) and the Canadian Space Agency (CSA), including contributions from 15 nations. The launch date is 2018 and the mission duration is 5 years minimum with a possibility of 10 years (see Fig. 2.4).

  Fig. 2.4Artists picture of what the James Webb Space Telescope looks like in space (Credit: NASA).

  Using Space Probes to Explore the Solar System

  Humans have always had the vision to 1 day live on other planets. This vision existed even before the first person was put into orbit. Since the early space missions of putting humans into orbit around Earth, many advances have been made in space technology. It is now possible to send unmanned space probes deep into the Solar system to explore other planets. Humans have only travelled as far as the Moon and back, but robotic space probes have been placed on the surface of planets like Venus and Mars, as well as on the surface of an asteroid. Probes have also been used to orbit distant planets like Jupiter and Saturn.

  Planetary probes travel in large orbits around the Sun. They often pass target planets (flybys), go into orbit around planets or land on planets. Instruments onboard space probes collect information about the planet and return it via radio signal
s back to Earth.

  The first planet to which humans sent a spacecraft was Venus, the closest planet to Earth. Venus is similar in size and mass to Earth and has always been of interest to humans. Mars was the next planet to which a probe was sent, followed by Mercury. Some of these early space probes were successful while others failed—all however provided information and experiences that lead to future success.

  Current Probes in the Solar System

  There are a number of significant space probes currently in operation exploring the solar system. Details of these probes and their missions will be discussed in the relevant chapters in this book. However, several recent probes are significant enough to deserve special mention here.

  The Messenger Probe

  Messenger (MErcury Surface, Space Environment, Geochemistry, and Ranging) is a robotic space probe launched by NSSA in August 2004 to study the planet Mercury. Messenger became the second mission after Mariner in 1975 to visit Mercury. Messenger made two flybys of the planet in 2008 and finally entered orbit around Mercury in March 2011.

  The primary mission was completed in March 2012 after the entire surface of the planet was mapped and 100,000 images were recorded. An extension of the program was made in July 2013. The probe made a final orbital correction in January 2015 leading to a termination of the mission with an impact into Mercury’s northern hemisphere in April 2015.

  Instruments on Messenger have collect valuable data on Mercury and its environment.

  Its surface looks much like that of Earth’s Moon.

  Messenger has a dual-mode, liquid chemical propulsion system that is integrated into the spacecraft’s structure to make economical use of mass. The structure is primarily composed of a graphite epoxy material. This composite structure provides the strength necessary to survive launch while offering lower mass. Two large solar panels, supplemented with a nickel-hydrogen battery, provide Messenger’s power (see Fig. 2.5).

  Fig. 2.5The Messenger probe is exploring the surface of Mercury (Credit: NASA/JHU/CIW).

  The New Horizons Probe

  The New Horizon probe was launched by NASA on 19 January 2006 on a mission to study the dwarf planet Pluto and the Kuiper belt (see Figs. 2.6 and 2.7). In July 2015, the craft was the first to fly by Pluto and its moons. Radio signals took over 4 h to travel from the craft back to Earth. The craft set the record for the highest velocity of a human-made object from Earth at 58,536 km/h. It flew by the orbit of Mars on 7 April 2006, Jupiter on 28 February 2007, the orbit of Saturn on 8 June 2008 and the orbit of Uranus on 18 March 2011. New Horizons flew within 10,000 km of Pluto with a velocity of 49,600 km/h. It also came as close as 27,000 km to Charon (Pluto’s largest moon). The spacecraft’s instrumentation includes a high resolution telescope, another scope with broadband spectroscopic capacity going into the near infrared and far ultraviolet, a particle and electron detector, a dust counter and radio science experiments using the communication channels.

  Fig. 2.6The New Horizons space probe was launched into orbit via an Atlas V rocket from Cape Canaveral Air Force Station in Florida, USA on 19 January 2006 (Credit: NASA).

  Fig. 2.7New Horizon’s space probe in its assembly hall (Credit: NASA).

  New Horizons was originally planned as a voyage to what was the only unexplored planet in the Solar System—Pluto. When the spacecraft was launched, Pluto was still classified as a planet, later to be reclassified as a dwarf planet by the International Astronomical Union (IAU). Some members of the New Horizons team disagreed with the IAU definition and still describe Pluto as the ninth planet. Pluto’s satellites Nix and Hydra also have a connection with the spacecraft: the first letters of their names (“N” and “H”) are the initials of “New Horizons”. The moons’ discoverers chose these names for this reason, in addition to Nix and Hydra’s relationship to the mythological Pluto.

  Pluto was discovered by Clyde Tombaugh in 1930. About an ounce of Tombaugh’s ashes are aboard the New Horizon’s spacecraft, to commemorate his discovery. A Florida-state quarter coin, whose design commemorates human exploration, is also included. One of the science packages (a dust counter) is named after Venetia Burney, who, as a child, suggested the name “Pluto” after the planet’s discovery.

  After passing by Pluto, New Horizons will continue farther into the Kuiper belt. Mission planners are now searching for one or more additional Kuiper belt objects (KBOs) of the order of 50–100 km in diameter for flybys similar to the spacecraft’s Plutonian encounter.

  The Stereo Probe

  The STEREO (Solar TErrestrial RElations Observatory) probe is a solar mission launched by NASA on 26 October 2006. It consists of two nearly identical spacecraft, one orbiting ahead of Earth (A) and the other behind Earth (B). Observations are made simultaneously of the Sun and then combined to provide a 3-D stereo image of the Sun. Spacecraft A takes 347 days to orbit the Sun while spacecraft B takes 387 days. Because the A spacecraft is moving faster than B, they are separating from each other and A is orbiting closer to the Sun than B. The images are adjusted to account for this difference.

  STEREO is used to image the inner and outer corona and the space between Sun and Earth, detect electrons and other energetic particles in the solar wind, study the plasma characteristics of protons, alpha particles and heavy ions, and monitor radio wave disturbances between the Sun and Earth.

  From February 2011, the two Stereo spacecraft will be 180° apart from each other, allowing the entire Sun to be seen for the first time. Such observations will continue for several years. By combining images from the STEREO A and B spacecraft, with images from NASA’s Solar Dynamic Observatory (SDO) satellite, a complete map of the Sun can be formed. Previous to the STEREO mission, astronomers could only see the side of the Sun facing Earth, and had little knowledge of what happened to solar features after they rotated out of view. In 2015 contact with the two spacecraft was temporarily lost for a few months as they both passed behind the Sun. Contact was re-established with Stereo A but not with Stereo B (see Fig. 2.8).

  Fig. 2.8Positions of the two Stereo probes as they orbit the Sun. From February 2011 the two craft will be 180° apart from each other, allowing the entire Sun to be seen in stereo (Credit: NASA).

  The Rosetta Probe

  Rosetta is a robotic spacecraft built by the European Space Agency (ESA) to study the comet 67P/Churyumov–Gerasimenko. The probe was launched on March 2004 on an Ariane 5 rocket and reached the comet in May 2014. The spacecraft consists of two main elements: the Rosetta space probe orbiter, which features 12 instruments, and the Philae robotic lander, with an additional nine instruments. Rosetta will orbit the comet for 17 months. The spacecraft has already performed two successful asteroid flyby missions on its way to the comet. In 2007 Rosetta also performed a flyby of Mars and returned useful images. The craft completed its flyby of asteroid 2867 in September 2008 and of 21 Lutetia in July 2010. On 20 January 2014, Rosetta was taken out of a 31-month hibernation mode and is continuing to its target.

  In May 2014, the Rosetta spacecraft entered a slow orbit around the comet and gradually slowed down before releasing the lander in November 2014. The lander approached the comet at speed around 3.6 km/h and on contact with the surface, two harpoons were to be fired into the comet to prevent it from bouncing off. However the harpoons didn’t fire and the lander bounced back up a kilometer into space, soaring for nearly 2 h before returning to the ground. After another small bounce, the lander settled somewhere in the shadow of a cliff, about a kilometre from where it was supposed to be. The lander was able to collect data with its suite of instruments, sniffing, hammering, drilling, and even listening to the comet (see Chap. 13).

  The Dawn Probe

  NASA launched the Dawn space probe on 27 September 2007 on a mission to study the two largest objects in the asteroid belt—Vesta and Ceres. The spacecraft uses ion propulsion to transverse space far more efficiently than if it used chemical propulsion. In an ion propulsion engine, an electrical charge is applied to xenon gas, and
charged metal grids accelerate the xenon particles out of the thruster. These particles push back on the thrusters as they exit; creating a reaction force that propels the spacecraft forward. Dawn has now completed over 5 years of accumulated thrust time, far more than any other spacecraft.

  Dawn was the first spacecraft to enter orbit around Vesta on 16 July 2011. The probe gave scientists a much closer view of this object. Dawn was originally scheduled to depart Vesta and begin its journey to Ceres on 26 August 2012, however, a problem with one of the spacecraft’s reaction wheels forced Dawn to delay its departure from Vesta’s gravity until 5 September 2012. Dawn successfully departed Vesta on this date and arrived at Ceres in April 2015, 3–4 months prior to the arrival of New Horizons at Pluto. Dawn is the first mission to study a dwarf planet at close range. It’s mission calls for it to enter orbit around Ceres at an initial altitude of 13,500 km for a first full characterisation. Dawn will then spiral down to a survey orbit at an altitude of 4430 km.

  Solar Dynamics Observatory

  The Solar Dynamics Observatory (SDO) is the most advanced spacecraft ever designed to study the Sun and its dynamic behavior. SDO is providing better quality, more comprehensive science data faster than any NASA spacecraft currently studying the Sun. The probe is aimed at providing data on the processes inside the Sun, the Sun’s surface, and its corona that result in solar variability. SDO will help scientists to better understand the Sun’s influence on Earth and near-Earth space through the use of many wavelengths simultaneously. SDO is investigating how the Sun’s magnetic field is generated and structured.