The Universe’s Last Stars

At present, the universe is in the stelliferous era where conventional star formation is an ongoing process and nuclear fusion in stars account for most of the energy generation in the universe.  However, star formation is not a perfectly efficient process and matter is continuously being lost due to incorporation into objects such as brown dwarfs, extremely long-lived red dwarfs and inert stellar remnants (white dwarfs, neutron stars and black holes). As a result, the stelliferous era is expected to end when the universe is around 10¹⁴ years old, about 10³ times its current age. The universe then enters the degenerate era which is expected to last until the universe is around 10³⁷ years old. This is the era where most of the mass in the universe is in the form of brown dwarfs, white dwarfs, neutron stars and black holes.

Brown dwarfs are objects not massive enough to become stars as their masses are below the hydrogen-burning limit. During the degenerate era, non-conventional star formation remains a possibility through 2 methods. The 1st method is when two brown dwarfs collide and merge into an object with a mass above the hydrogen-burning limit. The 2nd method occurs when a brown dwarf very slowly accretes matter from the rarefied interstellar medium until its mass is eventually pushed above the hydrogen-burning limit. At any given time, it is estimated that at least a few stars in the galaxy were created via the 1st method. As for the 2nd method, it is unobservable in the present universe because the timescale for it to happen is many orders of magnitude larger than the current age of the universe. Nevertheless, the every-increasing timescales in the far future of the universe may make the 2nd method a dominant mode of star formation in an almost starless universe.

The number of brown dwarfs in the galaxy is expected to be comparable to the number of hydrogen-burning stars. Over a very long period of time, a brown dwarf drifting through the rarefied interstellar medium can accrete enough matter to push its mass above the hydrogen-burning limit and become a red dwarf star. A more massive brown dwarf needs to accrete less material than a lower mass brown dwarf to arrive at the hydrogen-burning limit. Assuming the minimum mass necessary for hydrogen-burning is 7.7 percent the Sun’s mass and the interstellar medium has an average density of one proton for every 10 cubic centimetres, a brown dwarf with 5 percent the Sun’s mass needs to accrete for 10¹⁷ years before its mass arrives at the hydrogen-burning limit. After that, the newly born red dwarf star will shine for about 10¹³ years in a very dark universe. In comparison, our Sun has a lifespan of just 10¹⁰ years.

During the degenerate epoch, the accretion of matter from the interstellar medium by objects other than brown dwarfs can lead to some interesting phenomenon although such events are likely to be separated by exceedingly long periods of inactivity. For example, the accretion of matter from the interstellar medium by white dwarfs and neutron stars is expected to cause an occasional supernova via the accretion-induced collapse mechanism. On rare occasions, collisions between helium white dwarfs or between a brown dwarf and a white dwarf can create short-lived helium-burning stars and possibly even stars that burn elements heavier than helium. With fleeting lifespans of no more than a few 10⁸ years, such stars are expected to contribute little to the overall stellar luminosity in comparison to hydrogen-burning red dwarf stars in the degenerate epoch. Nonetheless, if the universe should obey the Copernican Principle, then the current universe has no special place in time and interesting things can still happen in the seemingly inactive universe of the very far future.

References:

1. Cirkovic, M. M. (2005), “Brown Dwarf Accretion: Non-Conventional Star Formation over Very Long Timescales”, Serbian Astronomical Journal, Vol. 171, p. 11-17.

2. Fred C. Adams and Gregory Laughlin (1997), “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects”, arXiv:astro-ph/9701131

3. Accretion-Induced Collapse (http://beyondearthlyskies.blogspot.sg/2013/01/accretion-induced-collapse.html), January 2013

Testing Life’s Cosmic Ubiquity

Is life common in the universe? The successful detection of a second independent origin of life within our own solar system would be absolute proof that life is common in the universe. Our solar system is just one out of billions of other planetary systems in the galaxy and if life can arise twice in a single planetary system, then the galaxy is expected to be teeming with billions of living worlds. Mars, Europa and Titan are undoubtedly the best places in the solar system to search for the existence of life. Of these three worlds, Titan may prove to be the best place to look for a second independent origin of life in the solar system and to test life’s cosmic ubiquity.

Throughout Earth’s history, hypervelocity impacts caused by asteroids and comets crashing into the surface of the Earth are known to be able to throw up rock material into space. These rocks can carry terrestrial life within them through the vacuum of space and a tiny fraction of these rocks do eventually find their way to the surfaces of other planets and moons in the solar system. Mars and Europa have environments just beneath their surface where the conditions are suitable to support terrestrial life and Earthly microbes that have hitched a ride on these rocks can indeed survive there. As a result, if any life is detected on Mars or Europa, it would be difficult to definitively proof if it has an origin that is independent from that of life on Earth unless the type of life turns out to be biochemically distinct from life on Earth.

Titan is the largest moon of Saturn and it has a thick nitrogen-rich atmosphere laden with a wealth of organic molecules. The mean surface temperature on Titan is a frigid -179 degrees Centigrade and this is so cold that any water on Titan is literally rock solid. In fact, frozen water makes up the crust of Titan, creating a geological landscape where features such as mountains, sand dunes and boulders are actually made of frozen water. Instead of liquid water, liquid methane and ethane are the working fluids in Titan’s “hydrological cycle”. The surface of Titan contains widespread fluvial features such as rivers and deltas that were created through the action of liquid methane and ethane. In the high latitudes of Titan, there are lakes and seas of liquid methane and ethane. This makes Titan the only other place in the solar system besides the Earth that has stable bodies of surface liquid.

Since all life on Earth is born of liquid water and sustained by liquid water, the absence of liquid water on Titan creates an environment that is completely inhospitable to terrestrial life. Any terrestrial life that may have hitched a ride to Titan will not survive. However, it would be naive to assume that liquid water is the only solvent suitable for biology just because all life on Earth is water-based due to water being the most common solvent on Earth. Like the Earth, Titan also has the basic requirements for life such as the presence of a fluid environment, a source of energy and abundant organic molecules. If any form of life exists on Titan, it is expected to use the abundant liquid methane and/or ethane as a biosolvent due to the absence of liquid water in such a cryogenic environment. This means that any form of life detected on Titan is expected to be so biochemically distinct from life on Earth that it would be proof for a second independent origin of life in the solar system.

The most common type of star in the universe are not G-dwarf stars like our Sun but the much less massive M-dwarf stars, also known as red dwarfs. M-dwarf stars vastly outnumber G-dwarf stars. A typical M-dwarf star is so much less luminous than the Sun that an Earth-sized habitable planet with surface liquid water can only be sustained at one-tenth the Earth-Sun distance from the star. However, such a close proximity causes the planet to be tidally locked where one hemisphere perpetually faces the M-dwarf star; creating permanent day and night sides. As a result, an Earth-sized habitable planet around an M-dwarf star is unlikely to resemble the Earth. On the contrary, a Titan-like habitable planet could exist at a distance equivalent to the Earth-Sun distance from a typical M-dwarf star. The planet would not be tidally locked and is expected to rotate freely about its axis. If life is indeed found on Titan, it means that Titan-like habitable planets are likely to be very common and may even be more common than Earth-like habitable planets, given that M-dwarf stars vastly outnumber G-dwarf stars.

When considering the exploration of Titan, the only disadvantage is the large distance between Titan and Earth. Apart from that, everything else about Titan such as its low gravity, dense atmosphere, low radiation environment and calm low altitude winds are advantages compared to the exploration of other places in the solar system. For future missions to Titan, the challenges associated with finding an exotic form of life that is biochemically distinct from life on Earth cannot be underestimated. If life is found on Titan, such life would have an origin that is independent from life on Earth and will be proof of life’s cosmic ubiquity.

References:

1. J.I. Lunine (2009), “Saturn’s Titan: A Strict Test for Life’s Cosmic Ubiquity”, arXiv:0908.0762 [astro-ph.EP]

2. Worlds Like Titan (http://beyondearthlyskies.blogspot.sg/2011/04/worlds-like-titan.html), April 2011

3. Titan Mare Explorer (TiME) (http://beyondearthlyskies.blogspot.sg/2012/02/sailing-titan-seas.html), February 2012

4. Aerial Vehicle for In-situ and Airborne Titan Reconnaissance (AVIATR) (http://beyondearthlyskies.blogspot.sg/2012/03/titan-airplane-mission-concept.html), March 2012

Mining the Asteroids

Near-Earth asteroids (NEAs) are asteroids that pass near to Earth and they provide attractive targets for the extraction of resources to support the expansion of activities in space. It is estimated that there are about 1000 NEAs larger than 1 km diameter and about 100,000 NEAs larger than 140 m diameter. For as many as 60 percent of the NEAs, the amount of energy required to make a round trip from the Earth to a NEA is less than a round trip from the Earth to the surface of the Moon. A 1 km diameter metallic asteroid is expected to contain a few times more platinum group metals (PGMs) than all that has been mined on Earth.

The large distance between the Earth and an asteroid means that a round-trip time for communications can be several minutes or more and transporting the extracted materials back to Earth can take up to a few years. As a result, one good strategy before mining a selected asteroid is to capture the asteroid and transport it into a stable orbit around the Earth. Having an asteroid in orbit around the Earth makes the resources on the asteroid a lot more accessible and effectively removes the problem of round-trip time for communications. A spacecraft with a solar-electric propulsion system can be used to haul in an asteroid since such a propulsion system contains many times more propulsive energy per unit mass than chemical propulsion systems. The strategy of capturing a NEA is likely to occur in 2 stages. Stage 1 involves transporting the asteroid until it is sufficiently close to the Earth for it to rendezvous with the Earth’s gravitational sphere of influence. Stage 2 involves placing the asteroid into a stable orbit around the Earth. With proper timing, a gravitational flyby with the Moon can be used to slow an incoming asteroid into a stable orbit around the Earth.

Materials extracted from an asteroid can have many useful applications both in space and back on Earth. Volatiles such as water can be extracted for use as rocket fuel and for biological uses. Precious metals such as gold, platinum and PGMs are in demand in the manufacturing of numerous products such as fuel cells, high-end electronics, catalytic converters and advanced medicines. These precious metals are vital to our advancing technological civilization but they are extremely rare on Earth because very little remains in the Earth’s crust as most of them have sank into the Earth’s core. However, asteroids are pristine objects that have undergone very little modifications. For this reason, a typical chunk of asteroid material is expected to have hundreds of times greater concentration of precious metals than in the Earth’s crust and tens of times greater concentration than in the richest mines on Earth. In addition to water and precious metals, other materials such as iron and aluminium can serve as useful materials for constructing structures in space. Even the leftover bulk material of the asteroid can serve as radiation shielding for long duration manned missions.

Mining an asteroid, especially one that has been transported to the vicinity of Earth reduces the need to launch large amount of materials from Earth to support the expansion of activities in space. Precious metals extracted from asteroids can reduce the environmental stresses that result from the mining of these metals on Earth as it creates another avenue of supply. One thing to note regarding the mining of precious metals from asteroids is that the profit margin needs to be high enough because an outside source of precious metals can lower prices sufficiently to doom the asteroid mining venture.

Torrential Rains on Titan

Figure 1: Bright tropospheric clouds on Titan. Credit: NASA/JPL/Space Science Institute.

The thick nitrogen atmosphere of Titan supports a methane hydrological cycle that is akin to Earth’s water cycle. Like the Earth, precipitation and storm activity appear to be quite common on Titan. On Titan’s surface, numerous fluvial features point towards a hydrology based on liquid methane. Much of Titan’s surface is kept wet by a light but persistent drizzle of liquid methane which forms an enduring component of Titan’s methane hydrological cycle. Data collected by the Huygens entry probe during the descend through Titan’s atmosphere in January 2005 suggests the presence of an upper cloud layer of methane ice between 20 km to 30 km and a lower cloud layer of liquid methane-nitrogen between roughly 8 km to 16 km. The upper cloud layer of methane ice is akin to terrestrial cirrostratus clouds while the lower cloud layer of liquid methane-nitrogen is akin to terrestrial stratiform clouds. A gap between the upper and lower cloud layers exists because that region is too cold to sustain liquid clouds, but slightly too dry for pure methane to condense.

Since the upper cloud layer of methane ice is at saturation (100 percent relative humidity), methane ice crystals can grow there and eventually precipitate out. As the methane ice crystals descend down the atmosphere, they warm up and melt before reaching the surface as a light drizzle. The supply of methane for the lower cloud layer upon comes from the melting of falling methane ice crystals. Once melted, the methane droplets also allow nitrogen in the atmosphere to dissolve in them and become droplets of liquid methane with dissolved nitrogen. As the drizzle of methane-nitrogen droplets fall towards the ground, they become less enrich in nitrogen due to the preferential evaporation of dissolved nitrogen. This basic cloud structure is believed to be widespread and represents more than half of Titan’s surface. The low precipitation rate of such a drizzle means that its minuscule erosive potential cannot explain the widespread fluvial features on Titan. However, it can easily account for the generally wet character of the surface material at the Huygens landing site and possibly on most of Titan.

Figure 2: Artist’s impression of a storm on Titan. Credit: Mark A. Garlick.

Precipitation events with strong erosive potentials are required to form the widespread fluvial features on Titan and such intense precipitation events do occur on Titan. Remote observations of Titan have revealed the presence of short-lived tropospheric clouds. These clouds are likely to be convective storms that bring significant amounts of methane precipitation. Three dimensional models show that methane convection storms accompanied by heavy precipitation can occur on Titan under favourable conditions. Such favourable conditions include the presence of aerosol particles that act as cloud condensation nuclei, updrafts provided by the Hadley cell convergence or forced lifting of air masses from topographic features.

When the relatively humidity is over 80 percent, a small temperature perturbation of just 0.5 K can trigger convection, causing methane in the air mass to condense. Latent heat released from the condensation of methane powers strong updrafts with speeds of up to 20 m/s. This establishes a convective cell with cloud tops attaining altitudes of up to 30 km. Within the convective cell, the condensed methane rains out over a period of 4 to 8 hours. During such a torrential downpour, the accumulated precipitation on the surface can be as high as 110 kg/m², comparable to severe storms on Earth. These three dimensional models show that intense downpours of methane can occur on Titan under the right environmental conditions and have the erosive power necessary to create the fluvial features on Titan’s surface. Such intense downpours are expected to leave signatures in the form of temporary liquid deposits and mild surface cooling. More observations of the cloud activity on Titan will provide a better understanding of the role convection plays in Titan’s methane hydrological cycle.

References:

1. Tetsuya Tokano, et al., “Methane drizzle on Titan”, Nature 442, 432-435 (27 July 2006)

2. R. Hueso1 and A. Sánchez-Lavega, “Methane storms on Saturn’s moon Titan”, Nature 442, 428-431 (27 July 2006)

Cloud Decks of Gas Giant Planets

Figure 1: Artist’s impression of a gas giant planet.

Brown dwarfs and gas giant planets such as Jupiter and Saturn have hydrogen-helium dominated atmospheres that are very different from the atmospheres of Earth-like planets. In the hydrogen-helium dominated atmospheres of brown dwarfs and gas giant planets, a wide variety of molecular species can condense to form clouds. This is expected to produce multiple cloud layers in the atmosphere, where each cloud layer is made up of a particular type of condensable species. By comparison, only water clouds produced from the condensation of water exist in the atmospheres of Earth-like planets.

Gas giant planets are known to exist over a wide range of distances from their parent stars, from “star-hugging” hot-Jupiters to isolated Jupiter-mass planets in the frigid depths of interstellar space. Also, depending on its age, a gas giant planet is much hotter when it is young and it will gradually cool over billions of years. As a result, gas giant planets can have a wide range of atmospheric temperatures. The atmospheric temperature of a gas giant planet strongly influences the number of cloud layers it has and the position of its cloud layers. With decreasing atmospheric temperatures, the more refractory cloud layers form at progressively greater depths in the planet’s atmosphere and cloud layers composed of more volatile condensable species become present at the upper portions of the atmosphere.

Figure 2: Illustration of cloud layers in the hydrogen-helium dominated atmospheres of gas giant planets. The three panels correspond roughly to effective temperatures of approximately 120 K (Jupiter-like, left), 600 K (middle) and 1300 K (right).

When a condensable species forms a cloud layer, the condensate is removed from the cooler overlying atmosphere and is no longer available to react with other molecular species higher up in the atmosphere. For example, the detection of hydrogen sulphide (H₂S) in Jupiter’s atmosphere by NASA’s Galileo entry probe indicates that iron (Fe) must be sequestered into a cloud layer much deeper in the planet’s atmosphere because the presence of Fe will lead to the formation of iron sulphide (FeS) instead of H₂S. In another example, the detection of monatomic potassium (K) in the atmospheres of some brown dwarfs suggests that rock-forming elements such as aluminium (Al) and silicon (Si) are removed from the atmosphere due to cloud formation deeper down in the atmosphere. If Al and Si were not removed, the potassium would have condensed into silicate minerals such as orthoclase (KAlSi₃O₈) which would remove the presence of monatomic potassium from the observable atmosphere of the brown dwarf.

In the atmosphere of Jupiter, the highest clouds are composed of a cirrus-like layer of ammonia (NH₃) ice crystals. Further down into the atmosphere are ammonium hydrosulphide (NH₄SH) and water (H₂O) cloud layers. The troposphere of Jupiter’s atmosphere is commonly defined from the 0.1 bar level (approximately 50 km above 1 bar level) down to the 10 bar level (approximately 90 km below 1 bar level). At the top of the troposphere, the temperature is about 110 K and at the bottom of the troposphere, the temperature is about 340 K. The NH₃ (0.6 to 0.9 bar), NH₄SH (1 to 2 bar) and H₂O (3 to 7 bar) cloud layers are all located within the troposphere of Jupiter. Although methane (CH₄) also exists in the atmosphere of Jupiter, the temperatures are too warm for it to condense to form clouds.

Deeper down into Jupiter’s atmosphere are alkali, halide and sulphide cloud layers. These are then followed by silicate and iron cloud layers at increasing atmospheric pressures and temperatures. Finally, the deepest cloud layers consist of refractory species such as perovskite (CaTiO₃) and aluminium oxide (Al₂O₃) crystals. In fact, in the atmospheres of the hottest gas giant planets, only cloud layers consisting of refractory species exist in their atmospheres because the temperatures are too high for other molecular species to condense. As a result, the atmospheres of the hottest gas giant planets resemble the deepest regions of Jupiter’s atmosphere.

References:

1. Visscher et al. (2010), “Atmospheric Chemistry in Giant Planets, Brown Dwarfs, and Low-Mass Dwarf Stars III. Iron, Magnesium, and Silicon”, arXiv:1001.3639 [astro-ph.EP]

2. Marley et al. (2013), “Clouds and Hazes in Exoplanet Atmospheres”, arXiv:1301.5627 [astro-ph.EP]

3. Loadders et al. (2006), “Chemistry of Low Mass Substellar Objects”, arXiv:astro-ph/0601381

Stratospheres on Hot-Jupiters

Figure 1: Artist’s impression of HD 209458b - a hot-Jupiter. Credit: NASA, ESA, and G. Bacon (STScI)

Hot-Jupiters are a class of extrasolar planets that have similar characteristics to Jupiter but unlike Jupiter, they orbit very close to their parent stars. As a result, hot-Jupiters exhibit high atmospheric temperatures ranging from hundreds to thousands of Kelvin. Additionally, hot-Jupiters are expected to be tidally-locked with respect to their parent stars and this means that a hot-Jupiter always presents the same hemisphere towards its parent star, resulting in a permanent dayside and nightside hemisphere. This establishes a large temperature contrast between the dayside and nightside. Condensable species such as titanium oxide (TiO) and silicates that are stable in the gas phase on the dayside may condense into particles on the much cooler nightside and gravitationally settle there. In the absence of vertical mixing in the atmosphere, condensation on the nightside can deplete the atmosphere of these condensable species. In this regard, the nightside can serve as a cold trap by depleting the atmosphere of these condensable species.

Observations of some transiting hot-Jupiters have revealed the presence of stratospheres on the daysides of these planets. A stratosphere is a layer in a planet’s atmosphere where the temperature increases as altitude increases (thermal inversion). In the atmosphere of a hot-Jupiter, a stratosphere is thought to result from the absorption of starlight by strong visible/ultraviolet absorbers and gaseous TiO is a good candidate for such an absorber. The presence of the nightside cold trap on a hot-Jupiter brings up the question of whether such a cold trap is effective enough to deplete the atmosphere of TiO and prevent the formation of a stratosphere. This is investigated by using 3D global circulation models of HD 209458b - one of the best studied hot-Jupiter which is also though to have a stratosphere and a large day-night temperature contrast.

Figure 2: Temperature and horizontal winds (arrows) in the global circulation model of the hot-Jupiter HD 209458b. The top three panels show the temperature and flow at three different pressures (0.1 millibar, 1 millibar and 10 millibar). The substellar point is located at longitude 0 degrees and latitude 0 degrees while the dayside is between the longitudes of -90 degrees and +90 degrees.

Based on the global circulation models of HD 209458b, a stratosphere is clearly visible on the dayside of the planet at altitudes above the 10 millibar level due to strong absorption by TiO in the atmosphere. Furthermore, at altitudes above the 10 millibar level, temperatures reach 2200 Kelvin around the substellar point on the planet and the day-night temperature contrast is as large as 1600 Kelvin. The large temperature contrast is due to the short radiative timescales at low atmospheric pressures. Centred along the equator of HD 209458b is a strong eastward superrotating jet. A pair of convergence/divergence shock-like features can be seen along the superrotating jet. The convergence/divergence feature located about 40 degrees west of the substellar point creates a region of strong upward flow while the convergence/divergence feature located about 150 degrees east of the substellar point creates a region of strong downward flow.

As shown from the global circulation models of HD 209458b, large scale circulation in the atmosphere naturally creates vertical flows that are strong enough to keep a condensable species such as TiO aloft if it condenses into particles no larger than a few microns on the planet’s nightside. If TiO is able to condense into particles larger than a few microns, then the nightside cold trap will be efficient enough to deplete TiO from the atmosphere. However, if TiO is unable to condense into particles larger than a few microns, there will be sufficient TiO on the planet’s dayside for a TiO-induced stratosphere to be present. Due to the relatively small abundance of TiO in the atmosphere, condensing TiO into particles larger than a few microns is difficult. However, if TiO can be efficiently incorporated into other more abundant condensable species such as silicates, then it can condense into larger particles. In such a scenario, the nightside cold trap can be effective enough to prevent the formation of a TiO-induced stratosphere on the planet’s dayside.

Reference: Vivien Parmentier, Adam P. Showman and Yuan Lian (2013), “3D mixing in hot Jupiter atmospheres I: application to the day/night cold trap in HD 209458b”, arXiv:1301.4522 [astro-ph.EP]

Characterizing Habitable Earth-like Planets

A major goal in the study of exoplanets is the detection and characterization of Earth-like planets in the Habitable Zones of nearby stars. However, direct imaging of exoplanets is extremely difficult because a star is typically billions of times brighter than the orbiting planet and from a distance of several light-years, an exoplanet will appear just tens of milli-arcseconds from its host star. This makes it almost impossible to directly image an exoplanet since it will be lost in the star’s overwhelming glare. A proposed mission known as the New Worlds Observer (NWO) will allow the direct detection and characterization of Earth-like planets around stars. NWO consists of a 50 meter diameter starshade placed 80,000 kilometres in front of a 4 meter aperture space telescope. The starshade suppresses the starlight by a factor of several billion and allows planets orbiting the star to be imaged by the telescope.

The starshade is attached to a spacecraft whose main purpose is to move the starshade to block different target stars and maintain precise alignment with the telescope during observations. To align with each target star, the starshade is expected to travel thousands of kilometres. For such retargeting manoeuvrers between target stars, the spacecraft uses a solar-powered ion propulsion system to move the starshade. This method of propulsion is selected to allow for the largest number of target stars with a given amount of propellant mass. During retargeting manoeuvrers by the starshade which can account for up to 70 percent of the telescope observing time, the telescope will be dedicated to general astrophysics. Typical starshade travel time between target stars is 5 to 10 days and typical observation durations at each target star ranges from 24 hours for imaging to 14 days for detailed spectroscopic observations. The planned mission duration for NWO is 5 years with the goal for an extended mission of an additional five years. Both the sunshade and telescope will be placed at the Sun-Earth L2 point, which is a low-acceleration environment in space. To prevent sunlight from illuminating the telescope-facing side of the starshade, the starshade will be tilted so that the telescope-facing side will always remain dark.

NWO can observe an entire planetary system around a target star at once. After suppressing light from a target star, NWO can image every planet from the Habitable Zone outward in a span of just several hours. Although the primary goal of NWO is to detect and characterize Earth-like planets in the Habitable Zones of nearby stars, it can also image and characterize planets beyond the Habitable Zone such as Jupiter-like and Neptune-like planets in long-period orbits. As a planet rotates, photometric observations by NWO will show variations in colour and intensity as different surface features rotate in and out of the telescope’s field-of-view. For Earth-like planets, this allows for the detection of surface features such as oceans, continents, polar ice-caps and clouds.

Spectroscopic observations of an Earth-like planet in the Habitable Zone of a target star can quickly reveal information about the planet’s atmospheric composition, surface conditions and even the presence of life. The spectrum of an Earth-analogue exoplanet observed by NWO for just several hours will reveal a brightening at short wavelengths that is indicative of Rayleigh scattering which accounts for the blue sky we see here on Earth. At longer wavelengths, the presence of molecular oxygen will produce two moderate-strength absorption features at the 0.76 and 1.26 micrometer wavelengths. Molecular oxygen is a key biosignature since it is chemically reactive in the Earth’s atmosphere and must be continuously replenished by the biosphere. The presence of water also produces six strong absorption features (0.72, 0.82, 0.94, 1.13, 1.41 and 1.88 micrometers) which get dramatically stronger towards longer wavelengths. NWO will allow astronomers to easily detect Earth-sized planets and image entire solar systems. This mission may prove to be the quickest and most affordable path to the discovery of life on other planets.

Searching for Nearby Earths

A complete census of planetary systems around Sun-like stars (FGK dwarfs) in the neighbourhood of the Sun out to a distance of 50 light years is highly desirable and will be a major milestone in the search for exoplanets. This can be achieved with a proposed mission called the Nearby Earth Astrometric Telescope (NEAT) which is sensitive enough to detect Earth-mass planets within the habitable zones of these nearby stars. NEAT is a space telescope that will carry out an astrometric survey of the 200 closest F, G and K-type stars in the Sun’s neighbourhood. Such an astrometric survey involves precisely measuring the position of a star and observing tiny changes in the star’s position caused by the presence of planets orbiting around the star. To accomplish this, NEAT performs extremely high precision astrometric measurements down to the 0.05 micro-arcsecond accuracy level.

The proposed mission architecture for NEAT consists of a pair of satellites flying in formation with one satellite being the telescope spacecraft and the other being the detector spacecraft. Both satellites are positioned precisely 40 metres apart since the telescope spacecraft consists of a one metre diameter parabolic mirror whose focal plane is located 40 metres away. The detector spacecraft will be at the focal plane where it will have ten 512 × 512 CCDs. Eight of the CCDs can be moved in the X and Y directions on the focal plane to image the reference stars while the central two CCDs are fixed in position - one CCD to image the target star and the other CCD to track the telescope’s axis. The use of 10 small CCDs instead of a billion-pixel focal plane dramatically reduces the mission cost.

Data obtained by NEAT will allow follow-up observations of planetary systems to be scheduled when the configuration of the planetary system is most favourable. In addition to detecting planets down to one Earth-mass around nearby stars, NEAT can also detect Jupiter-sized planets in long orbital periods around those planetary systems already discovered by the Kepler space telescope. Since the F, G and K-type stars in the Sun’s neighbourhood are all visible with unaided eyes or with simple binocular, the discovery of Earth-mass planets around these stars will dramatically change humanity’s view of the night sky.

Capturing an Asteroid

In April 2012, a report was published which investigated the feasibility of sending a robotic spacecraft to capture and haul back an entire Near-Earth Asteroid (NEA) to the vicinity of the Earth by the mid-2020s. The expected size of an asteroid that can be retrieved from such a mission depends on the overlap between the smallest NEAs that can be detected and characterized, and the largest NEAs that can be captured and brought back. This overlap appears to centre on NEAs roughly 7 m in diameter. Depending on its bulk density, a NEA with a diameter of roughly 7 m is expected to have a mass of about 500,000 kg. In comparison, the Apollo program brought back 382 kg of Moon rocks from six surface missions.

Capturing a 500 ton asteroid and placing it in high lunar orbit will provide an interesting and affordable destination for future astronauts since NASA expects a human presence in cislunar space to be well established by the mid-2020s. Having an asteroid in high lunar orbit provides a convenient test bed for new technologies and operational experience that will benefit future manned missions to larger NEAs. Returning samples from the captured asteroid will be easier since the flight time to and from high lunar orbit will be a lot shorter than a full-fledged mission into deep space to retrieve samples from even the most accessible NEAs. Finally, the extraction of water and other resources from the captured asteroid allows the asteroid to serve as a source of raw materials for manned deep space missions.

Figure 1: Illustration of an asteroid retrieval spacecraft in the process of capturing a 7 m, 500 ton asteroid. (Image Credit: Rick Sternbach / KISS)

The spacecraft proposed for this asteroid retrieval mission involves a robotic vehicle that is propelled by a 40-kilowatt solar-electric propulsion (SEP) system. At launch, the initial mass of the spacecraft is 18,000 kg and 12,000 kg of the initial mass is in the form of xenon propellant for the SEP system. The SEP system consists of five 10-kilowatt Hall thrusters that are powered by two solar array wings, each with a collecting area of 71 square meters. The spacecraft’s asteroid capture mechanism consists of a large inflatable bag measuring 10 metres by 15 metres that will deploy and envelope the asteroid.

The mission to retrieve an asteroid will be a rather long duration one. An Atlas V rocket will launch the spacecraft to low-Earth orbit (LEO). Once in LEO, the spacecraft will gradually spiral out to the Moon over a period of 2.2 years. Near the Moon, the spacecraft will undergo a lunar gravity assist followed by a 1.7 years cruise to the target asteroid. After rendezvous with the asteroid, the spacecraft will undergo 90 days of asteroid operations where it will capture and de-spin the asteroid. Finally, another 2 to 6 years is required to transport the captured asteroid to high lunar orbit. Hauling back a more massive asteroid will require a longer flight time. As a result, the total mission duration is expected to be between 6 to 10 years.

Figure 2: Asteroid return mission concept. Return flight time of 2 to 6 years depending on the asteroid mass.

NASA is already considering such an asteroid retrieval mission which is estimated to cost $2.6 billion - slightly more than NASA’s Curiosity Mars rover. The potential benefits of using a robotic spacecraft to snag and haul back a NEA to high lunar orbit for further study and exploration are expected to be huge. The team involved in this feasibility study mentioned: “Placing a NEA in lunar orbit would provide a new capability for human exploration not seen since Apollo. Such an achievement has the potential to inspire a nation. It would be mankind’s first attempt at modifying the heavens to enable the permanent settlement of humans in space.”

Accretion-Induced Collapse

When the mass of an accreting white dwarf grows towards the Chandrasekhar limit, the white dwarf eventually becomes unstable and destroys itself as a Type Ia supernova. In some cases, a white dwarf may not explode as a Type Ia supernova but instead undergoes a process known as an accretion-induced collapse to form a rapidly rotating neutron star. Accretion-induced collapse can occur for accreting oxygen-neon-magnesium white dwarfs or from the merger of carbon-oxygen white dwarfs. The occurrence rate for accretion-induced collapse is expected to be less than one percent of the occurrence rate of Type Ia supernova. No accretion-induced collapse event has yet been directly observed.

During an accretion-induced collapse event, a white dwarf collapses into a rapidly rotating neutron star. The ejected mass from such an event is expected to be small (less than 0.1 solar masses) and travelling at high velocity (up to about 10 percent speed of light). As such, the amount of emitted radiation will be orders of magnitude less than a typical supernova. Furthermore, the expected duration of observable optical radiation being emitted from such an event is expected to last for only a day or so.

As the white dwarf collapses, conservation of angular momentum and amplification of the magnetic field leads to the creation of a rapidly spinning magnetar. A magnetar is a type of neutron star with an exceptionally strong magnetic field. The spin-down of the newly formed magnetar powers a pulsar wind nebula which injects energy in the form of magnetic fields and relativistic particles into the ejecta surrounding it. Because the spin-down energy of the magnetar is so much greater than the initial kinetic energy of the ejecta, it is the spin-down energy that determines the ejecta speed. As the ejecta expand outwards, the low-density pulsar wind nebula pushing up against the high-density ejecta can lead to the development of Rayleigh-Taylor instabilities. The ejecta plough through the interstellar medium and will begin to decelerate after it has swept up a mass comparable to its own.

Reference: Anthony L. Piro and S. R. Kulkarni (2013), “Radio Transients from the Accretion-induced Collapse of White Dwarfs”, The Astrophysical Journal Letters Volume 762 Number 2