Earth-like Orbit Research Articles

A resonant family of dynamically cold small bodies in the near-Earth asteroid belt

Abstract Near-Earth objects (NEOs) moving in resonant, Earth-like orbits are potentially important. On the positive side, they are the ideal targets for robotic and human low-cost sample return missions and a much cheaper alternative to using the Moon as an astronomical observatory. On the negative side and even if small in size (2–50 m), they have an enhanced probability of colliding with the Earth causing local but still significant property damage and loss of life. Here, we show that the recently discovered asteroid 2013 BS45 is an Earth co-orbital, the sixth horseshoe librator to our planet. In contrast with other Earth's co-orbitals, its orbit is strikingly similar to that of the Earth yet at an absolute magnitude of 25.8, an artificial origin seems implausible. The study of the dynamics of 2013 BS45 coupled with the analysis of NEO data show that it is one of the largest and most stable members of a previously undiscussed dynamically cold group of small NEOs experiencing repeated trappings in the 1:1 commensurability with the Earth. This new resonant family is well constrained in orbital parameter space and it includes at least 10 other transient members: 2003 YN107, 2006 JY26, 2009 SH2 and 2012 FC71 among them. 2012 FC71 represents the best of both worlds as it is locked in a Kozai resonance and is unlikely to impact the Earth. These objects are not primordial and may have originated within the Venus–Earth–Mars region or in the main-belt, then transition to Amor-class asteroid before entering Earth's co-orbital region. Objects in this group could be responsible for the production of Earth's transient irregular natural satellites.

DEPLOYMENT AND SIMULATION OF THE ASTROD-GW FORMATION

Constellation or formation flying is a common concept in space Gravitational Wave (GW) mission proposals for the required interferometry implementation. The spacecraft of most of these mission proposals go to deep space and many have Earthlike orbits around the Sun. Astrodynamical Space Test of Relativity using Optical Devices optimized for Gravitation Wave detection (ASTROD-GW), Big Bang Observer (BBO) and DECIGO have spacecraft distributed in Earthlike orbits in formation. The deployment of orbit formation is an important issue for these missions. ASTROD-GW is to focus on the goal of detection of GWs. The mission orbits of the three spacecraft forming a nearly equilateral triangular array are chosen to be near the Sun–Earth Lagrange points L3, L4 and L5. The three spacecraft range interferometrically with one another with arm length about 260 million kilometers with the scientific goals including detection of GWs from Massive Black Holes (MBH) and Extreme-Mass-Ratio Black Hole Inspirals (EMRI), and using these observations to find the evolution of the equation of state of dark energy and to explore the co-evolution of MBH with galaxies. In this paper, we review the formation flying for fundamental physics missions, design the preliminary transfer orbits of the ASTROD-GW spacecraft from the separations of the launch vehicles to the mission orbits, and simulate the arm lengths of the triangular formation. From our study, the optimal delta-Vs and propellant ratios of the transfer orbits could be within about 2.5 km/s and 0.55, respectively. From the simulation of the formation for 10 years, the arm lengths of the formation vary in the range 1.73210 ± 0.00015 AU with the arm length differences varying in the range ±0.00025 AU for formation with 1° inclination to the ecliptic plane. This meets the measurement requirements. Further studies on the optimizations of deployment and orbit configurations for a period of 20 years and with inclinations between 1° to 3° are currently ongoing.

Last chance for a transit

TransiT June’s transit of Venus offers the last opportunity for a lifetime to watch an astronomical event that comes around twice every century or so. The transit is a relatively simple geometrical coincidence that is easy to understand, yet has complex and significant implications for historical and current astronomy. Transits allowed astronomers to take the first steps on the cosmic distance ladder, using parallax to measure some of the dimensions of the solar system. Historically, transits of Venus have triggered scientific thinking and launched scientific expeditions that discovered far more than astronomical data. The transit of 1639 was observed in Much Hoole, Lancashire, by Jeremiah Horrocks, possibly from the window of Carr House, as Kevin Kilburn and Clive Elphick discuss in this issue. And Captain Cook’s expedition to observe the 1769 transit resulted in the discovery – by the western world – of Australia, with significant and damaging legal consequences for its Aboriginal peoples, discussed in these pages by Ragbir Bhathal. Transits now have significance for exploration in the search for planets in other solar systems. The transit method sounds simple – using the dimming of light from a star as a planet passes in front of it to deduce orbital parameters and size of an exoplanet – but seeing the size of Venus as it crosses the disc of our Sun does highlight the precision needed to detect Earth-sized planets on Earth-like orbits. And it makes the fact that we have the technology to detect so many planets in this way, inspirational. Unfortunately, only the last hour or so of the transit will be visible from the UK in the early hours of 6 June this year, but if you are prepared to be up with the Sun, you may see Venus in silhouette for yourself. Good weather will help, of course, as will being in the north of the UK. Have a look at the transit pages on the RAS website or look online for transit times and local events where you live. EdiTorial nEws

A spectrograph for exoplanet observations calibrated at the centimetre-per-second level

The best spectrographs are limited in stability by their calibration light source. Laser frequency combs are the ideal calibrators for astronomical spectrographs. They emit a spectrum of lines that are equally spaced in frequency and that are as accurate and stable as the atomic clock relative to which the comb is stabilized. Absolute calibration provides the radial velocity of an astronomical object relative to the observer (on Earth). For the detection of Earth-mass exoplanets in Earth-like orbits around solar-type stars, or of cosmic acceleration, the observable is a tiny velocity change of less than 10 cm s(-1), where the repeatability of the calibration--the variation in stability across observations--is important. Hitherto, only laboratory systems or spectrograph calibrations of limited performance have been demonstrated. Here we report the calibration of an astronomical spectrograph with a short-term Doppler shift repeatability of 2.5 cm s(-1), and use it to monitor the star HD 75289 and recompute the orbit of its planet. This repeatability should make it possible to detect Earth-like planets in the habitable zone of star or even to measure the cosmic acceleration directly.

The search for life on Earth and other planets

As the NASA rover Curiosity approaches Mars on its quest to look for signs of past or present life there and sophisticated instruments like the space telescopes Kepler and CoRoT keep discovering additional, more Earth-like planets orbiting distant stars, science faces the question of how to spot life on other planets. Even here on Earth biotopes remain to be discovered and explored.

Delta-v requirements for earth co-orbital rendezvous missions

Earth co-orbital asteroids present advantages as potential targets for future asteroid rendezvous missions. Their prolonged proximity to Earth facilitates communication, while their Earth-like orbits mean a steady flux of solar power and no significant periodic heating and cooling of the spacecraft throughout the course of the mission. Theoretical studies show that low-inclination co-orbital orbits are more stable than high-inclination orbits. As inclination is the most significant indicator of low delta-v rendezvous orbits, there is the potential for a large population of easily accessible asteroids, with favorable engineering requirements. This study first looks at phase-independent rendezvous orbits to a large number of objects, then looks in more detail at the phase-dependent orbits to the most favorable objects. While rendezvous orbits to co-orbital objects do not have a low delta-v necessarily, some objects present energy requirements significantly less than previous rendezvous missions. Currently we find no ideal co-orbital asteroids for rendezvous missions, although theoretical Earth Trojans present very low-energy requirements for rendezvous.

Astro-comb: revolutionizing precision spectroscopy in astrophysics

AbstractSearches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision better than 60cm/s. To find a 1-Earth mass planet in an Earth-like orbit, a precision of 5cm/s is necessary. The combination of a laser frequency comb with a Fabry-Perot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration. Here we report the fabrication of such a filtered laser comb with up to 40 GHz (~1 Angstrom) line spacing, generated from a 1 GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb (astro-comb) is well matched to the resolving power of high-resolution astrophysical spectrographs. The astrocomb should allow a precision as high as 1cm/s in astronomical readial velocity measurements.

Asteroids on Earth-like orbits and their origin

The orbit of 1991 VG and a set of other asteroids whose orbits are very similar to that of the Earth have been examined. Its origin has been speculated to be a returning spacecraft, lunar ejecta or a low-inclination Amor- or Apollo-class object. The latter is arguably the more likely source, which has been investigated here. The impact probability for these objects has been calculated, and while it is larger than that of a typical near-Earth asteroid (NEA), it is still less than 1:200 000 over the next 5000 yr. In addition, the probability of an NEA ever ending up on an Earth-like orbit has been obtained from numerical simulations and turned out to be about 1:20 000, making this a rare class of objects. The typical time spent in this state is about 10 000 yr, much less than the typical NEA lifetime of 10 Myr.

A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s-1

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s(-1) (ref. 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of approximately 5 cm s(-1) is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (approximately 1-A) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s(-1) in astronomical radial velocity measurements.

LISA — An ESA cornerstone mission for the detection and observation of gravitational waves

The primary objective of the Laser Interferometer Space Antenna (LISA) is to detect and observe gravitational waves from massive black holes, galactive binary stars, and violent events in the Universe in a frequency range from 10 −4 to 10 −1 Hz which is totally inaccessible to ground based experiments. It uses highly stabilised laser light (Nd: YAG, λ = 1.064 μm) in a Michelson-type interferometer arrangement. A cluster of six spacecraft with two at each vertex of an equilateral triangle is placed in an Earth-like orbit at a distance of 1 AU from the Sun, and 20° behind the Earth. Three subsets of four adjacent spacecraft each form an interferometer comprising a central station, consisting of two relatively adjacent spacecraft (200 km apart), and two spacecraft placed at a distance of 5 × 10 6 km from the centre to form arms which make an angle of 60° with each other. Each spacecraft is equipped with a laser. A descoped LISA with only four spacecraft has undergone an ESA assessment study in the M3 cycle, and the full 6-spacecraft LISA mission has now been selected as a cornerstone mission in the ESA Horizons 2000 programme. The LISA Assessment Report is available as ESA document SCI(94)6, May 1994. Detailed information on the LISA cornerstone mission is contained in the LISA Pre-Phase A Report, available as MPQ Report MPQ 208 (1996) from the Max-Planck-Institut für Quantenoptik.

Discovery of an asteroid and quasi‐satellite in an Earth‐like horseshoe orbit

Abstract— The newly discovered asteroid 2002 AA29 moves in a very Earth‐like orbit that relative to Earth has a unique horseshoe shape and allows transitions to a quasi‐satellite state. This is the first body known to be in a simple heliocentric horseshoe orbit, moving along its parent planet's orbit. It is similarly also the first true co‐orbital object of Earth, since other asteroids in 1:1 resonance with Earth have orbits very dissimilar from that of our planet. When a quasi‐satellite, it remains within 0.2 AU of the Earth for several decades. 2002 AA29 is the first asteroid known to exhibit this behavior. 2002 AA29 introduces an important new class of objects offering potential targets for space missions and clues to asteroid orbit transfer evolution.

The planet search program at the ESO Coudé Echelle spectrometer

We present the complete results of the planet search program carried out at the ESO Coudé Echelle Spectrometer (CES) on La Silla, using the Long Camera from Nov. 1992 to April 1998. The CES survey has monitored 37 late-type (F8V – M5V) stars in the southern hemisphere for variations in their differential radial velocities (RV) in order to detect Doppler reflex motions caused by planetary companions. This led to the discovery of the first extrasolar planet in an Earth-like orbit around the young (ZAMS) and active G0V star ι Horologii (Kürster et al. [CITE]). Here we present the RV results for all survey stars and perform a statistical examination of the whole data-set. Each star is tested for RV variability, RV trends (linear and non-linear) and significant periodic signals. β Hyi and ϵ Ind are identified as long-term, low-amplitude RV variables. Furthermore, for 30 CES survey stars we determine quantitative upper mass-limits for giant planets based on our long-term RV results. We find that the CES Long Camera survey would have detected short-period (“51 Peg-type”) planets around all 30 stars but no planets with at orbital separations larger than 2 AU. Finally, we demonstrate that the CES planet search can be continued without applying velocity corrections to the RV results coming from the currently installed Very Long Camera at the CES.

LISA - an ESA cornerstone mission for a gravitational wave observatory

The European Space Agency has selected LISA, a gravitational wave observatory, as a cornerstone mission in its future science program Horizons 2000. This observatory will complement the development of ground-based gravitational wave detectors currently under construction. A spaceborne detector will enable the observation of low-frequency gravitational waves in a frequency range from to which is totally inaccessible to ground-based experiments. This frequency range is unique in that it is expected to contain signals from massive black holes, galactive binary stars, as well as the most violent events in the Universe. LISA will attain this low-frequency sensitivity by employing laser interferometric distance measurements over a very long baseline of . Three of these baselines form an equilateral triangle with spacecraft at each vertex. The cluster of spacecraft is in an Earth-like orbit around the Sun trailing the Earth by . The spacecraft contain infrared light-emitting Nd:YAG lasers and freely floating test masses made from a special platinum - gold alloy with vanishing magnetic susceptibility. The spacecraft are being kept centred on their test masses by using drag-free technology and field-emission electric propulsion, thus letting the test masses follow purely inertial orbits.

An Asteroid in a Earth-Like Orbit

The dynamical evolution of an asteroid with orbital elements strikingly similar to the Earth is analysed. The object, 1991 VG, was discovered by the Spacewatch telescope during a particular encounter with the planet. 1991 VG experienced a temporary satellite capture by the Earth, a phenomena that is recurrent in its dynamical history. The possible origin of this puzzling object is discussed, including the suggestion that 1991 VG could be a piece of lunar ejecta after a great impact.

LISA: laser interferometer space antenna for gravitational wave measurements

LISA (laser interferometer space antenna) is designed to observe gravitational waves from violent events in the Universe in a frequency range from to which is totally inaccessible to ground-based experiments. It uses highly stabilized laser light (Nd:YAG, ) in a Michelson-type interferometer arrangement. A cluster of six spacecraft with two at each vertex of an equilateral triangle is placed in an Earth-like orbit at a distance of 1 AU from the Sun, and behind the Earth. Three subsets of four adjacent spacecraft each form an interferometer comprising a central station, consisting of two relatively adjacent spacecraft (200 km apart), and two spacecraft placed at a distance of from the centre to form arms which make an angle of with each other. Each spacecraft is equipped with a laser. A descoped LISA with only four spacecraft has undergone an ESA assessment study in the M3 cycle and the full six-spacecraft LISA mission has now been selected as a cornerstone mission in the ESA Horizon 2000-plus programme.

Satellite Parallaxes of Lensing Events toward the Galactic Bulge

In order to understand the nature of the lenses that generate microlensing events, one would like to measure their mass, distance, and velocity. Unfortunately, current microlensing experiments measure only one parameter of the events, the characteristic timescale, which is a combination of the underlying physical parameters. Other methods are required to extract additional information. Parallax measurements using a satellite in an Earth-like orbit yield the projected velocity of the lens: = v/(1 - z), where v is the transverse velocity (speed and direction) of the lens relative to the Earth-source line of sight, and z is the ratio of the distances to the lens and the source. A measurement of could distinguish between lenses belonging to the bulge and disk populations. We show that for photometric precisions of 1%-2%, it is possible to measure the projected speed, , to an accuracy of ≤10% for over 70% of disk lenses and over 60% of bulge lenses. For measuring the projected velocity , the percentages are 40% and 30%, respectively. We find lines of sight greater than 2° away from the ecliptic are preferable, and an Earth-satellite separation in the range 0.7 AU-1.9 AU is optimal. The requirements of the satellite for measuring the projected velocities of events toward the bulge are similar to those for measurements toward the LMC.

Laser-interferometric gravitational wave detection — On Earth and in the Heavens

Worldwide activities in laser-interferometric detection of gravitational waves, so far concerned with building ever more sensitive prototypes, are beginning to bear fruit: several large ground-based interferometers have received funding, of which some are already under construction. The sensitivities achieved with prototypes so far, and the features of the large interferometer projects, are presented in tabular form. The regime of extremely low frequencies is not accessible with ground-based detectors, and it can be covered only with space-borne detectors. The project LISA, selected by ESA as one of the cornerstone space projects under the program Horizon 2000+, will deploy six spacecraft in Earth-like orbits, forming up to three interferometers with arm lengths of 5 million km. Launch is expected around the year 2015.

The Size and Shape of the Near-Earth Asteroid Belt

Evidence was recently reported for the existence of a near-Earth belt of small, Earth-approaching asteroids (SEAs) with diameters less than ∼50 m. This result was based upon observations made with the Spacewatch Telescope of the University of Arizona during the course of an ongoing search for Earth-approaching asteroids. Using a model to describe the effects of observational bias, it was shown that the orbits observed for SEAs are inconsistent with the orbits of Earth approachers larger than ∼1 km, and imply a relatively high fraction of Earth-like orbits among the SEAs. In this paper, new observations are included and the bias model is extended in order to quantify the number of SEAs within the near-Earth belt and to further constrain their orbital distribution. The calculation shows that relative to larger Earth approachers, SEAs are deficient in Aten-type orbits for which the semimajor axis is less than 1.0 AU. Instead, nearly all SEAs with aphelia less than 1.4 AU (5 ± 3% of the total population) have perihelia between 0.9 and 1.1 AU, thus defining a near-Earth belt. Those SEAs with aphelia > 1.4 AU, however, have a distribution of orbits that are indistinguishable from the orbits of larger Earth approachers. Taking the near-Earth belt into account does not significantly alter the previously determined enhancement in the number of SEAs compared to an extrapolation of the numbers of larger Earth approachers. At ∼10 m, the enhancement factor is 40 to within a factor of 2. Also, the RMS impact velocity of SEAs with Earth (17 km/sec) is nearly the same as for larger Earth approachers (18 km/sec).

5.1 Sources of Interplanetary Dust

AbstractAttention is centered on cosmic dust measures made by sensors on Pioneers 8 and 9 in Earth-like orbits. The conclusion follows Zook and Berg that the particles are largely “β-meteoroids,” interplanetary impact debris expelled by solar radiation pressure. An analysis of periodic comet orbits and comets observed during the missions failed to yield correlations, except possibly for debris from Comet Encke. A treatment of β-meteoroids from this stream is presented.