by Paul Gilster | Nov 14, 2019 | Missions |
We’re seeing our final images of asteroid Ryugu as the Hayabusa2 spacecraft leaves its orbit some 300 million kilometers from Earth. The Japanese Aerospace Exploration Agency (JAXA) intends to keep taking images of the receding Ryugu for several more days, after which it will be necessary to perform an attitude control maneuver to orient the craft for proper operation of its ion engines. An ion engine test period will culminate in cruise operations on December 3 to return the spacecraft to Earth.
Image: Asteroid Ryugu captured with the Optical Navigation Camera – Telescopic (ONC-T) immediately after departure. Image time is November 13 10:15 JST (onboard time), 2019. Credit: JAXA, Chiba Institute of Technology, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Meiji University, University of Aizu, AIST.
Happily, we are asked to join in JAXA’s ‘Goodbye Ryugu’ campaign by sending a #Sayonara_Ryugu tweet (https://twitter.com/haya2e_jaxa), although the agency also encourages old-fashioned cards and letters (as a collector of vintage fountain pens, I rather appreciate this). The address: Hayabusa2 Project, JAXA Institute of Space & Astronautical Science (ISAS), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara City, Kanagawa Prefecture, 252-5210, Japan.
Says JAXA: “5 years after launch and at the conclusion of about a year and a half of missions at Ryugu, we would love to hear your thoughts on Hayabusa2 and asteroid Ryugu.”
Departure images will be available here. As always, it’s exhilarating to see the crew at a spacecraft command center cheering, as they did after project manager Yuichi Tsuda confirmed the departure.
Now we think ahead to the sample return, scheduled for the end of 2020 when the Hayabusa2 re-entry capsule will be recovered at the Woomera Prohibited Area (WPA) located in the outback desert of South Australia. It’s a long way from there to here, but have a look at the candidate recovery site (image credit: JAXA). Final details are still being negotiated with the Australian government, according to Tsuda.
Remember that Hayabusa2 made two touchdowns on the asteroid, the first in February of this year, the second in July. We have surface material as well as underground samples to look forward to, and may learn valuable lessons about the distribution of carbon, organic matter and water in the Solar System. The underground materials, unaffected by radiation to which the surface was exposed, should offer insights into the formation of the Solar System some 4.6 billion years ago.
by Paul Gilster | Nov 13, 2019 | Exoplanetary Science |
The question of habitability on planets around M-dwarfs is compelling, and has been a preoccupation of mine ever since I began working on Centauri Dreams. After all, these dim red stars make up perhaps 75 percent of the stars in the galaxy (percentages vary, but the preponderance of M-dwarfs is clear). The problems of tidal lock, keeping one side of a planet always facing its star, and the potentially extreme radiation environment around young, flaring M-dwarfs have fueled an active debate about whether life could ever emerge here.
At Northwestern University, a team led by Howard Chen, in collaboration with researchers at the University of Colorado Boulder, NASA’s Virtual Planet Laboratory and the Massachusetts Institute of Technology, is tackling the problem by combining 3D climate modeling with atmospheric chemistry. The paper on this work, in press at the Astrophysical Journal, examines how general circulation models (GCM) have been able to simulate the large-scale circulation and climate system feedbacks on planets around red dwarfs, but these models have not accounted for atmospheric chemistry-driven interactions that the authors believe are critical for habitability. Thus so-called coupled chemistry-climate models (CCM) are needed to factor in how an atmosphere responds to the star’s radiation.
The study takes both ultraviolet radiation (UV) from the star and the rotation of the planet into consideration, noting how UV affects gases like water vapor and ozone. Says Chen:
“3D photochemistry plays a huge role because it provides heating or cooling, which can affect the thermodynamics and perhaps the atmospheric composition of a planetary system. These kinds of models have not really been used at all in the exoplanet literature studying rocky planets because they are so computationally expensive. Other photochemical models studying much larger planets, such as gas giants and hot Jupiters, already show that one cannot neglect chemistry when investigating climate.”
Image: An artist’s conception shows a hypothetical planet with two moons orbiting within the habitable zone of a red dwarf star. Credit: NASA/Harvard-Smithsonian Center for Astrophysics/D. Aguilar.
The researchers simulate the atmospheres of synchronously-rotating planets (i.e., with one side always facing the star) at the inner edge of the habitable zones of both K- and M-class stars. using numerical simulations of climate coupled with photochemistry and atmospheric chemistry through their 3D CCM. They find that the thin ozone layers produced on planets around active stars can render an otherwise habitable planet (in terms of surface temperatures) hazardous for complex life, as there is insufficient ozone to block UV radiation from reaching the surface.
Active photochemistry is a crucial issue, for according to Chen and team, planets can also lose significant amounts of water due to vaporization. Added to the ozone issue, we find boundaries beyond which a planet habitable in terms of liquid water on the surface is rendered lifeless. Understanding stellar activity becomes a predictive tool for gauging which M-dwarfs are most likely to merit precious telescope time for future missions looking for biosignatures. More active M-dwarfs appear far less likely to host life-bearing planets. From the paper:
…we find that only climates around active M-dwarfs enter the classical moist greenhouse regime, wherein hydrogen mixing ratios are sufficiently high such that water loss could evaporate the surface ocean within 5 Gyrs. For those around quiescent M-dwarfs, hydrogen mixing ratios do not exceed that of water vapor. As a consequence, we find that planets orbiting quiescent stars have much longer ocean survival timescales than those around active M-dwarfs. Thus, our results suggest that improved constraints on the UV activity of low-mass stars will be critical in understanding the long-term habitability of future discovered exoplanets (e.g., in the TESS sample…)
The effects of stellar UV radiation become a useful predictive tool as we narrow the target list. Vertical and horizontal winds in the upper atmosphere are strengthened as UV flux goes up. Moreover, the global distribution of ozone and hydrogen depends upon all these processes, which can affect the contrast between the dayside and nightside conditions under varying UV flux. The authors believe that only by bringing atmospheric chemistry into the picture of 3D modeling can we gauge whether a planet can attain true habitability and maintain it. Usefully, using their results, they show that both water vapor and ozone features could be detectable by instruments aboard the James Webb Space Telescope if we choose our targets carefully.
The paper is Chen et al., “Habitability and Spectroscopic Observability of Warm M-dwarf Exoplanets Evaluated with a 3D Chemistry-Climate Model,” in press at the Astrophysical JournaL (preprint).
by Paul Gilster | Nov 12, 2019 | Exoplanetary Science |
It’s good to see the European Space Agency’s ARIEL mission getting a bit more attention in the media. The Atmospheric Remote-sensing Infrared Exoplanet Large-survey was selected earlier this year as an ESA science mission, scheduled for launch in 2028. Here the goal is to cull a statistically large sample of exoplanets to examine their evolution in the context of their parent stars. Giovanna Tinetti (University College London) is principal investigator.
I would urge seeing ARIEL in the context of a different kind of evolution, that being the gradual growth in our technologies as we continue getting closer to studying the atmospheres of terrestrial-class worlds. For while ARIEL cannot achieve this feat — its focus is on exoplanets of Jupiter-mass down to super-Earths, all on close orbits, with temperatures greater than 320 Celsius — it leverages the fact that high temperature atmospheres keep their various interesting molecules in continual circulation, rather than letting them sink into obscuring layers of cloud. They are thus more easily detected and provide fodder for future work.
Image: Giovanna Tinetti (UCL), principal investigator for ARIEL.
The goal is to study hundreds of transiting exoplanets, looking at the spectra of their atmospheres as they pass in front of their host stars, allowing starlight to filter through the gaseous envelope for analysis. The light emitted by these atmospheres will also be analyzed just before and after the planets pass behind their primaries. Such transmission spectroscopy allows scientists to unlock the composition, temperature and chemical processes at work. No other spacecraft has been so tightly devoted to atmospheric analysis as ARIEL, and here we will be working with a large sample population in search of commonalities and differences. We go from just a few characterized atmospheres to hundreds.
I see that NASA is contributing fine guidance sensors in two photometric bands in an instrument called CASE — Contribution to ARIEL Spectroscopy of Exoplanets — which will observe clouds and hazes at near-infrared as well as visible wavelengths, complementing ARIEL’s other instrument, an infrared spectrometer that operates at longer wavelengths. It will be CASE that measures planetary albedo while examining how clouds influence the composition and other properties of the atmospheres under study. ARIEL should provide abundant insights into how future telescopes can home in on worlds much more like our own.
Image: This artist’s concept shows the European Space Agency’s ARIEL spacecraft on its way to Lagrange Point 2 (L2) – a gravitationally stable, Sun-centric orbit – where it will be shielded from the Sun and have a clear view of the sky. NASA’s JPL will manage the mission’s CASE instrument. Credit: ESA/STFC RAL Space/UCL/Europlanet-Science Office.
Remember that while we await the launch of the James Webb Space Telescope, JWST is by no means a dedicated exoplanet mission, though it will work with a small sample of exoplanets for detailed study as it shares observing time with other investigations. The ARIEL team should be able to draw from JWST’s experience as it homes in on a final target list. Keep in mind as well that ESA’s PLATO mission — PLAnetary Transits and Oscillations of stars — is also in the pipeline, slated for a 2026 launch. As I say, the tools are evolving as our focus sharpens.
by Paul Gilster | Nov 8, 2019 | Deep Sky Astronomy & Telescopes |
It’s always breathtaking to see the band of the Milky Way under good viewing conditions. I remember so well the night I saw it best, about 20 years ago on a cold, absolutely clear night from a boat in the middle of Lake George. This is up in New York’s Adirondacks, and when I glanced up as we crossed the lake heading back to our hotel, I was simply stunned by the vista. When you contemplate what you’re looking at and think of yourself within that ghostly band, you feel somehow a deep connection to all the myriad processes that put us here as observing beings.
Now we have another fine view of the Milky Way, this time from TESS. The scientists working data from the Transiting Exoplanet Survey Satellite have just given us a composite drawn from 208 TESS images taken during the mission’s first year of science operations, which ended July 18. Have a look at the southern sky, and realize what while TESS has found 29 exoplanets thus far, another 1,000 or so are in candidate stage and being investigated.
Image: This mosaic of the southern sky was assembled from 13 images taken by NASA’s Transiting Exoplanet Survey Satellite (TESS) during its first year of science operations, completed in July 2019. The mission divided the southern sky into 13 sectors, each of which was imaged for nearly a month by the spacecraft’s four cameras. Credit: NASA/MIT/TESS.
Lots of good things to see here. TESS has divided the southern sky into 13 sectors, each of which received almost a month’s worth of imaging by the four cameras aboard. The Milky Way’s band is easily recognized, but look in the center to see the Large Magellanic Cloud, and at the top of the image, you should be able to identify the Orion Nebula, a birthing place for stars.
Can you find Alpha Centauri in this image? Here’s a second image, one showing the confirmed TESS planets to date. I’ve inserted an arrow to identify our nearest star(s).
Image: The host stars of the 29 TESS planet discoveries to date are shown on this version of the southern sky mosaic. Credit: NASA/MIT/TESS and Ethan Kruse (USRA).
TESS is doing excellent work, capturing a full sector of the sky every 30 minutes as it hunts for exoplanet transits. In the first year of operations, its CCDs captured 15,347 30-minute science images. These make up a part of the more than 20 terabytes of southern sky data returned thus far. The TESS survey of the northern sky is now underway.
by Paul Gilster | Nov 7, 2019 | Missions |
It’s heartening to consider that the two Voyager spacecraft, though built for a 4 ½ year mission, have continued to function ten times longer than that. This fact, and data from other missions, will help us get a handle on longevity in spacecraft systems as we contemplate pushing out beyond the heliosphere with a spacecraft specifically designed for the job. Mission longevity is mysterious for it often seems to surprise even the designers, who would like to have a more concrete sense of how to ensure operations continue for decades.
Voyager 2 broke Pioneer 6’s record of 12,758 days of operation way back in 2012, but we can also consider spacecraft like Landsat 5, launched in 1984 and carrying two instruments, the Multispectral Scanner System (MSS) and the Thematic Mapper (TM). Managed by the U.S. Geological Survey (USGS), Landsat 5 completed over 150,000 Earth orbits and sent back more than 2.5 million images of Earth’s surface, with operations lasting almost three decades. Design life for Landsat 5 was estimated at three years, but it became, as Guinness World Records labels it, the ‘longest-operating Earth observation satellite.’
While the Landsat accomplishment is significant, the two Voyagers have actually taken us into a new realm, with Voyager 2 joining Voyager 1 beyond the heliosphere on November 5, 2018. Outside the protective ‘bubble’ blown by the stream of particles and magnetic fields from the Sun known as the solar wind, these craft are now the subject of five new research papers in Nature Astronomy describing the data Voyager 2 has returned since the crossing. Have a look at the relative position of the two spacecraft.
Image: This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in August 2012. Heading in a different direction, Voyager 2 crossed another part of the heliopause in November 2018. Credit: NASA/JPL-Caltech.
Note the plasma flow lines both inside and outside the heliopause. Plasma is a gas composed of charged particles, a ‘wind’ that differs in direction depending on whether its source is the Sun or the interstellar medium itself. The Voyagers have two instruments returning data on plasma at the borderline between the Sun’s domain and interstellar space. The data show hot and sparse plasma inside the heliosphere, while interstellar plasma is colder and denser. We learned from Voyager 1 that the heliosphere protects the Solar System from about 70 percent of the incoming cosmic ray radiation, which is made up of particles accelerated by exploding stars.
While Voyager 1 showed higher than expected plasma density just outside the heliosphere (an indication, researchers say, of compression), Voyager 2’s findings demonstrated slightly warmer plasma than expected, while confirming the compression at the edge of the heliosphere. Meanwhile, the spacecraft’s particle instruments (two of the five still operating instruments can detect particles in different energy ranges) showed some particles slipping across the boundary into interstellar space, indicating a more porous boundary in Voyager 2’s location outside the ‘flank’ of the heliosphere, as opposed to Voyager 1’s exit at its front.
Magnetic field issues still raise questions. Voyager 1 had shown that the magnetic field just beyond the heliopause is parallel to the magnetic field inside the heliosphere. Voyager 2’s magnetometer confirms this finding of field alignment. Ed Stone (Caltech) is the all but legendary project scientist for Voyager:
“The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy. Without this new data from Voyager 2, we wouldn’t know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed.”
Having two spacecraft leaving the heliosphere has been a tremendous boon for science. Voyager 1 and Voyager 2 exited the heliosphere at different locations as well as at different times in the 11-year solar cycle. The latter keeps the solar wind mutable and frothing, something to be borne in mind when we consider spacecraft designs that could ‘sail’ on this wind, and affects the shape of the heliosphere itself, whose boundaries vary with solar changes. We learn from the new papers that neither Voyager is in undisturbed interstellar space, but rather in a churning transitional region, outside the heliosphere but still affected by its presence.
The papers, all of them in Nature Astronomy‘s October 2019 issue, are Richardson et al., “Voyager 2 plasma observations of the heliopause and interstellar medium”; Krimigis et al., “Energetic charged particle measurements from Voyager 2 at the heliopause and beyond”; Stone et al., “Cosmic ray measurements from Voyager 2 as it crossed into interstellar space”; Burlaga et al., “Magnetic field and particle measurements made by Voyager 2 at and near the heliopause”; and Gurnett & Kurth, “Plasma densities near and beyond the heliopause from the Voyager 1 and 2 plasma wave instruments.”
