Last December I mentioned the ongoing work at the European Southern Observatory’s Very Large Telescope to modify an instrument called VISIR (VLT Imager and Spectrometer for the InfraRed). Breakthrough Initiatives, through its Breakthrough Watch program, is working with the ESO’s NEAR program (New Earths in the Alpha cen Region) to improve the instrument’s contrast and sensitivity, the goal being the detection of a habitable zone planet at Alpha Centauri. Exciting stuff indeed, especially given the magnitude of the challenge.
After all, we are dealing with a tight binary, with the two stars closing to within 11 AU in their 79.9 year orbit about a common center (think of a K-class star at about Saturn’s distance). The binary’s orbital eccentricity can separate the stars by about 35 AU at their most distant. The latest figure I’ve seen for the distance between Centauri A/B and Proxima Centauri is about 13,000 AU.
In an ESO blog post that Centauri Dreams reader Harry Ray passed along, planet hunter Markus Kasper explained that the performance goal for the upgraded VISIR requires one part in a million contrast at less than one arcsecond separation, something that has not yet been demonstrated in the thermal infrared. Alpha Centauri A/B will be a tough nut to crack. Kasper makes a familiar comparison: This is like detecting a firefly sitting on a lighthouse lamp from a few hundred kilometres away.
Image: Apparent and true orbits of Alpha Centauri. The A component is held stationary and the relative orbital motion of the B component is shown. The apparent orbit (thin ellipse) is the shape of the orbit as seen by an observer on Earth. The true orbit is the shape of the orbit viewed perpendicular to the plane of the orbital motion. According to the radial velocity vs. time  the radial separation of A and B along the line of sight had reached a maximum in 2007 with B being behind A. The orbit is divided here into 80 points, each step refers to a timestep of approx. 0.99888 years or 364.84 days. Credit: Wikimedia Commons.
But VISIR may be up to the task. Installed at Paranal, 2635 metres above sea level in Chile’s Atacama Desert in 2004, it has been used to probe dust clouds at infrared wavelengths to study the evolution of stars. The instrument is capable of compiling in a scant 20 minutes as many images or spectra as a 3-4 meter telescope could obtain in an entire night of observations. VISIR has also been used to study Jupiter’s Great Red Spot, Neptune’s poles, and the supermassive black holes that can occur at the centers of galaxies.
Ramping up VISIR’s capabilities for Alpha Centauri is a multi-part effort, as Kasper describes:
Firstly, Adaptive Optics (AO) will be used to improve the point source sensitivity of VISIR. The AO will be implemented by ESO, building on the newly-available deformable secondary mirror at the VLT’s Unit Telescope 4 (UT4).
Secondly, a team led by the University of Liège (Belgium), Uppsala University (Sweden) and Caltech (USA) will develop a novel vortex coronagraph to provide a very high imaging contrast at small angular separations. This is necessary because even when we look at a star system in the mid-infrared, the star itself is still millions of times brighter than the planets we want to detect, so we need a dedicated technique to reduce the star’s light. A coronagraph can achieve this.
Finally, a module containing the wavefront sensor and a new internal chopping device for detector calibration will be built by our contractor Kampf Telescope Optics in Munich.
Image: Adaptive optics at work. Glistening against the backdrop of the night sky about ESO’s Paranal Observatory, four laser beams project out into the darkness from Unit Telescope 4 (UT4) of the VLT. Some 90 kilometres up in the atmosphere, the lasers excite atoms of sodium, creating artificial stars for the telescope’s adaptive optics systems. Credit: ESO/F. Kamphues.
The exciting prospect, says Kasper, is that a habitable zone planet could be detected in just 100 hours of observing time on the VLT. To do this will first entail moving VISIR from Unit Telescope 3 to Unit Telescope 4 at Paranal. Hardware testing in Europe continues and the upgrade is expected to be implemented in VISIR by the end of 2018, with the Alpha Centauri observing campaign scheduled for mid-2019, a two-week run that will collect the needed 100 hours of observing time. We may not be far, then, from a planet detection at Centauri A/B.
Kasper points to Breakthrough Initiatives as key players in all this. Let me quote him:
…it is exciting to see how the Breakthrough Initiatives are managing to create momentum in the research field. By backing ideas and projects with a higher risk level than public funding agencies are ready to support, the Initiatives have motivated scientists to push the envelope and leap forward in their research.
Remember that Breakthrough Starshot not only achieved a great deal of media coverage but also caught the eye of a US congressman, with its goal to design and fly tiny probes by beamed sail to the Alpha Centauri system. NASA has begun to look at interstellar concepts again, a small but welcome effort long after the closing of the Breakthrough Propulsion Physics program. Breakthrough Listen is actively conducting SETI at Green Bank and Parkes in Australia. Breakthrough Watch now looks for habitable zone planets within 16 light years of Earth.
Pushing projects with higher risk levels is something that a private initiative can achieve, with repercussions for the broader effort to characterize nearby planetary systems and some day reach them with a probe. We’ll soon have the tools in place to study planetary atmospheres around many of these stars, so the timing of the VISIR effort could not have been better.
Given Centauri Dreams‘s interest in exoplanet studies, it’s no surprise that when I write about the James Webb Space Telescope, it’s usually to fit the observatory into the overall study of other stellar systems. But of course JWST has been conceived to study everything from the earliest stars and galaxies to the ongoing birth of stars out of massive clouds of dust, not to mention objects within our own Solar System. JWST also offers us a real chance to probe exoplanet atmospheres around nearby M-dwarfs, but it is certainly not a dedicated exoplanet mission.
So while we hope for a successful launch in 2020, according to the evolving schedule, and look forward to finding plenty of JWST targets with the upcoming Transiting Exoplanet Survey Satellite (TESS), let’s have a look at a new mission from the European Space Agency with a tight exoplanet focus. The Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL) has just been selected as an ESA science mission scheduled for launch in 2028. Its reason for existence is to look at how planetary systems form and evolve.
Giovanna Tinetti (University College London) is principal investigator here, which bodes well — I am a great admirer of Dr. Tinetti’s work and see ARIEL as being in excellent hands. Of the mission, Tinetti says this:
“Although we’ve now discovered around 3800 planets orbiting other stars, the nature of these exoplanets remains largely mysterious. ARIEL will study a statistically large sample of exoplanets to give us a truly representative picture of what these planets are like. This will enable us to answer questions about how the chemistry of a planet links to the environment in which it forms, and how its birth and evolution are affected by its parent star.”
Image: Giovanna Tinetti during a lecture at the Flamsteed Astronomy Society. Credit: Andy Sawers.
ARIEL’s targets are to be exoplanets from Jupiter-mass down to super-Earths, with the primary focus on planets in warm and hot orbits close to their parent star. The thinking here is that high temperatures keep different molecular species in circulation throughout the atmosphere, making them more easily detected because they do not as readily sink or form cloud layers that can obscure them. In that sense, a planet at 2000 degrees Celsius becomes a useful laboratory, churning with interesting molecules from the atmosphere and even from the planet’s interior.
Current plans call for a launch from Kourou in French Guiana into an orbit around the L2 Lagrange Point 1.5 million kilometers from Earth, where the Webb instrument is also headed. Here the balance of gravitational forces keeps the spacecraft in a fixed position relative to the Earth and Sun with a minimal expenditure of energy. Both spacecraft will essentially be ‘parked’ at the same region in space, unlike the Spitzer instrument, which is in orbit around the Sun and will eventually lose communications when its orbital path takes it behind Sol.
While JWST will view the universe in infrared, ARIEL’s meter-class primary mirror will also collect visible light, using a spectrometer to study planetary atmospheres through transmission spectroscopy, possible when a planet passes in front of or behind its star. One of its sensors will be capable of detecting the presence of clouds. Tinetti and team hope to observe 1,000 exoplanets, following up on the worlds discovered by upcoming missions like TESS, and paving the way for future European missions like CHEOPS and PLATO. A successful JWST launch will likewise result in new target options for ARIEL. The 1,300 kg spacecraft has a cost cap of 450 million euros, roughly $550 million at current rates.
Image: Artist’s impression of ARIEL on its way to Lagrange Point 2 (L2). Here, the spacecraft is shielded from the Sun and has a clear view of the whole sky. Credit: ESA/STFC RAL Space/UCL/Europlanet-Science Office.
You may have heard about an exoplanet mission called EChO (Exoplanet Characterization Observatory), which was a candidate for ESA’s Cosmic Vision program. ESA did not fund the mission, which led Tinetti and colleagues to design the similar ARIEL observatory around a spacecraft with a lighter, less complex payload. The PLATO mission that supplanted EChO as one of ESA’s medium-class missions is an exoplanet observatory slated for a 2026 launch.
No one said this was going to be easy. Delays involving the James Webb Space Telescope are frustrating, with NASA now talking about a launch in mid-2020 instead of next year, and the uncertain prospect of a great deal of further testing and new expenditures that could run the project over budget, necessitating further congressional approval. It’s hard to look back at the original Webb projections without wincing. When first proposed, estimates on the space observatory ran up to $3.5 billion, a hefty price tag indeed, though the science payoff looked to be immense. It was in 2011 that a figure of $8 billion emerged; the project now has a Congressionally-mandated cost cap of $8.8 billion.
And now, looking forward, we have Thomas Zurbuchen, speaking for NASA’s Science Mission Directorate, explicitly saying “We don’t really fully know what the exact cost will be…”
Image: Illustration of NASA’s James Webb Space Telescope. Credit: NASA.
Projects this big invariably take us into the realm of acronyms, with the project’s Standing Review Board (SRB) concluding that further time is necessary to integrate the various components of JWST. But we also learn that NASA is setting up an Independent Review Board (IRB) to complement the SRB findings. The space agency will look at the findings from both boards and consider their recommendations by way of taking us to a more specific launch schedule, with an assessment due in a report to Congress this summer. This NASA news release also talks about “Additional steps to address project challenges include increasing NASA engineering oversight, personnel changes, and new management reporting structures.”
There’s no question about the challenges JWST presents. Its spacecraft element is made up of the huge sunshield (the size of a tennis court), along with the spacecraft bus including the flight avionics, power system and solar panels. The collapsible sun shield must be folded and re-folded during the test process. Eventually it must be mated with the 6.5 meter telescope and science payload. The latter were successfully tested in 2017 at Johnson Space Center, with the telescope element being delivered to Northrop Grumman earlier this year.
Both halves of the observatory are now in the same facility for the first time. Ahead for the spacecraft element is vibrational, acoustic and thermal testing, necessary before the observatory can be fully integrated and pronounced ready for flight. The area of concern is the sun shield and bus, both developed by Northrop Grumman. Contributing to the delay, according to this Lee Billings essay, is a series of tears that appeared on the shield while being tested for deployment. The shield is said to have created a snagging hazard, forcing the addition of springs to prevent it from sagging. Other errors have involved the spacecraft thrusters.
What to do? Northrop Grumman teams are now working on the telescope 24 hours a day, while NASA is calling in the cavalry, as Marina Koren describes in The Atlantic:
Nasa announced some measures they would take at Northrop Grumman’s facility in California, where all of Webb’s parts currently reside. The space agency said they will increase engineering oversight at the facility in Redondo Beach and will track the company’s test reports on a weekly basis. Senior management from nasa’s Goddard Space Flight Center in Maryland, where much of the telescope was constructed, will work out of Northrop Grumman’s offices on a permanent basis. Northrop Grumman’s project manager for Webb will report directly to C-suite level executives at the company “to help remove roadblocks to success within the company,” the officials said.
Northrop Grumman is the prime contractor for JWST, but it appears that deeper NASA involvement in the process is forced by events. Ahead for the observatory is the tough environmental testing of sun shield and bus that the telescope and science instruments have already received. This in itself is a matter of several months, after which JWST can be assembled and tested in final form. The feeling at this end is that JWST has become so expensive, so pivotal in our astronomical roadmap, that it is too big and too expensive to fail.
That means its launch and deployment are going to be fraught with tension. 100 times more powerful than Hubble, JWST will operate 1.5 million kilometers from Earth, meaning that servicing missions by astronauts like Hubble has received will not happen. What would the path forward be if we lost JWST because the complex deployment process failed?
Also worth pondering: What will be the effect of any JWST overspending on the WFIRST mission? The Wide Field Infrared Survey Telescope has had its funding restored and operations continue, but we can’t rule out future attempts to cut the budget or even derail the program entirely. In the realm of technology, WFIRST may prove the easier of the two missions to complete, as this story in Nature points out:
JWST and WFIRST are very different technologically, says Jon Morse, chief executive of Boldly Go Institute, a space-exploration organization in New York City, and former head of NASA’s astrophysics division. JWST involves complex designs that have never been tested before, such as the enormous sunshield. WFIRST will use a well-understood 2.4-metre mirror design that does not require lots of new technology.
“WFIRST is not likely to develop the cost problems of the same magnitude as JWST,” Morse says.
Assuming its operations budget isn’t too severely raided to pay for JWST’s extra costs. By comparison, WFIRST’s own budget cap is a relatively svelte $3.2 billion.
Too much water helps planetary habitability not one bit. And while we find the availability of surface water a useful way of describing a potentially habitable world, we’re learning that some planets may have water in such abundance that life may never have the chance to emerge. It would be a shame if the numerous worlds orbiting TRAPPIST-1 fell into this scenario, but a multidisciplinary team from Arizona State University is making a strong case for the prospect.
What’s wrong with water? Let Natalie Hinkel (Vanderbilt University) explain. Hinkel worked with ASU’s Cayman Unterborn, Steven Desch and Alejandro Lorenzo on the question of water composition in these worlds. Coleridge’s “Rime of the Ancient Mariner” comes to mind — “Water, water, every where / Nor any drop to drink.” But in this case, there is plenty to drink, which is precisely the problem. Says Hinkel:
“We typically think having liquid water on a planet as a way to start life, since life, as we know it on Earth, is composed mostly of water and requires it to live. However, a planet that is a water world, or one that doesn’t have any surface above the water, does not have the important geochemical or elemental cycles that are absolutely necessary for life.”
Image: A nice visualization of the TRAPPIST-1 planets, here suggesting their relationship to the ‘snowline.’ As we’ll see below, what these planets are made of has implications for where they formed. Credit & copyright: NASA/Tim Pyle and Robert Hurt.
Not a good prospect, then, if the work of these researchers is any indication. What Unterborn et al. are saying in their paper in Nature Astronomy is that the TRAPPIST-1 planets are lighter than we would expect given our measurements of their mass and volume. All seven planets here appear to be less dense than rock. Remember, this is a transiting system, meaning we have constraints on mass and radius for all seven, allowing us to calculate density.
While low density worlds might well have a large gaseous envelope, the TRAPPIST-1 planets turn out to be not massive enough to hold onto the gas they would need to make up what Unterborn calls the ‘density deficit.’ If they somehow did hold onto the needed gas, they would be much puffier planets than what we see. The researchers argue that the low density component must be water, so the question becomes, how much water is there?
The numbers are daunting. Bear in mind as we look at them that the Earth, even with its magnificent oceans, is but 0.02% water by mass. Around TRAPPIST-1, the ‘dry’ inner planets b and c are likely to have less than 15 percent water by mass. Outer planets f and g are consistent with having more than 50 percent water by mass. These numbers will vary as we continue to constrain the masses of the planets, but the trend is clear enough.
“What we are seeing for the first time are Earth-sized planets that have a lot of water or ice on them,” said Steven Desch.
Which gets us to what planets like this can tell us about their formation and evolution. Planets with this much water — assuming water is the explanation for the density issue — should not have formed within the ‘snowline,’ that region within which water exists as a vapor and cannot be incorporated into a forming planet. Unterborn and team are clear on this point: The TRAPPIST-1 planets must have formed beyond the snowline and migrated to their current orbits. Indeed, these planets must have migrated from a position at least twice as far from the parent star as they are now. Have a look at the graph below to get the idea.
Image: This graph shows the minimum starting distances of the ice-rich TRAPPIST-1 planets (especially f and g) from their star (horizontal axis) as a function of how quickly they formed after their host star was born (vertical axis). The blue line represents a model where water condenses to ice at 170 K, as in our solar system’s planet-forming disk. The red line applies to water condensing to ice at 212 K, appropriate to the TRAPPIST-1 disk. If planets formed quickly, they must have formed farther away (and migrated in a greater distance) to contain significant ice. Because TRAPPIST-1 dims over time, if the planets formed later, they could have formed closer to the host star and still be ice-rich. Credit: Unterborn et al. / ASU.
This isn’t the first time we’ve seen migration discussed in relation to TRAPPIST-1. Simon Grimm (University of Bern Centre for Space and Habitability) and colleagues have looked at migration, noting that the resonant orbits here — the planets form a single resonant chain — is an indication of a slow migration consistent with the current perceived stability.
Other researchers have likewise addressed migration, including Chris Ormel (University of Amsterdam) and team, who look at planetary formation at the snowline itself in what they call a ‘resonant convoy,’ with the outer planets ‘pushing’ on the inner ones. So the idea of migration at TRAPPIST-1 is not new. What is new in the Unterborn et al. work is the use of planetary composition to add weight to the overall case for migration, which allows the team to quantify how much migration actually took place.
We’ve lucked out when it comes to nearby red dwarfs. TRAPPIST-1 will clearly be a primary source of data for red dwarf planets as we address the issue of habitability that their density and formation history implies. And then there’s that intriguing planet around Proxima Centauri…
The paper is Unterborn et al., “Inward migration of the TRAPPIST-1 planets as inferred from their water-rich compositions,” Nature Astronomy 19 March 2018 (abstract). The Grimm paper is Grimm et al., “The nature of the TRAPPIST-1 exoplanets,” in press at Astronomy & Astrophysics (preprint). The Ormel paper is Ormel et al., “Formation of Trappist-1 and other compact systems,” Astronomy & Astrophysics Vol. 604 (August 2017) (abstract).
Gregory Matloff’s contributions to interstellar studies need scant introduction, given their significance to solar-and beamed sail development for decades, and their visibility through books like The Starflight Handbook (1989) and Deep Space Probes (2005). A quick check of the bibliography online will demonstrate just how active Greg continues to be in analyzing the human future in space, as well as his newfound interest in the nature of consciousness (Star Light, Star Bright, 2016). The paper that follows grows out of Greg’s presentation at the 2016 iteration of the Tennessee Valley Interstellar Workshop, where he discussed ways to advance deep space exploration using near term technologies like Falcon Heavy, in conjunction with the solar sail capabilities he has so long championed. Read on for an examination of human factors beyond lunar orbit and a description of a useful near-term mission that could reach an object much closer than Mars relying on both chemical and sail capabilities. Dr. Matloff is a professor of physics at New York City College of Technology, CUNY.
The possibility of applying the Space-X Falcon-Heavy booster to human exploration of the inner solar system is discussed. A human-rated Dragon command module and an inflatable habitat module would house and support the 2-4 person crew during a ~1 year interplanetary venture. To minimize effects of galactic cosmic rays, older astronauts should conduct the mission during Solar Maximum. Crew life support is discussed as is application of a ~1-km square solar photon sail. The sail would be applied to rendezvous with the destination Near Earth Object (NEO) and to accelerate the spacecraft on its return to Earth. An on-line NASA trajectory browser has been used to examine optimized trajectories and destinations during 2025-2026. A suitable destination with well established solar-orbital parameters is Asteroid 2009 HC. Because the NASA Space Launch System (SLS) has a greater throw mass than the Falcon-Heavy, the primary propulsion for NEO rendezvous and Earth return would likely be a chemical rocket. The sail would be used in this case as an abort mechanism and a back-up for the primary propulsion system. In either scenario, a single Falcon-Heavy or SLS launch would be adequate.
Keywords: NEO Exploration, Falcon-Heavy, Dragon, BEAM, GCR, ECLSS, SLS
The United States is currently developing two separate approaches to launching inner solar system exploration by human crews beginning in around 2020: the NASA Space Launch System (SLS) and the commercial Space-X Falcon Heavy [1,2]. The advantage of the SLS is its greater throw-weight on interplanetary trajectories. A disadvantage of SLS is the cost permission and constraints imposed by the current state of the US federal government. Although Falcon-Heavy has less launch capacity than the SLS, this booster is a composite of three existing and mass produced Falcon-9 boosters. These have an excellent reliability record to date on NASA-sponsored resupply missions to the International Space Station (ISS) and commercial launches. Experiments indicate that the Falcon 9 first stage may eventually be recovered and reused, which promises to greatly reduce the cost of space missions to low Earth orbit (LEO) and beyond.
This paper concentrates upon interplanetary ventures using a single Falcon-Heavy launcher and a small crew (2-4 people). As well as a Dragon V2 capsule appropriately modified for interplanetary application (3), an inflatable Bigelow space habitat similar to the one to be launched to the ISS [4} in the near future will be used for crews habitability. Life support for the crew on their 1-2 year interplanetary venture will utilize recycling of oxygen and water. Food recycling by biological means will likely not be ready by 2020.
After the spacecraft is launched towards Mars, a state-of-the-art solar photon sail with a dimension of ~0.7 km will be unfurled. This will allow, as will be demonstrated, non-rocket accelerations of 1-2 km/s per month in the solar system region between Earth and Mars.
A recent comprehensive study of in-space radiation effects reveals that galactic cosmic radiation beyond LEO is reduced by a factor of ~5 above LEO, if missions are conducted during solar maximum. During solar flares and coronal plasma discharges, the crew could be protected by aligning the Dragon’s heat shield between the crew quarters and the Sun.
Human landings on Mars will not be possible using a single Dragon launch. But a host of Near Earth Objects of asteroidal and cometary origin and possibly the Martian satellites Deimos and Phobos will be open to human explorers.
But any human expedition beyond the Moon will likely require cruise durations of months to years. Solar and galactic cosmic radiation will certainly be a limiting issue. The possibilities and effectiveness of using the capsule and habitat mass for self shielding is discussed in the next section.
Falcon-Heavy Interplanetary Throw Mass and Cosmic Ray Shielding
According to Ref. 2, a Falcon-Heavy is capable of projecting 13,200 kg on a trans-Mars trajectory. From Ref. 3, the dry mass of a Dragon V2 capsule is 4200 kg and the endurance of this spacecraft is about 2 years in space. The mass of the BEAM inflatable module is 1360 kg .
From Ref. 3, the Dragon’s configuration can be approximated by a cone with a diameter of about 3.7 m and a height of about 6.1 m. From Ref. 4, the BEAM inflatable habitat can be approximated by a cylinder with a diameter of about 3.2 m and a length of about 4 m. Assuming that the base of the Dragon abuts one of the circular end caps of the BEAM, it is easy to demonstrate that the surface area of the spacecraft is about 100 m2. Assuming that all of the Falcon’s throw mass can be used for self-shielding against cosmic rays during an interplanetary venture, the approximate shielding areal mass thickness is 130 kg/m2 or 13 g/cm2.
There are two major sources of cosmic radiation beyond the Earth’s magnetosphere. These are eruptions of solar energetic particles (SEPs), which are most common during solar maximum and galactic cosmic rays (GCR), which are alway present. We first consider the effects of a 13 g/cm2 shield on SEPs.
Shielding From SEPs
A recent paper by an international team summarizes the latest results on cosmic ray shielding above LEO [5}. Figure 6 of that reference compares the Effective Dose Equivalent predicted to be incurred by an astronaut from four SEP events: a 20-year event, a 10-year event, a worst-case modeled event and a Carrington-event estimate. This data is plotted against aluminum shield thickness and compared with currently recommended European Space Agency (ESA) career dose limits. In all cases presented, a 13 g/cm2 aluminum shield is adequate.
Figure 7 of Ref. 5 presents similar information and compares predicted doses from the above SEP events with the 30-day and annual ESA limit. Once again, a 13 g/cm2 aluminum shield seems to be adequate, although the Carrington-event predicted dose rate is very close to the 30-day limit.
Therefore, SEPs do not seem to pose an insurmountable health risk to crews venturing beyond LEO with an equivalent a 13 g/cm2 aluminum shield. Additional shielding could be affected by orienting the Drago heat shield between crew and Sun during a major SEP event.
Shielding from GCRs
Galactic cosmic rays, on the other hand, pose a larger risk to the crew’s health. Since at least 1979, it was known that energetic galactic cosmic rays more massive than helium nuclei (high-Z GCR) are potentially dangerous to human health and very difficult to shield against . Figure 1 of Ref. 5 reveals that during solar maximum, the modeled flux of galactic hydrogen and helium nuclei are reduced by a factor of 5-10 when compared with fluxes of the same ions during solar minimum. But the fluxes of galactic lithium and iron nuclei are apparently independent of the solar activity cycle.
Cucinotta and Durante estimate that during an interplanetary transfer, the high-Z GCR dose might be 1-2 mSv per day or 0.4-0.8 Sv per year . From Tables 1 and 2 of McKenna et al, the NASA one-year dose limits for 40-year old female and male astronauts are respectively 0.7 and 0.88 Sv. For older astronauts, the limits are higher. Dose limits for men are higher than dose limits for women.
During a 1-year interplanetary voyage, the dose limits for 40-year old astronauts may be exceeded. Exposures beyond these recommended limits may result in a 3% increased risk of fatal cancers.
Health effects on interplanetary astronauts from high-Z galactic cosmic rays is an ongoing field of research. Mewaldt et al. also conclude that interplanetary voyagers will experience a higher galactic dose during solar minimum than during solar maximum. According to their study, a thin aluminum shield of about 3 g/cm2 may reduce solar minimum dose rates to the NASA LEO career limit of 50 cSv for a 1-year interplanetary round trip .
It should also be mentioned that it is not always possible to predict future GCR doses in interplanetary space from data obtained during previous solar cycles. Schwadron et al have noted that unusually high levels of GCRs were measured during a prolonged solar minimum in 2009 .
Crew Life Support on Interplanetary Ventures
We next consider the mass requirements to maintain a small crew (2-4 people) during a 1-2 year interplanetary expedition. According to the wikipedia entry on space-life-support systems and in substantial concurrence with one classic reference , daily average human metabolic requirements can be summarized:
oxygen: 0.84 kg
food: 0.62 kg
water: 3.52 kg.
If partial recycling were not built in to the mission, a two-person crew could not be supported in the proposed spacecraft for missions of one year or longer. Projections from International Space Station technology indicate that a near-term goal for water recycling is 85% and the oxygen recovery rate can be raised to 75% [11,12]. Applying these values for an interplanetary mission applying near-term recycling technology, the daily consumable requirement per astronaut is 0.21 kg oxygen, 0.62 kg food, and 0.53 kg of water. Each crew member consumes about 1.4 kg per day of these resources or about 500 kg per year. A 4-person crew therefore requires about 2,000 kg of these resources for a 360-day duration interplanetary voyage.
It is next necessary to estimate the mass of the Environmental Control and Life Support System (ECLSS) equipment, not including consumables, necessary to support the mission. In Table 4.3 of his monograph, Rapp estimates the mass of the water-recovery system for a 180-day transit to Mars at 1.4 metric tons or 1,400 kg and the mass of the oxygen recovery system at 0.5 metric tons or 500 kg for a 6-person crew . We are here considering a smaller crew and the 180-day return voyage as well as the 180-day flight to the interplanetary destination. Since we have no idea regarding ECLSS reliability on a deep-space mission, we will assume here that the required mass of ECLSS equipment is 3,000 kg. Including the 2,000 kg requirement for oxygen, food and water, the total ECLSS mass is about 5,000 kg.
Application of a Near-Term Solar Photon Sail
From the above discussion, the mass of the Dragon is estimated at 4,200 kg and the BEAM habitat mass is 1,360 kg. Since the Falcon-Heavy can project 13,200 kg towards Mars and our ECLSS mass projection is 5,000 kg, the remaining mass amounts to 2,640 kg. If 640 kg is required for scientific equipment, 2,000 kg remains to be allowed. We will therefore assume that the sail mass is 2,000 kg.
As an example of a large solar-photon sail that could be constructed in the not very distant future, we consider a 90% reflective (REF) opaque 1-km2 sail with an areal mass density of 2 g/m2. The sail mass is 2,000 kg and the areal mass density of the spacecraft (?eff) is 0.0132 kg/m2.
The lightness factor (?) of a solar-photon sail is the ratio of solar radiation-pressure acceleration on the sail to solar gravitational acceleration. It can be calculated by modifying Eq. (4.19) of Ref.14 for a solar constant of 1,366 W/m2:
Substituting in Eq. (1) for sail reflectivity and spacecraft areal mass density, we find that ? = 0.11.
Since the Earth is in a near-circular solar orbit at a distance of 1 Astronomical Unit (1 AU = 1.5 X 1011 m) and the Earth’s solar-orbital velocity is about 30 km/s, the Sun’s gravitational acceleration on the spacecraft at a distance of 1 AU is about 0.006 m/s2. The solar radiation-pressure acceleration on the sail is therefore about 6.6 X 10-4 m/s2, if the sail is normal to the Sun at a distance of 1 AU from the Sun. Such a sail configuration can result in a daily velocity increment of about 57 m/s. Every month, the sail can alter the spacecraft velocity by about 1.6 km/s, if it is oriented normal to the Sun at a solar distance of 1 AU. At the orbit of Mars (1.52 AU), this sail oriented normal to the Sun can alter the spacecraft’s solar velocity each month by about 0.69 km/s.
The effectiveness of this configuration for non-landing missions to Mars can be evaluated using Table 4.2 of Ref. 15. The duration of a Hohmann minimum-energy Earth-Mars trajectory is given in that table as 259 days. Although application of the sail can shorten this a bit on the Mars-bound trajectory, sail or some other form of propulsion must be used to accomplish Mars rendezvous. Aerocapture with a deployed sail of this size will be difficult or impossible. Table 4.2 of Ref. 15 also presents Earth-Mars transit times for spacecraft flying a logarithmic spiral trajectory at a sail pitch angle relative to the Sun of about 35 degrees, as a function of lightness factor. For ? = 0.1, the time required for a return journey on such a trajectory is about 431 days. To accomplish logarithmic-spiral Earth-return trips from Mars approximating the Hohmann-trajectory duration with the spacecraft configuration considered here would require a substantial increase in sail area without an increase in spacecraft mass. More advanced sails, smaller crews, or less massive ECLSS would be required.
Application of Falcon/Dragon/BEAM for NEO Exploration
So the configuration presented here is marginal at best for Mars-vicinity missions such as exploration of the natural Martian satellites Phobos and Deimos. Instead, it might see more immediate application to near Earth objects (NEOs) orbiting the Sun close to the Earth’s solar orbit.
A suitable target NEO for such an expedition would be in a near-circular, low-inclination orbit approximately 1-AU from Earth. If the mission is timed appropriately, the Hohmann trajectory duration should be considerably less than the time required to reach Mars and the Falcon-Heavy payload should be greater than that estimated for a Mars mission. One NEO class of potential targets is Earth’s quasi-satellites in “corkscrew orbits” .
The principal use of the sail on the out-bound trajectory leg would be deceleration for rendezvous with the NEO. The sail would be used to accelerate the spacecraft for Earth rendezvous on the return trajectory leg. Although the Sail and BEAM inflatable habitat could be maneuvered into Earth orbit for possible reuse, the Dragon would return crew and payload (including NEO samples) to Earth in a ballistic reentry.
It is possible to investigate mission possibilities with the aid of the NASA on-line Trajectory Browser (trajbrowser.arc.nasa.gov). After accessing this site on May 23 and 25, 2015, we specified a mission to a NEO with a well established orbit during the next solar-max (2025-2026) to reduce crew exposure to GCRs. Two mission types were considered: a round-trip with a maximum duration of 360 days and a one-way, rendezvous mission with a maximum duration of 180 days. For the one-way rendezvous mission, the maximum delta-V for departure from a 200-km low-Earth orbit was 4 km/s. For the round-trip. the maximum delta-V was 5 km/s.
The results of this exercise are presented in Table 1 [following the references]. The destination NEO determined by the Trajectory Browser software is Asteroid 2009 HC. Browser output is summarized in Table 1. This table also includes physical data on this NEO from the NASA Jet Propulsion Laboratory NEO data base (ssd.jpl.nasa.gov).
It is assumed that the pre-rendezvous propulsion requirements will be met by the Falcon upper stage, since this configuration is capable of reaching Mars, a more distant destination. Since the post-injection delta-V is small and the sail’s characteristic acceleration at 1 AU is greater than 1 km/s, the sail can be used to match velocity with the destination NEO without significantly increasing pre-rendezvous mission duration.
Note from Table 1 that the difference between post-injection delta-V for one-way rendezvous and round-trip missions is less than 1 km/s. So it is safe to assume that use of the sail to power Earth-return maneuvers does not significantly increase mission duration.
Another way to consider use of the sail on the Earth-bound trajectory is to estimate time required for the sail to be used for orbital inclination adjustment or “cranking”. From Eq. (5-74 of McInnes’s monograph ), a sail operating at the optimal cone angle can change its inclination by
? i = 88.2 ? degrees/orbit. (2)
According to McInnes, Equation (2) is independent of solar-orbit radius. Inclination correction for Earth-return will add a few months to the duration of the round-trip mission.
Conclusions: Use of the Sail with the NASA Space Launch System on NEO-Visit Missions
From the work presented here, it seems that round-trip visits to selected Near Earth Objects with durations not much greater than one year can be accomplished using a single Falcon-Heavy launch and a combination of a Dragon spacecraft, an inflatable habitability module, state of the art partially closed environmental system and a state of the art square solar photon sail with a 1-2 km dimension.
To reduce the Galactic Cosmic Radiation risk to the 3-4 person crew, it is advisable to conduct lengthy voyages above Low Earth Orbit during periods near the maximum of the solar activity cycle and to position the Dragon heat shield facing the Sun to shield against solar flare events. It may also be advisable to select older astronauts to crew such interplanetary ventures.
If the NASA Space Launch System is available to conduct human visits to suitable NEOs, the sail could serve at least two functions. Because the SLS has 2-3X the throw mass of the Falcon-Heavy, the sail could be accommodated as a pre-rendezvous abort option or as a back-up to the SLS propulsion module for Earth-return maneuvers.
It should also be mentioned that with either launch alternative, the sail and inflatable could be steered into high-Earth orbit for reuse after the Dragon or Orion is directed on its Earth reentry path.
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Table 1. Details for a NEO Visit in 2025-2026