Commercial Space Tourism and Space Weather: An Update

Abstract

Numerous developments have occurred in commercial space tourism since a feature article in Space Weather first addressed the growing interest in this facet of human space flight [Turner, 2007]. For example, in July 2012, at the Farnborough International Air Show, Virgin Galactic announced that SpaceShipTwo is on the verge of the first commercial space tourism suborbital flight (Virgin Galactic press release, 10 July 2012, available at http://www.virgingalactic.com/news/item/xxx/): Virgin Galactic Founder Sir Richard Branson revealed that the company has now accepted deposits for suborbital flights on SpaceShipTwo from 529 future astronauts, a number greater than the total count of people who have been to space throughout human history. This news comes following a flurry of recent test activity and confirmation that all major components of SpaceShipTwo's rocket system have been qualified for powered flight, on track to begin before the year's end. In addition, progress is being made toward operating the first commercial orbital space tourism flights. Bigelow Aerospace launched and still monitors the flights of two subscale inflatable modules, Genesis I (launched 12 July 2006) and Genesis II (launched 28 June 2007), and continues to develop its BA 330 orbital habitation module (Bigelow Aerospace home page, 3 October 2012, available at http://www.bigelowaerospace.com/index.php). Meanwhile, on 31 May 2012 and again on 10 October 2012, SpaceX successfully demonstrated its ability to deliver a commercial resupply module, the Dragon, to the International Space Station via its own launch vehicle, the Falcon 9. The Dragon allows for both cargo and crew variants (SpaceX Dragon Capsule, 11 October 2012, available at http://www.spacex.com/dragon.php): To ensure a rapid transition from cargo to crew capability, the cargo and crew configurations of Dragon are almost identical, with the exception of the crew escape system, the life support system and onboard controls that allow the crew to take over control from the flight computer when needed. This focus on commonality minimizes the design effort and simplifies the human rating process, allowing systems critical to Dragon crew safety and space station safety to be fully tested on unmanned demonstration flights and cargo resupply missions. Also in May 2012, Bigelow and SpaceX announced that they would begin working together on a joint effort to provide a fully commercial launch-to-orbit-to-reentry orbital space experience (Bigelow Aerospace and SpaceX joint venture, available at http://www.spacex.com/press.php?page=20120510): Space Exploration Technologies (SpaceX) and Bigelow Aerospace (BA) have agreed to conduct a joint marketing effort focused on international customers. The two companies will offer rides on SpaceX's Dragon spacecraft, using the Falcon launch vehicle to carry passengers to Bigelow habitats orbiting the Earth. Thus, the conclusion of the 2007 Space Weather journal article remains valid today, with a growing sense of urgency [Turner, 2007]: As society itself expands into the frontiers of space, the space weather community must be prepared to welcome increasing roles and responsibilities to ensure the safety of these intrepid pioneers. A 2008 report prepared for the Federal Aviation Administration (FAA) Office of Commercial Space Transportation summarized space weather impacts on suborbital space flight [Turner et al., 2008]. Among its conclusions were that under typical conditions the radiation exposure to crew and passengers on a suborbital flight is less than that of a long-duration airline flight. Although the radiation risk for crew and passengers is minimal except possibly at high latitudes and during solar and geomagnetic disturbances, crew and passengers should be briefed on the radiation risks in the spirit of informed consent, and they should be monitored during the mission for radiation exposure. This means that the providers of suborbital flights will require, as part of their support staff, experts familiar with the space environment (space weather), techniques for monitoring the radiation environment, techniques and models for estimating the doses with the body under realistic shielding, and the biological effects of radiation in order to ensure crew and passenger well being. The transition to commercial orbital space flight will substantially increase participants’ exposure to radiation and will lead to increased demands for space weather expertise. Even during a nominal space environment (400 kilometers altitude, with orbit inclination less than 30 degrees) under reasonable shielding, a week's exposure will be on the order of 100 cross-country airplane flights, or close to the typical U.S. resident's annual exposure [Turner, 2012]. This exposure could increase significantly if the mission altitude or inclination is increased or if there is a solar storm. Here we address the biological effects of radiation exposure, the radiation exposure of suborbital and orbital flights, and the role of the space weather community in commercial space flight. The biological impacts of exposure to the unique space radiation environment are discussed in Turner [2007, and references therein]. Recent reports that discuss the details of space radiation exposure include National Research Council [2012], Cucinotta et al. [2010, 2011], and Durante and Cucinotta [2011]. The key points are summarized here. The general U.S. population's annual exposure to radiation is less than 0.5 cSv per year [National Council on Radiation Protection and Measurements, 2009]. Exposure from a typical midlatitude cross-country air flight is about 25 ?Sv. Limits for radiation workers are generally less than 5 cSv per year. The cumulative experience of U.S. astronauts has ranged from 0.1 to 10 cSv. The exposure rate varies with altitude, inclination, solar cycle, solar activity, vehicle shielding, vehicle orientation, and location within the vehicle. Mission-averaged rates have ranged from 0.01 to 0.4 cSv per day. A high dose of radiation over a short period can lead to acute effects such as headaches, nausea, and skin “burns.” In extreme cases, the effects of high dose rates can be severe, either directly through radiation sickness or indirectly, as from vomiting in a space suit. Acute effects generally do not manifest at exposures below 0.7 Gy-Eq attained within a few hours. In 2010, NASA developed an acute effects projection model combining radiation transport codes and a probabilistic model of acute radiation risk (ARRBOD) [Kim et al., 2010]. Table 1 shows the scale of effects and threshold doses in gray equivalent to induce the indicated response within 24 hours, with no medical intervention, according to that model. To date, astronaut exposures have not approached even the lowest acute exposure thresholds. A lower dose over a prolonged period can have long-term impacts, including increased risk of cancer, effects on genetics or fertility, development of cataracts, and cumulative damage to tissue (particularly the central nervous system, digestive system, cardiovascular system, and immunological system). A good general rule, if used with caution and applied only to doses above a few mSv, has held that a 20 cSv (equivalent dose) exposure increases the probability of a fatal cancer by 1 percent [National Council on Radiation Protection and Measurements, 1993]. In 2012, NASA increased funding for studies on these “noncancer” effects, particularly impacts on the central nervous system (NASA Space Radiation Element of the Human Research Program, 2012, available at http://www.nasa.gov/centers/johnson/slsd/about/divisions/hacd/hrp/space-radiation.html). The effects of radiation on occupants of future commercial space vehicles may vary with the occupant's role in the mission. Typical passengers may be the least affected if they experience only one mission lasting tens of minutes (suborbital) or on the order of a week (orbital). However, there are cases where radiation exposure poses an elevated risk. For example, the National Council on Radiation Protection and Measurements recommends that pregnant females not fly in space, noting that [National Council on Radiation Protection and Measurements, 2000, p. 145] “The special radiation risks for the embryo/fetus are malformation and mental retardation…” In addition, crew members, who may fly many suborbital flights in a single month or may spend significant time in an orbiting space station, will face a correspondingly larger risk. Although not the focus of this article, radiation also poses a risk to spacecraft systems. Risks include a threat to avionics if a vehicle is launched during periods of enhanced energetic radiation, single-event effects to electronics in orbit, and spacecraft charging effects [Turner et al., 2008]. It was reported that 5 months after its launch, Genesis I (a prototype of the Bigelow Aerospace inflatable habitation module) suffered an anomaly during a solar storm that could have led to the end of the mission [David, 2007]. A study for the FAA Office of Commercial Space Flight [Turner et al., 2008] looked at both biological and systems impacts of the radiation environment on commercial suborbital space flight. The study examined four different suborbital trajectories at three different launch latitudes. The estimated participant doses from background radiation during solar minimum (galactic cosmic radiation maximum) are summarized in Table 2. The study also examined the impact of solar radiation events. A key conclusion was that for launch latitudes within 45 degrees of the equator, the solar storm impacts were less than the background cosmic radiation (Figure 1). The same report examined the risk of impact to vehicle avionics and concluded that with reasonable attention to component selection and good space system design practices, the risk was negligible during nominal radiation conditions. However, it would be prudent to avoid launch during a solar particle event or during disturbed geomagnetic conditions. Likely delay times to wait for more favorable launch conditions would be on the order of a day and only rarely more than a few days. Virgin Galactic suborbital flights will launch from SpacePort America, near Truth or Consequences, N. M. Located at approximately 33 degrees north latitude, the trajectory, reaching an altitude just over 100 kilometers, will be well within the protection of the Earth's magnetosphere, significantly reducing the radiation threat from solar particle events to both passengers and avionics. Compared to suborbital flights, the advent of commercial orbital space flight will significantly increase the radiation risk to passengers and avionics. The radiation exposure will depend on mission duration, orbital altitude, inclination, shielding, and details of the dynamic space environment. Figure 2 shows how the effective dose will vary with altitude and inclination under nominal 10 grams per square centimeter shielding during solar minimum conditions [Turner, 2012]. The impact of a solar storm is harder to quantify because it depends not only on the strength of the solar particle event and the altitude and inclination of the orbit but also on the relative timing of the peak of the event and the vehicle's possible transit of high-latitude regions above the geomagnetic cutoff for high-energy protons in the magnetosphere. For sufficiently low inclination (less than 30 degrees) or for sufficiently advantageous orbit phasing, the exposure can actually go down due to the effects of a “ Forbush decrease” in the galactic cosmic rays during a significant coronal mass ejection (http://science.nasa.gov/science-news/science-at-nasa/2005/07oct_afraid/, 7 October 2005). However, for sufficiently high inclination (above 50 degrees) and particularly unfortunate phasing, the effective dose could be 10-20 percent of the deep space exposure, or as much as a few tens of mSv. While it would be prudent to delay launch if a solar storm is in progress or deemed to be impending, a mission already under way has very little flexibility in its response to a storm. The crew and passengers could be restricted to a more shielded portion of the vehicle, reconfigurable shielding could be deployed, or the vehicle could return to Earth once orbital conditions for a favorable reentry exist. The choice of orbital altitude and inclination will likely be driven not by radiation risk considerations but by more immediate engineering and commercial considerations. Higher inclinations cover more of the Earth's population, and space tourists may desire to “see their house from orbit.” Providers may also want to go to high inclination to offer wider Earth coverage to the research community (for a fee). However, higher inclinations come at a mass performance penalty to the launch vehicles. Bigelow Aerospace has not published a destination altitude or inclination for its BA 330 module. However, its prototype vehicles Genesis I (http://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=2006-029A) and Genesis II (http://nssdc.gsfc.nasa.gov/nmc/spacecraftOrbit.do?id=2007-028A) both have an orbit inclination of 64.5 degrees with an altitude between 500 and 600 kilometers (300 and 365 miles). Both prototypes were launched by Russian rockets from near 51 degrees north latitude. If SpaceX is the launch provider for BA 330, it would launch from Kennedy Space Center (28.5 degrees north latitude). The radiation exposure will also depend on vehicle shielding. According to Bigelow Aerospace (home page, 3 October 2012, available at http://www.bigelowaerospace.com/index.php), “Bigelow Aerospace's shielding is equivalent to or better than the International Space Station and substantially reduces the dangerous impact of secondary radiation.” So what does this mean to the space weather community? Most significantly, opportunities will arise for small enterprises to provide the necessary consulting support. There will be a need to provide the participants with an understanding of the space environment and the effects of radiation. This will contribute to “informed consent” documents prior to flight. It will be prudent for providers to monitor the radiation exposure in the vehicle and to personalize information to the participants. Thus, there will be a need for simple, efficient, and effective dosimeters as well as improved simulation tools to estimate the radiation environment in the vehicle. Commercial flight operators will need tailored space situation awareness, so there will be a need for targeted space weather forecasts and nowcasts, leading to an expanding market for “Accu-Space-Weather” firms. Most importantly, these are not “someday” opportunities: with the emerging space tourism market, suborbital and orbital, these are opportunities that need to be filled today. Definitions of units are from the National Council on Radiation Protection and Measurements composite glossary (available at http://www.ncrponline.org/PDFs/NCRP%20Composite%20Glossary.pdf http://www.ncrponline.org/PDFs/NCRP%20Composite%20Glossary.pdf, updated July 2011). Effective dose (E): The sum over specified tissues of the products of the equivalent dose in a tissue (HT) and the tissue weighting factor for that tissue or organ (wT) (i.e., E = ?T wTHT). Effective dose (E) applies only to stochastic effects. The unit is the joule per kilogram (J kg-1) with the special name sievert (Sv). Equivalent dose: The mean absorbed dose (gray) in a tissue or organ modified by the radiation weighting factor for the type and energy of radiation incident on the body. The SI unit of equivalent dose is J kg-1 with the special name sievert (Sv); 1 Sv = 1 J kg-1. For low linear energy transfer radiations (e.g., gamma rays and electrons), the radiation weighting factor is assigned a value of unity; therefore, 1 Gy is numerically equivalent to 1 Sv. Gray (Gy): The SI unit of absorbed dose of radiation; 1 Gy = 1 J kg-1. Gray equivalent (Gy-Eq): The special name for the unit of the quantity gray equivalent (GT). Also given as the product of DT and Ri, where DT is the mean absorbed dose in an organ or tissue and Ri is a recommended value for RBE for deterministic effects for a given particle type i (i.e., GT = Ri × DT). An Ri value applies to the particle type incident on, or emitted from radioactivity within, the body. The dose limits for deterministic effects from space radiation are given in terms of the quantity gray equivalent. Sievert (Sv): The special name for the SI unit of dose equivalent, equivalent dose, and effective dose. 1 Sv = 1 J kg-1. J kg-1 is also the SI unit for the International Committee on Radiation Units and Measurements operational quantities. Ronald Turner is a fellow at Analytic Services Inc., Suite 800, 2900 South Quincy Street, Arlington, Va 22206. E-mail: ron.turner@anser.org mailto:ron.turner@anser.org