These are relatively dark days for the U.S. human spaceflight program, as Congress prepares for another funding fight over 's budget. Aside from growing utilization of the International Space Station with Russia playing the role of taxi driver, there just isn't much “boldly going” right now. But a couple of recent robotic missions have laid down some pretty useful data points against the day humans move beyond low Earth orbit (LEO) to continue checking out the Solar System.
One is's Gravity Recovery and Interior Laboratory (Grail), which performed exactly as advertised in generating valuable data about the Moon and its environs—the next place humans will operate when we leave LEO and head for Mars. Astronauts who take that ultimate trip to the red planet will stand a much better chance of surviving, thanks to the Mars Science Laboratory's measurement of radiation loads along the way. As my colleague Mark Carreau reports, the doses Mars explorers can expect mean a lot more work must be done on shielding and/or faster in-space propulsion. Now the engineers have some in-situ numbers to define the problem.
Gravity-mapping data from the Grail orbiters and computer simulations of millions of years of lunar cooling have given planetary scientists a better understanding of the variations in the Moon's gravity that can throw lunar orbiters off course. Presented in the peer-reviewed journal Science, analysis of data from the year-long orbital mission low over the lunar surface shows the mass concentrations (mascons) observed since the Apollo era probably were caused by an upwelling of the Moon's dense molten interior when ancient asteroids smashed into the relatively light crust, and then remained there in pockets as the Moon cooled.
That would explain the characteristic bulls-eye shape of the mascons mapped with unprecedented resolution by Grail's matching spacecraft Ebb and Flow from January-December 2012 (see illustration, corrected for terrain height). The dense center of the target is surrounded by a ring where there is a “gravity deficit,” which is surrounded at the perimeter of the impact crater by another ring of high-density material.
Grail was designed to provide precisely the level of data detail that would allow the type of simulation and extrapolation reflected in the Science paper. The mission's gravity maps will help guide NASA's Orion multipurpose crew vehicle through cislunar space on flight tests scheduled later in this decade.
Designers of the early human Mars missions, probably with Orion as the crew capsule, will have the very first direct measurements of galactic cosmic rays from high-energy events outside the Solar System and solar energetic particles (SEPs) associated with solar flares and coronal mass ejections from the Sun. NASA's Mars Science Laboratory collected the data as it delivered the Curiosity rover to Mars in 2011-12.
The measurements suggest a roundtrip exposure of 0.66 sieverts, plus or minus 0.12 sieverts, over the 180-day outbound and inbound legs of the deep-space voyage using current shielding and propulsion techniques, according to a team of researchers from the Southwest Research Institute (SwRI), Germany's Christian-Albrechts University and NASA. The exposure approaches two-thirds of the 1-sievert career exposure limit that carries a 5% increased risk of fatal cancer recognized by the Russian, European and Canadian space agencies.
NASA observes a stricter, individually tailored standard that equates to a 3% increased risk of fatal cancer over a career. Protected by shielding comparable to that available on the current Orion design, the SwRI instrument on Curiosity showed not just when in the voyage radiation loads spiked (as in the time plot, below, of the effects of solar activity), but where within the vehicle the loads were highest.
“Understanding the radiation environment inside a spacecraft carrying humans to Mars or other deep-space destinations is critical for planning future crew missions,” says Cary Zeitlin, SwRI scientist and the lead author of the radiation report, which also was published in Science. “[U]nless propulsion systems advance rapidly, a large share of mission radiation exposure will be during outbound and return travel, when the spacecraft and its inhabitants will be exposed to the radiation environment in interplanetary space, shielded only by the spacecraft itself.”