CubeSats to the Moon
by Jeff Foust
|As its name suggests, Lunar Flashlight’s solar sail will reflect sunlight into permanently shadowed regions of craters at the south pole, looking for water ice.|
But what about CubeSat uses beyond Earth orbit? There’s growing interest in applying CubeSat technologies for scientific missions, with the same promise of being able to do missions less expensively, and with larger numbers of spacecraft, than a conventional larger spacecraft mission. The Moon in particular has become a target of interest to CubeSat developers, with several proposals for missions to look for lunar volatiles and test technologies to enable more ambitious future missions.
The lunar CubeSat concept that is perhaps the farthest along is a proposed mission called Lunar Flashlight. The mission was one of three selected last year by the Advanced Exploration Systems (AES) division of NASA’s Human Exploration and Operations Mission Directorate for continued study, with plans to fly the missions as secondary payloads on the first Space Launch System (SLS) mission, EM-1, in late 2017.
As currently planned, Lunar Flashlight will be a 6U CubeSat, 10 by 20 by 30 centimeters in size. Its key technology is a solar sail with an area of 80 square meters. The sail will serve as the spacecraft’s main propulsion, maneuvering it after release from the SLS into a lunar capture orbit, and then into a final orbit that takes the spacecraft within about 20 kilometers of the lunar south pole. That process takes time, given the solar sail’s weak thrust: about six months to achieve the initial capture orbit, and then another year to spiral down into to its desired orbit.
Once in that science orbit, the solar sail takes on a new role. As the mission’s name suggests, the sail will reflect sunlight into permanently shadowed regions of craters at the south pole. A spectrometer on the spacecraft, operating in the near infrared and including absorption lines associated with water, will measure the reflected light, allowing scientists to identify those shadowed regions with deposits of water ice.
“We’re pretty confident that we’re going to be able to distinguish water ice from dry regolith at about 0.5% by weight,” said Barbara Cohen of NASA Marshall Space Flight Center, the principal investigator on Lunar Flashlight, in a presentation about the mission at the NASA Exploration Science Forum at NASA Ames Research Center on July 23. It should also be possible, she added, to distinguish between water ice and carbon dioxide ice.
While such detections have obvious scientific interest, they’re also useful for any future human exploration missions, filling what NASA dubs a “strategic knowledge gap” in such plans and hence of interest to AES. That 0.5% level that is the goal of Lunar Flashlight is also the threshold for what in situ resource utilization experts call “operationally useful quantities” of water that can be easily extracted, Cohen noted.
Cohen said the mission is working to better understand the ability of the solar sail to effectively reflect sunlight into those shadowed regions. (The mission, and another CubeSat-class mission selected by AES to fly by a near Earth asteroid, plan to make use of solar sail technology from a project called Sunjammer, which Cohen said was recently descoped from a flight project to a ground test.) A mission concept review and system requirements review for Lunar Flashlight is scheduled for later this month.
Lunar Flashlight is not the only concept under study to look for water ice on the Moon using CubeSats. Two presentations Sunday at the CubeSat Developers Workshop, held prior to the start today of the annual AIAA/USU Conference on Small Satellites at Utah State University in Logan, described alternative mission concepts to use CubeSats to search for lunar water.
|“We can cheaply sample a whole range of different places” with CubeSat impactors, said Hermalyn. “The initial trade study seems promising and hopefully this is something we can continue to pursue.”|
One proposed mission, called Lunar Water Distribution (LWaDi), would use a 6U CubeSat equipped with a near infrared spectrometer to map the distribution of water and possibly other volatiles. The spacecraft would revisit the same regions at different times in the lunar day over a six-month mission to understand variations in volatiles as a function of time and location.
Walter Holemans of Planetary Systems Corporation, who presented the LWaDi concept at the workshop, said the decision to use the relatively untried 6U form factor was based on a cost-benefit analysis. Going from a more common 3U CubeSat to a 6U one does increase launch costs, he said, noting current market rates for launching CubeSats as secondary payloads of about $85,000 per 10x10x10 cm unit. “You get double the volume, so your systems engineering for things like packaging, electric power, and propulsion get a lot easier,” he said.
The LWaDi concept is still in its early stages, with many of its technologies, like its spectrometer, still at low technology readiness levels. Holemans estimated the mission would cost about $10 million. “The launch opportunities are manifold,” he said, with options ranging from flying as a secondary on SLS to hitching a ride on a Google Lunar X PRIZE mission to flying as a secondary payload on a commercial geostationary orbit communications satellite. In that last case, LWaDi would deploy from the satellite after it reaches GEO, and then use its own electric propulsion system to head to the Moon.
CubeSats could also have a more literal impact on the search for lunar water. In 2009, NASA’s Lunar Crater Observation and Sensing Satellite (LCROSS) mission detected water ice in the ejecta plume created by the impact of the probe’s upper stage. Could a CubeSat-based mission do something similar?
One challenge, said Brendan Hermalyn of the University of Hawaii and NASA Ames, is that CubeSats are far less massive less than a Centaur, which weighed more than 2,000 kilograms at impact, and thus would throw up less ejecta to altitudes high enough to be detected by other spacecraft. A 1U CubeSat, he said, is too small to generate enough ejecta to be observable. However, a “heavy” 6U CubeSat would produce just enough ejecta to be observable.
A mission design study at NASA Ames showed that a lunar ice detection mission using a series of 6U CubeSats is “relatively feasible” with current technology, he said. In that mission scenario, a series of such CubeSats would impact a shadowed crater, each observing the plume produced by its predecessor and transmitting those observations to an orbiting satellite before impacting.
|NASA’s first launch of its new heavy-lift rocket could, in an ironic twist, also help pave the way for exploration using very small spacecraft.|
The advantage of this approach, he said, it that it allows a cost-effective way to study a range of locations that might otherwise be inaccessible, particularly to any future human missions. “We can cheaply sample a whole range of different places” with this approach, he said. An initial set of impactor and follower spacecraft would cost on the order of $20 million, he estimated, with additional spacecraft in the set costing $10 million or less. “The initial trade study seems promising and hopefully this is something we can continue to pursue.”
In the question-and-answer session after his presentation, an audience member asked if the ability of the CubeSat impactors to generate ejecta could be enhanced by packing them with explosives. “I’m all for making bigger craters,” he said to laughter from the audience, “but there’s a lot of concerns about contamination, particularly if you’re looking for hydrocarbons.”
A new prize competition NASA is about to start may also stimulate the development of lunar CubeSats. In June, NASA released a request for information (RFI) seeking input on a pair of proposed new prizes involving CubeSat missions to lunar orbit and in deep space. The RFI, whose deadline for responses was July 11, was intended to gauge interest in the proposed prizes and to get feedback on the draft rules.
That feedback convinced NASA to proceed with their plans. “Today, we are announcing two new challenges,” said Eric Eberly, deputy program manager of NASA’s Centennial Challenges prize program, at the CubeSat workshop on Sunday. Registration for the competition will open in the fall, he said, with a “kickoff summit” likely to be held at that time.
The lunar CubeSat challenge is the more lucrative of the two, with a prize purse of $3 million. Half of that will be shared among the teams whose CubeSats achieve lunar orbit. A $1-million prize will go to the CubeSat that demonstrates the largest volume of error-free communications, and $500,000 for the CubeSat that lasts the longest time. A separate deep space CubeSat challenge will offer $1.5 million in prizes: $1 million for error-free communications, and $250,000 each for the CubeSats that last the longest and operate at the greatest distances from the Earth.
Besides the prize purses for the two challenges, NASA is offering $1 million in prizes for ground qualification tests of CubeSats. Those qualification tests will also be used by NASA to select several CubeSats that will get free rides as secondary payloads on the SLS EM-1 launch. The exact number of slots available is “still up for determination,” Eberly said: the competition is guaranteed at least one slot for a 6U CubeSat but could have six or more. Competitors can also choose to make their own launch arrangements.
These missions are designed to be technology demonstration, but Eberly said competitors can elect to place instruments or other experiments on them as well, something that drove them to the larger 6U size. “It allows teams to potentially seek out somebody who wants a ride for their experiment,” he said.
Specific rules for the competition are still under development, he said, and competition deadlines will be based on the timeline for the EM-1 mission, currently planned for late 2017. NASA’s first launch of its new heavy-lift rocket could, in an ironic twist, also help pave the way for exploration using very small spacecraft.