The L2 alternative
by Jeff Foust
|“We’re starting with something a little less ambitious” than a return to the Moon, said Farquhar.|
A bigger political challenge, though, lies two years down the road, when a new president is sworn into office. Depending on the status of the VSE (including how closely it is hewing to current budgets and schedules) and the new president’s interest in the program and NASA in general, he or she could take the program in a very different direction, or kill it in any recognizable form. Anticipating this change in administrations, one group of experts is studying an alternative to NASA’s current plan that would do away with lunar exploration entirely.
In 2004, the International Academy of Astronautics (IAA), a group of over 1,000 space scientists and engineers from 75 countries, published a report titled “The Next Steps in Exploring Deep Space”. The study, led by former NASA associate administrator Wes Huntress, provided a general blueprint for space exploration for the first half of the new century, with an emphasis on identifying the key destinations and roles for humans in space. The study, whose origins predate the announcement of the Vision for Space Exploration by nearly five years, identified four destinations of interest for human space exploration: the Moon, the Sun-Earth L2 point (about 1.5 million kilometers from the Earth), near Earth objects (NEOs), and Mars.
The IAA is now engaged in a follow-on to the “Next Steps” study intended to provide a distinct alternative to NASA’s current exploration architecture. “We’re starting with something a little less ambitious,” said Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory (APL). Farquhar, who is leading the new study, discussed the effort during a public colloquium at APL on December 1.
The alternative Farquhar and his international team of colleagues are investigating involves bypassing the Moon for more distant destinations. Instead, human missions would initially go to the Sun-Earth L2 (SEL2) point. That location would serve as a staging point for more distant expeditions, initially to NEOs and, later, to the Martian moon Phobos. Eventually the architecture would support human missions to the surface of Mars, just as such missions are an ultimate long-term goal of the VSE.
Why SEL2? The libration point, located in the direction opposite the Sun from the Earth, has become an increasingly popular destination for space observatories, most notably NASA’s James Webb Space Telescope (JWST), the successor to the Hubble Space Telescope. It’s particularly attractive to infrared telescopes like JWST that want to shield themselves from the Sun and Earth while remaining relatively close to the Earth. “The L2 libration point is going to be a place of intense astronomical activity in the next 20 to 30 years,” said Joseph Veverka, a professor of astronomy at Cornell University who is also involved in the IAA study. “There are at least 19 or 20 missions over the next 20 or 30 years that want to go to the L2 point.”
The current generation of missions that plan to operate at SEL2 are intended to be completely autonomous, with no capability of being serviced, unlike Hubble. However, Veverka believes that more ambitious, complex observatories currently being contemplated for SEL2 will require, or at least benefit from, human servicing missions. “Most of these missions are very elaborate, very expensive ones that represent large investments,” said Veverka. “I think there’s going to be a lot of pressure eventually to keep them going as long as possible, to fix any problems that arise, and to upgrade their instrumentation.”
With that initial destination and purpose in mind, Farquhar described how an alternative human space exploration program might operate. The IAA study assumes the existence of two key components of NASA’s current program, the Ares 1 launch vehicle and the Orion spacecraft. “We’re starting with those pieces of hardware and saying, ‘Well, what else can we do with them?’” he said.
|Human missions to L2 could be used to service a new generation of advanced observatories planned for operation there, Veeverka believes. “I think there’s going to be a lot of pressure eventually to keep them going as long as possible, to fix any problems that arise, and to upgrade their instrumentation.”|
The notional mission design that Farquhar presented is based on what he calls the “Deep Space Shuttle”, which is similar to Orion but features a reusable service module that would aerobrake into Earth orbit at the end of the mission. The vehicle would also have “drop tanks” carrying the propellant needed to send the spacecraft to SEL2 and back; as the name suggests, the tanks would be jettisoned after use. A 35-day round-trip mission to SEL2, including five days at the libration point to carry out telescope servicing, could be carried out with a total delta-v only marginally higher than a round-trip mission into lunar orbit.
While missions to SEL2 can carry out servicing of observatories based there, the libration point can play an even bigger long-term role, Farquhar believes. “The L2 libration point is the logical place to stage missions either to near Earth asteroids or to Mars,” he said. An “Interplanetary Transfer Vehicle” would slowly spiral out from SEL2, phasing its orbit to head to a NEO or to Mars. A Deep Space Shuttle carrying the mission’s crew would dock with the vehicle when it’s in position to head to its destination; both would be reusable after returning to Earth.
The idea of human missions to NEOs is not new (see “Near Earth asteroids: the third option”, The Space Review, January 26, 2004). However, the advantage of staging such missions from SEL2, Farquhar said, is that it requires far less delta-v than carrying out the same mission from low Earth orbit. As an example, Farquhar discussed a hypothetical 2025 mission to the near Earth asteroid 1999 AO10, a five-month round-trip that includes a 30-day stay at the asteroid. Staging the mission from SEL2, Farquhar said, would require less than half of the total delta-v than if the mission flew from LEO. The total energy requirements for a mission to Phobos would not be much higher, he added, although the trip time would be much longer.
“There’s a tremendous performance advantage doing it this way,” he said. “I did talk to [NASA administrator] Mike Griffin about this once, and I think it went over his head.”
One disadvantage of this proposed architecture is that rather than going to a universally-known destination like the Moon or Mars, it involves, at least initially, going to quite literally an empty location in space, a concept that may be a little too abstract and difficult to grasp for the general public or politicians. As one person noted in a Q&A session after the talk, “once you’re there, there’s nothing to plant a flag in.”
“Well, we’ve already got lots of flags on the Moon,” Farquhar countered. “But you can put flags on the near Earth asteroids, and then we’re going out to Mars, and I think this is the main goal.”
|“A lot of people think that all of these decisions have been made for the next 20 to 30 years,” said Farquhar, “and I don’t believe that.”|
Farquhar added that the expense involved with going back to the Moon could prevent missions to Mars or elsewhere in the solar system for decades. “I think that if we get stuck on the cul-de-sac of going to the Moon, I don’t see us going anywhere else for the next 75 to 100 years, because that’s such a huge program,” he said. His alternative, he said, “is not going to be cheap, but it’s a lot cheaper than what they’re talking about as far as the lunar program is concerned” because a lot less hardware is involved.
Such a study would, at first glance, seem to be little more than an academic exercise since NASA is currently committed to a lunar-centric exploration architecture. However, given that NASA’s efforts are in their earliest stages, there may still an opportunity to change direction. “A lot of people think that all of these decisions have been made for the next 20 to 30 years,” said Farquhar, “and I don’t believe that.”
Farquhar said he’s discussed this study with people at NASA Headquarters who are, at the very least, not opposed to the concept. “I’m not going to mention any names, but a lot of people at NASA Headquarters think it’s a good idea to look at these alternatives.”
That interest, he suggested, stems from the uncertainty that a change in presidents—and, most likely, change in NASA administrators—will bring to the space agency and the current exploration program. Farquhar said the IAA plans to complete this study and present it at the 2008 International Astronautical Congress, just a few months before the next president takes office. People at NASA, he said, “know things are going to change in just two years, with a new administration coming in, and what they’d like to have are several options.”