In a recent presentation to the human exploration and operations committee of the NASA Advisory Counsel (NAC), William Gerstenmaier, Associate Administrator for Human Exploration and Operations Mission Directorate, over-viewed the challenges associated with the recently announced mission to capture an asteroid.
EM-2 Asteroid Mission:
The mission involves three segments: the first is the detection and characterization of the candidate Near Earth Asteroids (NEAs) phase; the second segment involves a robotic rendezvous, capture, and redirection of a target asteroid to the Earth-Moon system; the third segment involves a crewed mission to explore and sample the captured asteroid using the Space Launch System (SLS) and the Orion crew capsule.
Based on the initial target date presented to lawmakers in the FY14 Budget Proposal, the Orion trip to the captured asteroid is likely to be a realigned version of the 2021 Exploration Mission -2 (EM-2).
Concerning the redirection segment, Mr. Gerstenmaier – presenting the mission to the NAC – said that the high powered (40-kW) Solar Electric Propulsion (SEP) hardware “plays a key role” in this aspect of the mission.
The word “redirect” is being used for this part of the mission, because the asteroid cannot be maneuvered into cislunar space, it “must be heading back to cislunar space on its own.”
“(The asteroid is) continually thrust upon by the SEP, essentially deflecting it or redirecting it into the earth gravity or lunar gravity. With gravity assist – and then with some continued thrusting – you end with a deep (distant) retrograde orbit around the moon. That is a stable orbit that can be stable for up to a hundred years.”
Mr. Gerstenmaier admitted the team will have to work with a large set of criteria, in order to find the correct target asteroid.
“(We) have to find an object that is naturally of the right size, that has the right spin characteristics, right mass and which has to be heading back into cislunar space at the time.”
Interestingly, the Associate Administrator also noted the search for a candidate asteroid may result in some of the discarded Saturn IV upper stages from the Apollo era being spotted in deep space.
The S-IVB’s used during Apollo 8 through 12 are currently believed to be wandering around in a Heliocentric orbit, with Apollo 12’s S-IVB actually mistaken for an asteroid in 2002.
However, Mr. Gerstenmaier believes they can now determine where the Saturn upper stages are located and will be able to identify them, given NASA have calculations on their current positions. The density of the upper stage is also fairly low, when compared to an asteroid.
It was also noted NASA conducted a preliminary study with object 2009 BD, although it was determined that this asteroid could not be brought back to cislunar space until 2024, even if it met all of the conditions.
Per the proposed asteroid mission, a large body of work will have to be conducted on all of the mission elements.
One example provided by Mr. Gerstenmaier related to the SEP thrusters, that would need to fire for almost a year, requiring 12mT of xenon gas. Since the channel wall of the hall thrusters can erode, magnetic protection will also be needed.
Another challenge involves designing “a capture device that takes the wobble out” of the asteroid. Because of this, you need “a soft enough capture device like a spring tamper kind of device,” added Mr. Gerstenmaier.
“(NASA is) looking at the schematics of some inflatable device, that takes out (the) nutation slowly enough that it doesn’t overload (the) solar array. There is a lot of spacecraft motion. (The spacecraft is going to) essentially ride with this asteroid for a while.”
The spacecraft would use a “hydrazine system to despin this asteroid”. After this, the asteroid is redirected, using the thrusters, with the biggest challenge relating to capturing it – something that “couldn’t be guaranteed.”
The third part of the mission is the estimated 20 day crewed flight to explore and sample the captured asteroid, utilizing the SLS and Orion – with the centerpiece of the mission being the EVAs to document and take samples of the asteroid.
Mr. Gerstenmaier mentioned NASA teams “were looking at potentially using the launch and entry suits with a modified TMG (thermal meteoroid garment) to provide an EVA capability.
“We envision two EVAs opportunities during the attached period, each with a duration of about 3 to 4 hours. (The spacewalks) would not be sophisticated EVAs, they would be simple, to go and grab a sample.”
To allow the spacewalkers to translate across to the captured asteroids, they would need “to put some booms across and modify the robotic spacecraft to have some handles for translation path.” Mr. Gerstenmaier added that the Orion EVA capability “was a design case for Orion, but not many people thought that we were going to do it fairly early in the (exploration roadmap) sequence.”
It was also mentioned that NASA could conduct an EVA at the ISS, with the new suits, in order to buy down some risks.
According to Mr. Gerstenmaier, the mission makes sense, since it involves a lot of activities that were already being conducted by various directorates, unifying them into one mission: asteroid identification and characterization, solar electric power (solar arrays and hall thrusters), in-space robotic activities, heavy lift and Orion.
The mission also builds NASA’s “tool box” for the ultimate goal, Mars.
“The robotic spacecraft that we build may be the robotic bus that (NASA is) going to use for propulsion in the future,” adding he had spoken to the international community about this mission on two occasions, and there is “general interest in it,” – ranging from observation opportunities to technology or mission design pieces.
Using Orion also has its advantages, given “it serves a rescue function, or a safe haven function, for the habitation module. It can take a three quarter inch hole on the other side of the Moon and still return four crew safely.”
The asteroid capturing mission itself will not need a habitation module, but later missions are likely to need one. The habitat itself could be derived from Orbital’s Cygnus spacecraft or Japan’s HTV, incorporating a next generation life support system which will also involve demonstrations on the ISS.
The history behind the decision to use ESA’s ATV for the service module was discussed at the same meeting by Kirk Shireman, Deputy Manager, International Space Station Program.
“Initially, there was discussions about (additional) ATVs, but ESA doesn’t really want to build any more,” Mr. Shireman noted.
“ESA was really interested in building something new. They actually wanted to build a human capsule. We told (ESA) that (NASA) didn’t have any interest (in this), that they are welcome to do that but not with (NASA’s) money.”
Shireman explained that he sees “common system ops costs” as NASA money, and added that he didn’t see “why NASA would invest in ESA building a crewed vehicle for (NASA).”
However, per the arrangement for ESA to provide ATV technology for Orion’s Service Module, this is now something he sees as “a wonderful solution.”
The new mission will now progress towards a feasibility review, sometime this summer.
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