Mission Design for Setting up an Optical Telescope on the Moon
Yuki D. Takahashi
California Institute of Technology, Pasadena, CA, USA
Summer trainee at Tsukuba Space Center, National Space Development Agency of Japan (NASDA)
To observe the universe with revolutionary clarity, we humans can utilize the unique environment on the Moon. The Lunar surface has so many significant advantages for a telescope that it may be the best place around to observe the universe from. This paper shows the preliminary process of designing a mission to set up a telescope on the Moon. First, mission requirements are defined and a general flow of the mission is outlined. Then, possible tradable options (such as telescope site and aperture size) are investigated and system drivers are identified. After characterizing the mission further, one combination of tradable options is chosen through a tradeoff process. Initial space mission design process is presented so that it may be referred to in developing an actual design that will someday put a telescope on the Moon.
Humans have always wanted to look deeper into the universe. We have always wanted to look clearer at an endless variety of objects and phenomena that surround us. We have always wanted to look for something new - something we could not see before because of the environment we have on Earth. When placed above the atmosphere, the Hubble Space Telescope and others amazed us with what are out there. Even though space telescopes were expensive effort, money was spent to satisfy our curiosity for the unknown. If we want to find out what we are still missing out in this awesome universe, Moon is the place to be. A workshop on the topic"Astronomical Observations from a Lunar Base" in 1986 concluded that "the Moon is very possibly the best location within the inner solar system from which to perform front-line astronomical research". Today, having a telescope such as the one here proposed is within our technical reach. This baseline design can be a starting point of the actual design of the mission to set up a revolutionary telescope on the Moon.
To observe the universe with revolutionary clarity utilizing the unique environment on the Moon.
To set up the first one of several telescopes required for interferometry in the future.
To increase cooperative relationship between nations by doing this as an international effort.
Reason for choosing the Lunar surface as the site for a telescope
Compared to telescopes on Earthís orbits:
* Data delivery
- Scientific data are collected with CCD cameras, compressed and transmitted to the ground and archived in a central location for access through the Internet.
- Housekeeping data are automatically processed on observatory.
* Tasking, scheduling, control
Like Hubble, commands will be sent from a central ground station.
* Communications architecture
Ground antenna -> observatory antenna, with or without relay.
Data rate fast enough to transmit data acquired almost continuously.
2005~2020.......... Building & testing
2020.................. Launch, land, set-up
- Launch on a rocket to escape Earth
- Travel to Moon in a landing module
- Orbit Moon to prepare for precise landing
- Precise landing using retrorocket
- Setting up solar panel & antenna with rover
2020~................. Full operation, upgrade every 10 years
Area of interest - performance.
* Telescope aperture
* Location of the telescope
* Data rate
- Data sources
CCDs, other instruments
- Sampling rate
- Quantization level: 8 bits/sample
Thermometer (system temperature)
- Storage location: central data archive for public access
- Access time: – ~6 hours / day (depending on availability of ground antenna)
- Data quantity: transmission rate * access time
- Transmission delay – 2 seconds
- Availability, reliability
Orders will be sent from the ground, assigning such tasks as where to point, what instrument to use, how long to collect data for, when to store/transmit data, when to receive next command, and when and how to re-calibrate coordinates.
Fixed or tracking. The tracking option will require a sensor for tracking using guide stars.
Far-side or polar region.
If fixed mount is chosen, the telescope should be placed so that the most interesting objects will pass through the view. Landing the telescope to point at precisely the desired direction may be difficult. Also, since integration would be possible only within the beam width (as the Moon slowly rotates), the telescope must be pointed at such declination that integration time can be long enough for observation of faint objects. This means placing it at the Lunar latitude corresponding to the desired declination, assuming the telescope will point directly up. For example, if the field-of-view is 2.5’ (like the Hubble’s wide field camera), only up to 5 minutes of integration is possible for telescope pointed directly up at the Lunar equator (because the apparent motion of the sky there is about: ).
If polar region is chosen, south pole will be preferred because the universe is less explored from the southern hemisphere and interesting places like the galactic center can be observed from that side. In addition, the regions around the south pole have more permanently shadowed places (for the telescope) and spots that receive sunlight almost permanently (for solar cells). In placing the telescope, we must find a permanently shadowed spot that is sufficiently close to an almost permanently lighted place for acquiring solar power. This may be difficult. Moreover, if direct communication with the ground from the telescope site is desired, the site must be sufficiently close also to a place with at least occasional view of the Earth. Finding such an ideal spot may be demanding.
Large or small.
An upper limit to the telescope aperture would be about 4 meters. This is because of the limited fairing size of the largest currently available launchers (4.5m for the U.S. Space Shuttle or Titan IV, and 5.5m for Energia).
If a telescope with a certain aperture is chosen, its mass and power requirement can be estimated by scaling from a model. Since this telescope will be made using ultra lightweight optics as in NASA’s Next Generation Space Telescope (NGST), one of the potential NGST designs was used as a standard model for the telescope. According to this design, an NGST observatory with an 8-meter aperture is estimated to weigh 3000kg and consume 760W of power on average. Then, a telescope with an aperture of A meter will weigh
, and consume
The factor of K attempts to compensate for the fact that the Lunar ground-based telescope requires a stable mount (maybe tracking) unlike the NGST floating observatory. The results of these estimations are as follows:
Cameras (visible, infrared, ultraviolet): Must consider wavelength range, resolution, sensitivity, field of view, focal length, data rate, command memory size, heat rejection, …
Cameras only or additional instruments: Like spectrographs for each range of frequencies.
Must consider: range, accuracy, coordinate calibration method, stability (change rate of angular orientation), and tracking mechanism.
To determine where the telescope is pointing after the initial landing and to guide the telescope thereafter, wide-field guidance sensors will be required. Low gravity and light optics will ease the development of mechanism for pointing the telescope mirror. The telescope will be tracking the same spot unless otherwise ordered by command. Pointing direction will be relative to certain guide stars.
Range: If the telescope is fixed, only the field of view throughout Moon’s rotation is in the observable range. If the telescope is steerable, one in the polar region can cover almost a semi sphere while one on the far side can cover almost the entire sky.
If the far side is chosen for the observatory, a relay satellite is required; if the right spot on the polar region is chosen, direct communication with the ground may be possible without a relay.
If communication through a relay satellite:
If direct communication with Earth:
20cm, 10kg, 50W (2)
Command processing, data acquisition, compressor, temporary storage
Need to supply power for telescope (tracking mechanism), antennas, lander, rover, computers
If a polar site is chosen, solar panels must be placed in a spot with almost constant sun-shine.
If far side is chosen, a large battery is required for the Moon’s 14-day nights.
If 300W on average is required for the observatory operation, the battery needs to store (2). Even assuming that the NaS re-chargeable battery will be developed with extremely high specific energy density (~200Wh/kg), the battery would weigh (2).
Payload capacity and thrust for transfer to Moon.
If a large telescope (aperture ~ 4m) is chosen: Shuttle / Titan IV / Energia
If a smaller telescope (aperture ~ 2m) is chosen: almost any system
Upper stage(s) will be required for transfer to Moon.
Depends on vehicle: Kennedy Space Center / Bailkanor
Data will be compressed, and stored until opportunity for transmission.
Telescope antenna, relay satellite (if chosen), ground antenna (Deep Space Network).
One existing ground control center
Data rate: >10Mbit/s
Users (scientists) submit instructions and controllers send commands.
Central automatic command processing & transmission
Full ground command & control
Automatic environment adjustment/control (temperature)
Scientists, Controllers, Commanders, Communicators
* Performance better than Hubble
- Sufficient coverage, high resolution, high sensitivity
* Lifetime >100 years
- Long-term reliability, survivability against Lunar environment
In the following, each option will be evaluated first in terms of performance (p), then in terms of cost (c). [O = good, V = normal, X = bad]
+ Any object of interest can be observed within the range.
+ Virtually infinite integration time is possible.
- Much more expensive and failure is possible.
+ Much less expensive and easy.
+ Scan survey could return useful results.
- Very limited observing range.
- Allows integration only within beam width.
- Pointing the telescope at precisely the desired direction may be difficult.
- Application is very limited ñ far inferior to Hubble -> (Since this telescope should not be inferior to Hubble, this option is out of question.)
+ Virtually infinite integration time possible.
+ Permanently shadowed and cryogenic.
+ Solar panel could be placed in a constantly lit spot4.
+ Antenna could be placed in a spot with view of Earth so that direct communication is possible.
- Finding a good and safe spot may be very difficult*.
- Even if a good spot found, may be dangerous to set up.
- Only a hemisphere observable.
+ Most sky observable.
- Severe temperature variations.
- Full observation only during nights.
- Must have a very large battery for nights.
+ Greater gain and resolution (better than Hubble).
- More massive and power-consuming.
+ Light and easy
* Less gain and resolution, but still great.
+ Will be sufficient.
+ Much more useful.
+ Red-shifts can be measured.
- More expensive.
Communications architecture (p-V c-V)
* Must for far side.
+ Have been done before.
- Communication time with Earth limited
* May be possible for pole.
+ More communication time with Earth.
- Placing the solar panels and antennas with rovers will be difficult.
3rd row from bottom is evaluation in terms of performance.
2nd row from bottom is evaluation in terms of cost (including cost for assuring reliability).
1st row from bottom is the overall evaluation.
Since performance is of higher priority than cost for such a revolutionary effort, the following was chosen for the baseline of the mission.
Although this is only the beginning of the mission design process, what is on this paper has a hope of developing into a rigorous and realistic design of a Lunar telescope mission if worked out further. That is my plan. In collaboration with experts in fields related to telescope design and space mission design, I would like to make earlier the day we can observe the universe from the Moon.
Calculations for Antenna Parameters
Possible Telescope Site in the Polar Region
Other Advantages of Setting up Observatories on the Moon 1
- Low radiation background
- Cryogenic surroundings (<70K at poles)
- Stable platform - low seismicity (10-6 times Earth)
- Wider frequencies range observable
- Better resolution achievable
- 14-day nights (long integration time)
- Reduced gravity (1/6 Earth)
- Week magnetic field (10-3 Earth)
- Near and easily accessible from scientific stations (when Lunar base established)
- Large areas, craters for fitting antennas in
- Raw materials for construction
* Micrometeorites: 10mm-craters form at a rate of 300/m2 per year 1.
* Electronics in cryogenic environment.
* High-precision placement of telescope.
* Vibration resistance upon landing.
Why Optical/Infrared/Ultraviolet Telescope was Chosen 1,5
Other reasonable choices:
* Very low-frequency array
* Cosmic-background radiation
X-ray, gamma-ray, cosmic-ray, Moon-Earth radio interferometer, gravitational waves, optical interferometer, radio, radio very large array, radio very long-baseline array, millimeter-wave array, neutrino
1NASA. Future Astronomical Observatories on the Moon. NASA Conference Publication 2489, 1986.
2Larson, Wiley J., James R. Wertz. Space Mission Analysis and Design. Microcosm, Inc., Kluwer Academic Publishers, 1992.
4Bussey, D. B. J., P. D. Spudis, M. S. Robinson, Illumination conditions at the lunar south pole, Geophys. Res. Lett., 26, No 9, 1187-1190, 1999.
5Advanced Mission Research Center. Proposal of SELENE2. NASDA, 1998
This work was done during my 2-week training in September 1999 at Tsukuba Space Center of Japan (with SELENE group). I would like to thank Iwata-san for always being willing to answer questions and help me not only with mission design, but also with my learning of Japanese culture and language. I would like to thank Kawakatsu-san for planning a very interesting and productive time and for always being cheerful, patient, and responsive. I would like to thank Yokoyama-san for trying to help me as much as possible in his busy schedule. I would like to thank Hirata-san for taking an interest in my project and referring me to related articles. I would like to thank Togai-san for talking to me and checking out reference materials from Tokyo University for me. Finally, I would like to thank many other people at Tsukuba Space Center, especially in the SELENE Project Team, who were very friendly to me and were willing to help me have a memorable time.