The viability of a Lunar Observatory

Graeme Ing, HET606, Swinburne Astronomy Online

Introduction

The world’s observatories have performed incredible science in the last few hundred years, and in the last quarter century, sophisticated space-based telescopes have provided even greater image resolution. Every year instrumentation increases in size and sensitivity. Why then, should we consider building observatories on the Moon? What does the Moon offer astronomers?

This paper outlines the advantages and disadvantages of a lunar observatory, suitable sites and recent proposals for building one.

Many visionaries have championed the cause for a lunar observatory, including Dr. Von Braun. The USSR included an astronomy lab in their proposal for a lunar base as early as 1969 (ILOWeb). Perhaps the first and only lunar telescope takes the form of a tiny Ultraviolet (UV) camera-telescope left by Apollo 16 in 1972 (NASAWeb).

The Advantages

The list of advantages is long and compelling, the most significant being the lack of lunar atmosphere. Earth-based telescopes are hampered by atmospheric extinction – the attenuation in strength and quality of light as it passes through Earth’s atmosphere. Similarly, our atmosphere causes scintillation and image distortion; effects that can seriously reduce the resolution of ground-based telescopes. Storms, cloud cover and precipitation all reduce viewing time. A lunar telescope would escape these problems, enjoying 100% clear, un-attenuated and undistorted viewing. It would not require expensive adaptive optics to compensate for atmospheric distortion, or be sited upon a mountain peak to reduce extinction (NASAWeb; ILOWeb).

Earth-bound observatories have to rely upon atmospheric windows. The atmosphere is opaque to most of the electromagnetic (EM) spectrum with the exception of some Infrared (IR), visible light and some radio frequencies. To observe UV, X-Rays and Gamma Rays we must move our telescope above the atmosphere and into orbit. The Moon has no such atmospheric windows, allowing observation across the entire EM spectrum (Lester 2004).

Another advantage for a lunar observatory is the very low lunar seismicity and lack of plate tectonics. NASA estimates an average lunar ground motion of less than one micron (NASAWeb), compared to several centimeters per year for some Earth tectonic plates. (USGSWeb).Telescope stability is vital for long baseline interferometers whose accurate results depend upon precise, stable distances between component telescopes.

Many of these advantages also apply to space-based telescopes, but they require a fuel reservoir for orbital corrections and must deal with obstruction from thousands of orbiting objects. The lifetime of a space-based telescope is typically 5 to 10 years (ILOWeb). In comparison, a lunar telescope offers operational lifetimes as high as 100 years, suffers no elemental weathering and few if any orbital objects transiting the field of view. Its power source is free solar energy rather than expensive chemical fuel.

A lunar “night” is 14 days long, allowing very high integration times, the length of time that a telescope can continually observe an object, gathering even faint EM radiation. Long integration times are critical for deep field extra-galactic observing. Compare this with Earth’s 12 hours and a mere 45 minutes for the Hubble Space Telescope (HST) (NASAWeb). Using baffles for shade, a lunar telescope can continue to observe during the lunar day without the telescope becoming swamped with atmospheric light scattering, nor are observations hampered by synthetic light pollution.

The Disadvantages

The major disadvantage to establishing a lunar observatory is cost. The HST cost $2b to place into orbit (ILOWeb), yet its 2.4m mirror is small by terrestrial standards. Landing a telescope on the Moon is an order of magnitude more complicated and costly. Getting the payload into Earth orbit is only the first stage; the launch vehicle must transfer the payload to the Moon and ensure that its components survive the landing. Both America and the USSR have landed science probes on the lunar surface – e.g. the Surveyor series – but a proper lunar observatory demands more than a space probe loaded with sensors; it requires one or more highly sensitive instruments to be firmly mounted on the surface.

The maximum size for a pre-formed mirror or detector is limited by the maximum diameter of today’s largest launchers, e.g. the Space Shuttle (Takahashi 1999). The James Webb Space Telescope (JWST) will launch with a 6.5m mirror. Larger diameters would require unfolding or assembly on the lunar surface, increasing mission complexity and risk.

The lunar surface is subject to bombardment by micrometeorites. No significant study has measured the strike frequency at a variety of surface locations, so we cannot assess the risk of micrometeorite damage to the surface of a telescope or its support equipment. Even a non-direct hit may disturb enough lunar regolith to cover and degrade delicate instrumentation over time. Again, no study has measured the movement of regolith (Mendell 1993).

A more continual threat is damage to the telescope from solar and cosmic radiation, and harsh temperature shifts of up to 350 degrees between lunar day and night (ILOWeb).

Lester (2004) comments that space-based observatories have a lower launch risk, are human accessible for maintenance and can track objects more effectively with less weight than a lunar surface-based telescope, even in the low lunar gravity.

A Suitable Location

Most lunar observatory proposals limit their landing site to one of two major areas, the lunar poles or the far side.

A far side site is attractive to non-optical telescopes chiefly because the mass of the Moon itself shelters instrumentation from man made noise. Almost the entire sky is observable throughout the lunar day/night cycle, although daytime observations would require baffling to shade instrumentation from the Sun’s heat and light. Drawbacks include a lack of permanent sunlight for power, requiring either batteries or a nuclear power source. Without line-of-sight to Earth, lunar satellites would be required to relay data to and from the observatory (Takahashi 1999).

A polar site offers the ability to position solar panels on a mountain high enough to receive permanent sunlight, as well as a direct line of sight to Earth for communication Within a permanently shadowed crater the telescope can enjoy almost infinite integration times, allowing extremely deep field observations. The drawback is that observation is limited to a single hemisphere (Takahashi 1999). The southern pole provides visibility toward the galactic center, whereas the North Pole allows for extragalactic observations.

International Lunar Observatory (ILO)

One of the most ambitious proposals is that of a partnership of Space Age Publishing, SpaceDev Inc., and the Lunar Enterprise Corporation. Their plan is to position a multi-wavelength dish of 2m diameter at the south lunar pole. They propose a 1 to 5 year time frame at a cost of $50M to $75M (ILOWeb). This figure is remarkably low given the $2b cost of the HST and the proposed $500M to $1b cost of the JWST, which is already over budget (Atkinson 2005). ILO’s business plan is to encourage the eight leading international space agencies to split the cost. $10M per agency is likely to be a tiny fraction of 1% of their yearly budget. The ILO instrument itself will be a single unit 3m high, housing the dish, solar panels and antenna for direct communication with Earth (ILOWeb).

Radio Observatory at the South Lunar Pole

Takahashi (2002) proposes an elegant design for a radio interferometer to be located at the south lunar pole. Comprised of 50 dipole antennae with a baseline of 50km, it would become operational by 2015. A south polar site offers visibility to nearly all coordinates within the Milky Way and high integration times with minimal tracking. Radio antennae are less susceptible to temperature extremes, dust and meteorite strikes than other telescope types. Hundreds of craters provide suitable, low cost “mounts” for radio dishes, similar to Arecibo in Puerto Rico (Takahashi 1999).

The Moon offers plentiful real estate upon which to construct numerous radio interferometers with baselines of hundreds of kilometers. An interferometer could include far more antennae than space on Earth allows, providing significantly higher gathering power. The potential also exists for an Earth-Moon interferometer, providing the longest baseline yet.

Observations in the 2 to 30MHz band are extremely difficult on Earth or Low Earth Orbit due to man made and solar interference at these frequencies, 10 thousand times as strong as a deep space signal. Shielding against this noise must be larger than the wavelength of the noise, several kilometers in this case. The Moon itself provides excellent shielding (Takahashi 2002).

The low lunar gravity allows for larger steerable dishes than on Earth, and the Moon’s low seismicity provides the stability for a high angular resolution interferometer (Hannula et al 1991).

International Lunar Far side Observatory and Science Station (ILFOSS)

ILFOSS proposes optical and radio interferometers comprising 1 to 3 telescopes of 1.5m diameter each and five dipole radio antennae forming a Very Low Frequency Array (VLFA), all located within 100m of the central “base” unit. At a cost of $26b over 20 years, Mendell (1993) points out that this amount is about the same as a single year’s spending for all world space agencies.

The ILFOSS plan emphasizes the need to adopt a simple phased approach to a lunar observatory. Additional instruments, such as X-ray and Gamma ray detectors, are added over the 20-year period. In addition to the initial instrumentation, the first phase of ILFOSS will include a communications satellite placed at the L2 Lagrange point to maintain communication with Earth. Within 7 years, the VLFA would expand from five to 280 antennae in a 17-km circle.

Liquid Mirror Telescope

Scientists from the USA and Canada have proposed constructing a deep field Infrared observatory in a lunar crater shielded from sunlight. A Liquid Mirror Telescope (LMT) would be a perfect instrument in the lunar environment. The mirror in an LMT is formed by rapidly spinning a pool of liquid metal such as mercury. On Earth, an LMT has a maximum diameter of about 6m due to interference effects from turbulent air above the spinning liquid, but in a vacuum, a lunar LMT could utilize a much larger mirror such as 20m in diameter. Important for lunar operations, an LMT is inexpensive to construct, since it is non-steerable and always points at the same position above the horizon. There is no tracking mount and no complicated adaptive or active optical systems. (Atkinson 2005).

A dark crater at the south lunar pole would house the telescope in temperatures as low as 40K. Extremely cold temperatures are essential for an IR telescope. Another advantage for a lunar IR telescope is the absence of airborne moisture. Moisture blocks IR radiation to the extent that even at the high altitude of Mauna Kea in Hawai’i, only 40 to 80 nights a year are suitable for IR observing (ILOWeb). A 20m LMT at the lunar south pole would provide three times the resolution of the JWST along with very long integration times, allowing IR objects hundreds of times fainter than ever before to be seen (Atkinson 2005).

Conclusion

Currently, cost prohibits us from taking our science to the Moon with any kind of permanence. I believe it is inevitable that we will do so, for the numerous advantages outlined in this paper, although I concur with Atkinson (2005) that science might have to wait for human lunar exploration to come back into vogue so as to piggy-back upon the cost of manned lunar missions. It is going to be significantly easier for humans to construct lunar observatories than design robotic craft to do the complex task for us.

A 1986 workshop considering lunar observatories concluded, “The Moon is very possibly the best location in the inner solar system from which to perform front-line astronomical research.” (Takahashi 1999).

I conclude that future astronomers will situate telescopes on Earth, in orbit, on the Moon and beyond, each location serving its own purpose in the never-ending pursuit of imaging the universe.

References

Atkinson, N. 2005 “A Pristine View of the Universe... from the Moon”

http://www.universetoday.com/am/publish/pristine_view_universe_moon.html?2812005

Hannula, et al. 1991 “Far Side Lunar Observatory”

http://www.tsgc.utexas.edu/archive/design/farside.html

ILOWeb: International Lunar Observatory

http://www.spaceagepub.com/ilo/ilo.home.html

Lester, D. 2004

http://www.spaceref.com/news/viewsr.html?pid=12416

Mendell, W. 1993 “An International Lunar Farside Observatory and science station”

http://ares.jsc.nasa.gov/HumanExplore/Exploration/EXLibrary/DOCS/EIC039.HTML

NASAWeb: NASA Aerospace Scholars

http://aerospacescholars.jsc.nasa.gov/HAS/cirr/em/8/3.cfm

Takahashi, Y. 1999 “Mission Design for Setting up an Optical Telescope on the Moon”

http://www.ugcs.caltech.edu/~yukimoon/MoonTelescope/

Takahashi, Y. 2002 “A concept for a simple Radio Observatory at the Lunar South Pole”

http://panic.millennium.berkeley.edu/~yukimoon/Takahashi-VLFA.doc

USGSWeb: USGS - Understanding Plate Motions

http://pubs.usgs.gov/publications/text/understanding.html