Yuki D. Takahashi

2001/2/22

Radio Astronomy from the Lunar Farside

PROLOGUE

Humans on planet Earth have yet to look out at the universe through the longest-wavelength part of the electromagnetic radiation spectrum, even though they have explored all the rest of the spectrum. The ionosphere on their planet prevents any observation from the ground at wavelengths greater than about 20m. Also, radio communication on the planet is growing so that they will soon need to do all radio astronomy from outer space. For some reason, they have yet to take advantage of their moon even though it would provide the most ideal platform for the study of the universe. Particularly, the farside of the Moon is the only place within their current reach where they can avoid all radio noises from Earth. They should really construct a low-frequency radio array on the Lunar farside so they can find out what they have been missing in this great new spectral region. Plus, by avoiding the noise from Earth, they will be able to better detect signals from other civilizations, which would be one of the most significant events in the history of humans on Earth.

Why radio astronomy?

Radio astronomy is likely to lead to a breakthrough in history of astronomy and a breakthrough in history of Earth.

Exploring through a new spectral range

We, humans on Earth, have essentially never observed the universe at any wavelengths greater than 20m (frequencies below 15MHz) because of absorption and scattering by the Earth’s ionosphere.Even at 30MHz (10m), ionospheric phase effects limit the interferometry baseline to only 5km, corresponding to only about 10 arcmin resolution.Observing through this new spectral range will lead to discoveries of new phenomena and new classes of objects.

http://sgra.jpl.nasa.gov/html_dj/ALFA-intro.html

Through this very wide spectral range, we will discover totally new processes and new object classes.For example, we may find new sources of coherent emission with sharp upper-frequency limits.Some pulsars are expected to be among the strongest sources for frequencies below 10MHz so that studies at very low frequencies will probe the extreme environment of pulsar emitting regions.

Finally, we may even learn something completely unexpected.I stay hopeful about the possibility of studying the very low frequency end of the cosmic background radiation by subtracting all the resolved objects.Fluctuations in the background at such low frequencies might tell us something about the early universe.I could also imagine the possibility of very low frequency radiation not being scattered by photons before recombination so that observations of such radiation will tell us about the universe even before the surface of last scattering.

In addition, we will be able to study intrinsically low-frequency emissions and strongly frequency-dependent processes as outlined (Weiler, 2000, The Promise of Long Wavelength Radio Astronomy):

Astrophysics

· Cosmology and large scale structure

· Formation and evolution of galaxies

- Cluster halos and intergalactic magnetic fields

- Evolutionary studies of radio galaxies

· Very high redshift radio galaxies

· Emission mechanisms and jet physics

· Star forming galaxies

- Galaxy halo emission

· Cosmic ray diffusion times away from galactic disks

- Emission and absorption mechanisms in galaxies

- “Fossil” radio galaxies

· Interstellar processes

- The origin, acceleration, distribution, and energetics of cosmic rays

- The structure and distribution of diffuse ionized hydrogen

- The origin and transport of turbulence in the ISM

- Interstellar refractive and diffractive scattering

· Structure and properties of HII regions

- Temperature, density, and clumpiness

- Separation of thermal and non-thermal components

· Structure, properties, and statistics of supernova remnants (SNRs)

- Relation to cosmic ray acceleration sites

- Location of “missing” SNRs

- Improvement of stellar birth/stellar death statistics

- Test of diffusive shock acceleration models.

· Discovery of new phenomena

- Searches for millisecond pulsars

- Searches for coherent emitters

- Searches for bursting extra-solar planets

Solar Physics

· Solar variability

- Type II and Type III burst studies

· Solar-terrestrial interaction

- Solar wind-Earth’s magnetosphere interaction

· Space weather forecasting

- Coronal mass ejection (CME) tracking to 1 AU

· Solar radar

Note in particular: The origin of cosmic rays is perhaps the most fundamental question still remaining from the classical physics era. The galactic diffuse ionized hydrogen (HII) is the only major component of the interstellar medium not yet surveyed (Reynolds, 1990). Accurately forecasting the arrival of coronal mass ejections at Earth days in advance will be important for protection (from severe geomagnetic storms) of high latitude power distribution systems, satellite systems, and users of shortwave communications and GPS navigation systems.

Discovering extraterrestrial intelligence

If we want to experience the first detection of signals from civilizations outside Earth during our lifetime, we better take action.Such a discovery will likely be a significant experience for all 6 billion people of Earth (if we can share the information well to everyone).Imagine how exciting it would be to try to find out what information the signal contains.It may start a whole new field of science.“Extraterrestrial intelligence” may become a major field of study, for example.It will be one of the most significant changes humans will ever experience.

Why Lunar farside?

Only location free from terrestrial interference

The farside of the Moon is the only place in the vicinity of Earth where we can avoid radio interference from Earth.Radio astronomers are working with higher and higher levels of sensitivities, now easily down to 10^-28 W/m²/Hz.Anywhere in view of any telecommunication emitters is contaminated in frequency by their harmonics and in propagation by stray emissions in their secondary lobes (Heidmann, 2000).

Surface for permanence & expansion (Douglas & Smith, 1985)

Unlike any setup in space, the Lunar surface provides a stable platform that leaves room for future upgrades of the setup.Such a platform allows us to start modestly and expand with time.

Surface has many additional advantages over free space for setting up an observatory:

We can place as many elements as we like in perfectly stable relative positions; we do not need to continually monitor and control their positions.
Such observatory would last virtually forever (Lifetime would be limited only by fuel for the relay satellite).
Access from scientific stations
There are craters to avoid lunar-surface interferences
stable: interferometry
only half of celestrial sphere need imaging.
Sun is blocked half the time
long integration time (slow rotation)
We can lay antenna elements wires directly on regolith.

How? (science requirements -> technical implementation)

How can we construct an observatory to open the new window to the universe and to actively seek signals from the extraterrestrial intelligence?We decide on the details of technical implementation based on science requirements and constraints.

Modified from http://rsd-www.nrl.navy.mil/7214/weiler/kwpdf/report-long6.pdf

Spectral range

We want to observe the universe through the spectral range inaccessible from Earth, at wavelengths longer than 20m (frequencies lower than 15MHz).To search for extraterrestrial intelligence, we will want to cover microwaves down to wavelengths of about 1cm (Heidmann, 2000).This is because microwave band is relatively low in background noise and contains the 21cm hydrogen emission line (extraterrestrial people will likely send out signals near this well-known line).Covering the microwave band will also allow us to study the cosmic background radiation at a revolutionary resolution.We have a lower bound for observable frequencies because the local interplanetary plasma cuts off frequencies below 30kHz and the ionosphere of the Moon cuts off frequencies below 90kHz (Douglas & Smith, 1985).

To do spectroscopy, we will also need to decide on the number and widths of spectral channels.These will depend on spectral resolution desired, but are constrained by the available data rate.

Sensitivity

Since the Lunar platform allows us to observe a source for days, we can improve the sensitivity as much as we want by increasing the integration time.Sensitivity also improves with wider observing bandwidth, but bandwidth is constrained by maximum data rate.We must decide on the bandwidth by compromising between integration time, sensitivity, and data rate.To get a very rough idea, we expect to be able to achieve sensitivities on the order of 10~100 Jy with integration time on the order of 10 minutes to 1 day using bandwidths on the order of 0.01~1 MHz.

Dynamic range & rapid imaging

We will consider the desired dynamic range and speed of simultaneous aperture plane sampling to design an array configuration.At low radio frequencies, the array images the entire sky at the same time because individual antennas have very low directivity at these wavelengths.For imaging with high dynamic range, we must remove the effects of strong radio sources from all sky directions because they will create sidelobes even in directions far from their positions.So we must design an array geometry that produces highly uniform aperture plane coverage in all directions simultaneously.We want the maximum number of different baselines (different directions and lengths) with minimum number of elements.

Resolution

Using interferometry, we want to observe the universe with the best possible resolution.We want the resolution limited only by scattering and scintillation in interstellar and interplanetary media, which we cannot control.Such scattering broadens the apparent angular size of source by a factor roughly proportional to the wavelength squared (see plot by Kuiper and Jones, 2000).We can see that a baseline of a few 100 km would give as much resolution as possible for frequencies below ~10MHz.

(Kuiper and Jones, 2000)

Location

We have several criteria in choosing the site for the radio array:

To be permanently shielded from Earth, longitude must be >98^o E/W and latitude must be <60^o N/S.
To be able to observe the entire sky, the site must be within 20^o of the equator.
As people develop bases on the Moon, we can avoid interferences from the lunar ground by choosing a site inside a strongly walled crater.
The site must be large enough to accommodate the desired baseline (a few 100 km, at least).

Crater Saha Proposal (SETI, Jean Heidmann, Claudio Maccone)

100km diameter, 3000m high circular rim, 102E, 2S

Tsiolkovsky: 113km, 128.5E, 20.5S

Data handling

Data handling is considered one of the greatest challenges for radio astronomy from the Lunar farside.This section describes the process (Weiler, 1998).

Sidelobe suppression

To reduce the delay beam sidelobes, all spectra will be multiplied by a combination of Gaussian and cosine functions to filter the frequency response of the array.

Cross-correlation of baselines

To obtain the cross-power spectra for each baseline and for each phase center, data from each satellite will be Fourier transformed into a time series of frequency spectra and then pairs of spectra (after sidelobe suppression) will be cross-multiplied.

Calibration

Phase calibration will be done by the X-band uplink carrier, to which all satellite oscillators are locked.Amplitude calibration will be done by periodically injecting a known calibration signal into the signal path between the antennas and low frequency receivers.We will also use comparison with know astronomical sources at the high end of the LFSA frequency range and comparison with ground-based observations of solar bursts using antennas of known gain.

Sensitivity & dynamic range

Confusion noise and dynamic range, rather than Galactic background, will limit the number of detectable sources.Confusion effects will be minimized by imaging all strong sources on the sky simultaneously so their flux can be taken into account for each field of view.Dynamic range is determined mainly by the number and distribution of visibility samples, the data signal-to-noise ratio, the quality of calibration, and the complexity of the sources being imaged.

Prospect

Yuki will form an international team to prepare a proposal for Radio Array on the Lunar Farside.

References

Works cited

Heidmann, J., Recent progress on the lunar farside crater Saha proposal. Acta Astronautica, 46, 661-665, 2000.

Heidmann, J., A proposal for a radio frequency interference-free dedicated lunar far side crater for high sensitivity radioastronomy. In 45th Congress of the International Astronautical Federation, IAF-94-O.1.330, Acta Astronautica, 46(8) 555-558, 2000.

Kuiper, T. B. H.Lunar Surface Arrays.Radio Astronomy at Long Wavelengths, Geophysical Monograph Series, Vol. 119, pp. 351-357, 1998.

Landecker, P. B. et al.Telerobotically Deployed Lunar Farside VLF Observatory.Robotic telescopes in the 1990s, ed. A. V. Filippenko, ASP Conference Series (ASP: San Francisco), vol. 34, p. 335, 1992.

Reynolds, R. J., in Low Frequency Astrophysics from Space (Lecture Notes in Physics,

vol. 362), ed. N. Kassim and K Weiler (Berlin: Springer-Verlag), 121, 1990.

Schrunk, D. et al. The Moon: Resources, Future Development and Colonization, 1999.

Weiler, K. W. and D. L. Jones. Low Frequency Astrophysics from Space (Opening the Last EM Window on the Universe).Summary of Space Initiatives to the Radio/Submm Panel, 16 February 1999.

Groups

International Lunar Exploration Working Group

IAA Sub-committee “A Lunar SETI Study”

ESA Lunar Study Steering Group