Harvard astronomers, who for 200 years have studied the stars and planets through the telescope, the spectroscope, and the photograph, are developing a new group of electronic instruments for searching the heavens.
Until the last decade, nearly everything we know about the planets, stars, and galaxies came to us on light waves in the narrow, visible portion of the spectrum. Astronomers have likened the visible region of the electromagnetic spectrum--which stretches from short-wave gamma rays at one end and the long radio waves at the other--to a tiny window looking out on the universe. Most of the revolution in modern astronomy stems from the design of instruments that have opened up new "windows" in the electromagnetic spectrum.
Many of these telescopes, cameras, radiometers, spectrometers and spectrographs have been built, and others are being built by astronomers at the Harvard College Observatory and the closely associated Smithsonian Astrophysical Observatory. The laboratories on Observatory Hill have become modern electronic workshops.
Some of the devices built in Cambridge are for use with telescopes and other instruments at the half-dozen or so stations in the United States and abroad where University astronomers are working; others are designed to observe the universe from satellites above the earth's obscuring atmosphere. The following is a report on three of these new electronic instruments, each of which peers through a different window in the electromagnetic spectrum: the infrared, the far ultraviolet, and the radio.
How Hot the Moon
Nearly all that we know about the moon has come to us in visible sunlight, reflected from the moon toward the earth. There are other, longer waves that travel invisibly between the moon and the earth, which can add knowledge about our satellite: the infrared, or radiated heat, rays. Although thermal photography is not new, the Harvard College Observatory has recently built one of the most sensitive instruments in existence for making thermal "pictures" of the moon.
The instrument, about the size of a TV camera, is called a radiation pyrometer. It was built under the supervision of Donald H. Menzel, Director of the Harvard College Observatory, with the collaboration of Hector Ingrao, Research Engineer and Lecturer on Astronomy, who designed the device. The pyrometer is a product of the Observatory's Infrared Laboratory, established with funds from the National Aeronautics and Space Administration.
The pyrometer, which has already performed well on the 61-inch reflector telescope at Harvard's Agassiz Station, measures the temperature of a small area of the moon's surface at a time. On a map of the moon some five feet in diameter, this area is about the size of a postage stamp. As the pyrometer scans the moon from side to side and from top to bottom, the record of intensity maps the distribution of temperature over the whole disc of the moon.
The heart of the instrument is a tiny square "thermistor," one-tenth of a millimeter on a side, small than a pin head, and attached to two hairlike platinum wires. The electrical resistance of the thermistor changes in response to tiny fluctuations in temperature. Special filters allow only a very narrow band of the infrared, between eight and 14 microns, to fall on the thermistor. The filter rejects the shorter infrared waves omitted by the sun and reflected by the moon. The earth's atmosphere, where water vapor, ozone and carbon dioxide absorb other portions of the infrared, also acts as a sort of filter. However, one can easily derive the temperature of the moon from the intensity of infrared radiation in the band transmitted between eight and 14 microns.
Attached to the pyrometer is a 35-millimeter camera accurately aligned to take pictures of the exact area of the moon's surface from which the thermistor is recording. As the pyrometer scans, the camera and the thermistor record alternately.
The moon's thermal profile will be compared with a conventional map of the moon to see of any correlation exists between the temperature variations and such surface features as mountains, "seas," and craters.
Russian astronomers have reported signs of volcanic activity on the moon, and Menzel hopes to arrange with Russian observatories to report immediately to the Cambridge Observatory any signs of volcanic activity so that the observation may be corroborated, using the Harvard pyrometer.
In other experiments, the pyrometer will measure how fast the surface of the moon cools. From the cooling rate, astronomers hope to estimate the size of the particles composing the moon's surface--rocks or dust.
Under favorable conditions, the thermistor can detect a change in temperature on the moon of less than one degree Fahrenheit. The Infrared Laboratory has invented and built another detector, called a ferroelectric bolometer, which promises to be even more sensitive. It should be able, for example, to detect the heat given off by a human hand flashing across it from across the room.
The Flares of the Sun
Another "window to the universe, until recently closed to astronomers on earth, lies at the ultraviolet end of the electromagnetic spectrum. The earth's atmosphere absorbs most of the ultraviolet light reaching it from the sun and other stars. A Harvard ultraviolet spectrometer, designed to look closely at the sun from a satellite outside the earth's atmosphere, has already been tested aboard a rocket and following further test, will join a number of other instruments aboard an orbiting solar observatory sometime this year.
This solar observatory, called S-17, will be the second such platform in space. S-16, which went up about a year ago, is the first of a series that the National Aeronautics and Space Administration plans to orbit during the current solar activity cycle of 11 years. The Harvard instrument which S-17 will carry was made at the College Observatory by a group directed by Leo Goldberg, Higgins Professor of Astronomy. William Liller, Robert Wheeler Willson professor of Applied Astronomy, is assistant director of the project, and the instrument was built and tested by Edmond Reeves, William Parkinson, Donald Buckley, and Nathan Hazen.
The ultraviolet spectrometer, about the size and shape of a window box, will provide new information about the solar flares that erupt now and then from the sun's atmosphere, appearing as tongues of luminous gas flicking outward around sun spots. During a flare, clouds of ionized hydrogen gas--protons and electrons--shoot out, filling interplanetary space with intense radiation. When these clouds encounter the earth and pass through the earth's magnetic field into the polar regions, they produce the northern lights, and cause short-wave radio transmission to fade or black out.
A flare is accompanied by a burst of ultraviolet radiation, and the Harvard instrument can record this radiation in two ways. First, it can concentrate on a small spot in the center of the solar disc and record, in about 27 minutes, the intensity of radiation over the whole ultraviolet spectrum. During its other "mode" of operation, the eye of the spectrometer will scan the whole disk of the sun, back and forth, bottom to top, recording at just one wavelength. Each complete scan will take about four and one-half minutes and will provide a crude ultraviolet picture of the whole sun. Both these kinds of information will be recorded on magnetic tape and relayed to earth on command.
The intensity of the ultraviolet radiation that accompanies a solar flare varies from one point of the spectrum to another. Also, at any one wavelength, the intensity varies as the flare moves through the sun's atmosphere. Ultraviolet light is a kind of thermometer, and these changes reflect temperatures varying from about 10,000 to about one million degrees centigrade. As it scans, the Harvard instrument will be recording the occurrence and spread of flares in every region. When the instrument is fixed on the center of the sun, it will provide more detailed information on how and where flares originate and move.
Our Noisy Universe
When tuned in on sensitive radio telescopes, the apparently silent universe becomes a hissing, noisy chorus. This discovery led to the science of radio astronomy, which has added to knowledge of the universe through radio telescopes tuned to various frequencies at the radio end of the electromagnetic spectrum. (The Harvard 60-foot radio telescope, for example, listens to 21-centimeter waves, the wavelength of emissions from neutral hydrogen gas. Hydrogen is the most abundant element in the universe.)
But the very long, low-frequency waves from the universe never reach terrestrial receivers because they are blocked by the earth's ionosphere, the outer layer of the atmosphere composed of ionized, or charged, molecules and free electrons. These long radio waves convey a great deal of information--about the origin of cosmic rays, magnetic fields in space, the mechanism of solar flares, and radio storms on Jupiter, for example. To record these long waves, Harvard, in cooperation with the Air Force Cambridge Research Laboratory, has built and orbited a series of small radio telescopes above the opaque ionosphere.
The Harvard Space Radio Project is three years old and is led by Edward Lilley, Associate Professor of Astronomy. To date, some 13 radio telescopes have orbited the earth aboard seven satellites; the instruments, each about the size of a pocket radio, are tuned to various wavelengths from about 20 meters to six kilometers (four miles). The Project so far has been chiefly concerned with finding out just how noisy the earth itself is, a necessary prelude to low-frequency radio studies of other planets, the sun, and the galaxies.
Radio waves from earth usually bounce off the ionosphere and return to the ground, but astronomers suspect that the ionosphere has "holes" through which some radio waves can pass. The University group is investigating the homogeneity of the ionosphere by aiming radio pulses of various wavelengths from a transmitter at the Harvard College Observatory to the satellites as they pass over Cambridge. The information received by the radio telescopes is then relayed on command to a receiver at the Air Force Base in New Boston, N.H.
Astronomers suspect that the ionosphere itself may be a radio transmitter, and University astronomers hope to confirm this. They are also trying to find out how radio noise from the earth and its ionosphere fluctuates between day and night, and from latitude to latitude, and how such phenomena as sun-spot activity and solar flares influence its intensity.