MIRO at Comet Wirtanen: what a gas!
10 August 2001What do a comet, the Voyager mission, the Big Bang and the search for extraterrestrial intelligence have in common? Answer: Dr. Samuel Gulkis from NASA's Jet Propulsion Laboratory, Principal Investigator for the Microwave Instrument for the Rosetta Orbiter (MIRO), has played a leading role in each of them.
Here Dr. Gulkis describes the unique science that MIRO will undertake and provides some other fascinating insights into his career in space science.
Q. Tell us about MIRO and what it will measure.
A. The flight model of MIRO - one of 11 experiments on the Rosetta Orbiter - was delivered to Alenia Spazio in Turin at the beginning of August. Since then it has been bench tested and integrated with the spacecraft.
MIRO has two radiometers. One operates at a wavelength of 2 mm and is used to measure temperature alone. The second operates at 500 microns, or 0.5 mm. It can measure temperature and is also a spectrometer.
The two channels can obtain temperatures at different depths. By obtaining two different temperature measurements, we will be able to find out how rapidly the temperature increases or decreases with depth below the surface of the comet's nucleus.
Each radiometer can penetrate the nucleus to a distance 10 - 25 times its wavelength, so we will be able to measure to a depth of a few millimetres at a wavelength of 500 microns and perhaps down to 1 cm with the radiometer operating at a wavelength of 2 mm. We would need a longer wavelength to go deeper. However, by combining our data with infrared measurements of the surface (from other instruments) we will obtain a range of temperatures and depths. These will tell us whether the nucleus has an insulating surface - possibly a dark, organic crust.
Q. What about the spectrometer?
A. This will measure some of the most abundant volatiles or gases found in comets - water, carbon monoxide, ammonia and methanol. We will try to tie together the temperature with the volume and numbers of molecules coming off the nucleus. The MIRO instrument will provide the first simultaneous measurements of nucleus temperature and production rate of molecules. We will also measure the velocities of the gas molecules.
One of MIRO's principal characteristics is that it will measure three different isotopes or types of oxygen atoms in the water of the nucleus and the coma. Their ratios can be compared with similar measurements taken throughout the Galaxy and the Solar System. This may help to tell us where the comet originally formed and whether it originated outside the Solar System in another star-forming region.
There are still many things that we don't understand about the formation of a coma. With Rosetta we will see the nucleus temperature change from about 100K (-173°C) to more than 200K (-73°C). This will cause large changes in the gas production rate.
By observing the comet for more than a year, we will be able to map the sources of the gases. If the spacecraft is at a distance of 2 km from the comet, we will have a 'footprint' on the nucleus of about 5 metres. At a distance of 10 km, the resolution will be about 25 metres. The gas sources will be mapped over the entire surface. By co-ordinating our observations with other instruments, such as VIRTIS, we should be able to see hot spots or gas jets and see them develop. At present we don't know whether they suddenly erupt or gradually seep out. Our understanding of these jets is very rudimentary and we don't know whether all comets look alike.
MIRO will also enable us to study the evolution of a comet. How much gas does it lose in each orbit around the Sun and how DO its mass and size change?
Q. I believe it will also be switched on during the flybys of two asteroids and the planet Mars?
A. When we pass by the asteroids, we will measure their temperatures and look for possible clouds of water accompanying them. If they contain water deep beneath their surfaces and this is slowly escaping it is possible we would see that.
Similarly, we know that there is a lot of water in the Martian atmosphere. MIRO is very sensitive and it will be able to detect water vapour at very high altitude - up to 140 km - in the Martian atmosphere. This cannot be observed from the ground. However, we will only have one glance since we fly by very quickly.
Q. Apart from Rosetta, you have also worked on COBE (Cosmic Background Explorer) and Voyager. These are all very different missions. How did you come to be involved in them?
A. The link is radio technology. I earned my PhD in radio physics and started my career as a ground-based radio astronomer. I observed Jupiter as part of my dissertation project, so going on to Voyager was very natural. I joined a team of ground-based radio astronomers on the Voyager planetary radio astronomy experiment.
As for COBE - in the early 1970s, theoreticians thought that the 3K (-270°C) background radiation should be very grainy. Temperature fluctuations on the sky should be quite large and easily measured. I realised that we could do an experiment that would answer that question by using the 210 ft radio telescope at Goldstone. Two colleagues and I did the experiment but the graininess was not there. That led me into COBE. In 1974, NASA announced an opportunity for a scientific spacecraft and I submitted a proposal to measure the graininess of the 3K background radiation. A number of other groups independently proposed the same thing and NASA brought us all together. I was one of the original six in the COBE team. The satellite was eventually launched in 1989.
I decided to come back to the Solar System after COBE and put in a proposal for Rosetta in 1995.
Q. You were also involved in SETI - the Search for Extraterrestrial Intelligence. How did that happen?
A. A group of scientists and engineers at JPL , including Bruce Murray, who was then the director of JPL, held a series of weekly seminars to address the question of whether or not JPL could make a major contribution to the search. We reached the conclusion that JPL had a number of resources that could be applied to the search. These included a world-wide network of radio telescopes, first rate digital and communication engineers, and a number of interested scientists. Encouraged by Bruce Murray, we designed a comprehensive all-sky search.
We needed to build a multi-channel spectral analyser with some one million channels - it seemed a huge leap forward. We could then study the entire sky over a reasonable period of time in the most likely frequency range that would be used for interstellar communications: a wide window from 1 GHz to 10 GHz. We had just built the first spectral analyser when Congress eliminated all funding. This terminated all SETI activities within NASA.
As for ET - I've always had an open mind. I always felt that extraterrestrial life could not be ruled out. It may be common, but we don't have enough knowledge to know one way or the other. Because it is easy to communicate over interstellar distances with modest-sized radio telescopes, I have always supported SETI as a means towards answering the question, "Are We Alone?".