Interview with Hans-Ulrich Käufl

Hans-Ulrich (Ulli) Käufl is since 1989 staff astronomer at ESO, in Garching (Germany). He works in the infrared instrumentation department on projects such as TIMMI-2, SOFI, VISIR and CRIRES. He is mostly interested in the application of molecular physics in astronomy. A short bio is available below.

Ulli, we all know the Deep Impact movie. But what is the Deep Impact mission?
H.-U. Käufl: The Deep Impact mission is the next logical step in the exploration of comets by spacecrafts. Back in 1986, ESA sent the Giotto spacecraft to the nucleus of comet Halley. This was a fantastic success. Then, there have been other flybys, but now the objective of the Deep Impact mission is to actually perform an experiment on the nucleus of the comet. That is, to find out things that we cannot find out by just looking from the outside. In particular, Deep Impact was conceived to produce something like an artificial crater, an artificial meteoritic impact. The idea is to follow the consequences of this 'creating a crater' with all possible means available to us.

Why is this important?
Cometary science is characterized today by a paradox: we know a lot about comets, very detailed things, like their X-ray emission or their magnetic field, but on the other hand, we are missing some important information. We don't know what is the mass of a comet, we don't know what the crust of a comet is composed of, how thick it is, how it reacts when subjected to pressure,... All these things are important to understand and the Deep Impact mission will help in this regard.

But can all be done from space?
No. Ground-based observations are very important and this, for various reasons. The spacecraft will move very fast by the comet and will only be able to observe the comet for no more than, say, a quarter of an hour after the impact. Observing it longer can only be done from ground-based telescopes as well as by a few satellites or space probes. Moreover, by using telescopes on the ground, all over the world, we will have a continuous coverage of the comet, and see whatever will happen.

Why was Comet 9P/Tempel 1 chosen?
Comet Tempel 1 was chosen mostly for celestial mechanics. You have to be able to reach it with the technology available. And even if NASA uses a very advanced launch vehicle, there are limits. So there were a few comets that met the criteria. Tempel 1 has another nice feature in that its orbits has a closest approach to the sun that is relatively far away from the sun, more or less at the mean distance of Mars. This ensures that the comet will not have changed too much by previous orbits around the sun.

Have observations already begun?
As soon as it was clear that Deep Impact would fly towards comet Tempel 1, the comet was observed from the ground intensively. First of all, we had to be sure that the spacecraft would indeed hit the comet, i.e. that the comet would be at the right place. We also had to make sure that we understand the size and the rotation of the nucleus of the comet. So, for the preparation of the mission, ESO was already heavily involved in observing this comet. Since the International Halley Watch, there has not been such a major coordinated effort. Comet Tempel 1 is certainly the best-observed comet that there is.

And what have you learned?
Since January this year, we are observing the comet to prepare the study of the effect of the impact itself. We have learned that the comet now behaves differently from how it behaved during its previous apparitions. It has started its gas and dust production at a later point than the previous years.

So, Ulli, what will happen to Comet Tempel 1 after the impact?
There is a most likely scenario but I must say that we are not entirely sure of what will happen to the comet. There is realistically a minor chance that the spacecraft will actually miss the target. After all, we try to hit something but we don't know what it really looks like. Then, it might well be that even if the spacecraft hits the comet, nothing happens except that the comet just swallows the impactor, which is by itself an interesting result even though, admittedly, we will all be disappointed. But we would learn a lot from that. On the other hand, the most extreme thing that could happen is that as a result of the impact, the crust gets modified in such a way that the comet becomes unstable and would break to pieces. So, there is a wide range of possibilities.

But what is the most likely scenario?
We think that as a result of this artificial crater, a hole of 50m in diameter and depth will be created and the equivalent amount of cometary material will be dispersed or vaporized, corresponding to the output the comet would normally give away in about one week. Moreover, we hope that as the result of this impact, a kind of chimney will form - what astronomers call a vent.

Will the telescopes in Chile be able to observe the impact?
From Chile, the direct impact will not be visible. However, we do not consider this a disadvantage. On the contrary, we are actually quite pleased. Indeed, the impact is best observed by the spacecraft itself and by the Hubble Space Telescope. We know from the breaking up of comets that the phenomena that happen normally develop to reach a peak something like half a day or one day after the comets have actually broken up. And so we expect the maximum effect observable from the ground, more or less exactly when we get the first sight of the comet in Chile.

And this maximum effect is?
We expect the equivalent of one week outgasing will be dispersed. This material, hopefully, is different from what the comet would normally produce. As you know, on Earth, if you drill a very deep hole or if you go in a very deep mine, the temperature raises when you go towards the centre of the Earth. In a comet, we expect the opposite. We think the comet is warmer on the outside and becomes colder and colder to the inside. Actually, we expect the very centre of the comet to be as cold as the solar system was when it formed, i.e. -250 degrees centigrade.
Due to the cratering, we hope that we can get access to deeper parts of the comet, where it is colder, and that we will find conditions there that normally do not exist on the surface of the comet.

What kind of observations do you plan?
I am planning to do infrared observations of dust. The most common model of comets, suggested by Fred Whipple in the 1950's, is called the 'dirty snowball'. The comet consists of a mixture of dust in a matrix of water ice, which includes a lot of volatile elements like formaldehyde, alcohol, methane, and other hydrocarbonates that are frozen in that matrix. We know there will be dust coming out of this matrix and I, together with my colleagues, will observe this dust. We look at it at it is irradiated by the sun but we will also observe the thermal emission coming from this dust.

You will observe in the infrared. Why?
The infrared is the only possibility to get a direct signal from a solid body. Normally when astronomers observe light emitted in the universe, they look at something similar to a discharge lamp, i.e. very hot gas (a 'plasma'). In the visible, it is not possible to see light coming from a solid body, like dust. This can only be done in the infrared.

When will you obtain the first data?
We will try to get our telescopes ready to observe already in day time. This is one of the advantages of observing in the infrared, in that you have a better image quality, and that you can observe even through the perturbing effect of daylight. At La Silla, we will try in the early afternoon to get a signal from the comet as soon as it rises above the Andes. We expect to have the first science data at about 15:00 local time in Chile (21:00 CEST). This is a big advantage for us because it will allow us to fine-tune our observing programmes on all ESO telescopes for the rest of the night, as it gives us three hours or so of warning time to refine the observing strategy.

And what will you see?
The observing campaign is already on-going since many months. Moreover, we will have two days before the impact, the full instrumentation of ESO available to study the comet before the impact. So, of course, the first thing we will see is what has changed following the impact. What we think we will see is extra outgasing of the comet and then we can check if there is a new vent or not. We will then use spectroscopic techniques - i.e. dispersing the light as in a rainbow - to analyse the chemical composition of the material. We can then check if we see different material and also how and if the material released will change with time.

What is so particular about ESO's campaign?
ESO is particular in the sense that it is the only observatory worldwide that can offer simultaneously seven telescopes with something like fifteen different instruments. This was also a big advantage of the ESO campaign of the Shoemaker-Levy comet crash into Jupiter in 1994 and it contributed in those days a lot to the scientific success. So, in that sense, ESO is unique and we have the best set-up to handle the unexpected.

Short Bio of Hans-Ulrich (Ulli) Käufl

Born 1954 in Landshut (Bayern, Germany). Graduated in 1981 from the Technische Universitaet Muenchen with an experimental thesis in nuclear physics. From 1981 to 1984, he completed his PhD at the Max-Planck-Institut fuer Extraterrestrische Physik (Garching, Germany), where he built a Laser-Heterodyne-Receiver to do extremely high resolution spectroscopy of atmospheres in the Solar System. Using many different telescopes, he also studied the physical properties and circulation of the Martian and Venusian's atmospheres and discovered polar warming above the Martian poles. He then held a two years post-doctoral position at the NASA Goddard Space Flight Center, Greenbelt, Maryland, where he worked on Comet Halley, winds on Venus and the oscillations in the Sun. He was part of the group which discovered the acoustic modes of the Solar Chromosphere. After, he worked in the industry, developing a a very high definition scanning reprographic camera, as well as being involved in micro-film laser printers and in a conceptual study for high definition large format color laser printing. In 1989, Ulli Käufl joined the infrared instrumentation department of ESO. He has been involved with the TIMMI, TIMMI2, SOFI, VISIR and CRIRES instruments. He is scientifically interested in the application of molecular physics in astronomy.

During his free time, he likes sailing, cooking, gardening and listening music. He is also a member of the Parliament of the district "Schwabing-Freimann" of the city of Munich.