Interview with Olivier Hainaut

Olivier Hainaut is a Belgian astronomer working at ESO in Chile since 1989 - with some interruptions back in Belgium, at the ESO headquarters in Germany, and the University of Hawaii in Honolulu. He is now in charge of the "Science Operations" department of Paranal. When not working on the telescopes, his main field of research is cometary nuclei and tran-neptunian objects. Olivier Hainaut is one of the principal investigators of the observation campaign of Comet Tempel 1 using ESO telescopes.

What is your role in the observation campaign at ESO?
I have various roles. Firstly, I am a cometary astronomer and the observations are thus very interesting for my research. I am involved at different level in the four observing programmes that are or will be executed with ESO's telescopes. I am also leading the programme that is doing the long-term monitoring of the comet, before, during and after the impact, in order to provide the long baseline reference for the other observations. Secondly, I am a collaborator of Karen Meech, from U. Hawaii, who is in turn the general coordinator of all the ground-based observations in support of the Deep Impact mission. In that way, I am kind of an ambassador between ESO and NASA. Finally, my job at ESO is to organise and supervise the operations of the telescopes of the Very Large Telescope (VLT) array on Paranal. In that way, I can make sure that the Tempel-1 observations will be feasible and executed in the best possible conditions.

Why is this campaign important?
This is one of the great events in cometary science! There are just a few such events in an astronomer's carreer. So, I'm very enthousiastic and happy to be able to participate directly in it!

How will telescopes in Paranal and La Silla be working during the days close to the impact?
All ESO telescopes will be observing the comet during the week of the impact. This means, the four 8.2m telescopes forming the VLT on Paranal, and the ESO 3.6m, the NTT (3.58m) and the ESO/MPI 2.2m on La Silla. Three groups of astronomers will share these seven telescopes and the complete collection of instruments that are equipping them, covering all possible optical to infrared observations possible from Earth. That is, from high resolution imaging (with Adaptive Optics, which corrects for the atmosphere's turbulence and give images similar to what can e obtained from space) to wide field imaging (with WFI the largest camera or ESO, that could take a picture of the full Moon in a single exposure). The observations will also cover the range from the near ultraviolet through all the colours of the visible light to the infrared, all the way to thermal infrared.

What will be your work routine?
All the astronomers on the two ESO sites will work in very close collaboration: we will have real-time video connections between the two control rooms from where the 7 telescopes are operated. This will ensure that we will be able to discuss what we see and take decisions in real time to optimize the observations, possibly changing strategy and even changing instruments in real time. ESO is the only observatory with such a broad and organised collaboration, so it has the potential to give a very complete portrait of the event. In addition to the internal coordination, we also plan to be in close contact with K.Meech, who will be on Mauna Kea ensuring the worldwide coordination with the spacecraft control at NASA JPL. Deep Impact ground based support and ESO also have a very close collaboration with the PLANET ( international team, that is operating 3 telescopes well spaced in the Southern Emisphere at La Silla, Boyden (South Africa) and Canopus (Australia). PLANET is providing us with a daily set of images of the comet. As they have three sites, this ensures that the images will keep coming even if one of the observatory is clouded out.

You have started observing the comet already some times ago. Why?
Because we needed to know the comet we are targeting at. The ground-based support started already several years ago: NASA/JPL needed very, very accurate measurements of the position of the comet in order to get its orbit properly defined. This is critical as the goal of Deep Impact is to collide with a 6 km object that is at ~200 million kilometres from the Earth. Moreover, the comet is not spherical, it looks more like an elongated potato. The size and shape of that potato needed to be determined properly well in advance. For the collision to be success, it is much better to collide with the "big" side of the nucleus rather than on the small side. As the time of the collision has to be set long in advance, it is therefore very important that the duration of the rotation of the nucleus be well known.

But the space probe is now well under way...
More recently, we started a monitoring of the comet on a monthly basis with medium-sized telescopes (3-4m), and on a daily basis with smaller telescopes (1-2m). Indeed, the comet is now "active": the ice making the nucleus is heated by the Sun, and it is sublimating into gas. That gas drags the dust that was mixed with the ice, forming the coma and the tail of the comet (the gas is mostly invisible).

And do we need to know this?
Cometary activity can result in quite complex features around the comet: the coma can display jets (caused by a small region of the surface where the comet is very active, like an "ice volcano"), fans (caused by the combination of a jet with the rotation of the comet), outbursts (if a region of the nucleus that is very rich in ice suddenly receives the heat of the Sun - this causes like a small explosion). We want to know well these behaviour in order to be sure to recognize the effect of the impact, and not to confuse it with a normal feature of the comet. Fortunately, until now, the comet is very quiet, with very few changes and few jets: the effect of Deep Impact should be very clear, with no confusion.

These observations will continue long after the impact...
We plan to continue the monitoring at least till October. This is for us a unique opportunity to follow the evolution of a surface feature (the crater caused by the Deep Impact missile). We will know exactly when it started, and we will have pictures of its beginning taken by the mother ship of Deep Impact. Will that crater cause a jet that will be visible just for a few hours, or will it last for days, weeks, months? Usually, we follow jets of other comets without really knowing anything about them. This time, we will know all the details in advance. That will help us understanding how other comets behave.

Will the impact be visible from our garden/terrace...?
Unless you live in Hawaii or in Australia and some places in California, the answer is NO. The comet will be above the Pacific ocean at the time of the impact. The Deep Impact mother ship will be downloading all the data from its cameras and instruments in real time so that most of the data are rescued even in case the spacecraft is destroyed by the comet's dust. For more security, NASA want to have 2 "Deep Space Network" antennas to receive these data on Earth. The only place where that is possible is over the Pacific, where both Goldstone (California, USA) and Canberra (Australia) can see the same spacecraft at the same time.

So the impact isn't visible from Chile?
Seen from Chile, the comet will just be setting at the time of the impact; the ESO telescopes will start observing the next day, when the comet becomes visible again. While it would certainly have been interesting (and fun!) to observe the impact in real time, most of our ground-based contribution will come from observations of the plume of dust and gas that will be released by the impact. During the time the comet will not be visible, this cloud of dust and gas will have time to expand and develop into something large enough to be observable. So, stay tuned on the web and TVs for updates!

What if the impact changes the orbit of the comet so that it collides with the Earth?
A collision between a 6 km comet and the Earth would be very bad. Fortunately, Tempel 1 (the comet of Deep Impact) is absolutely safe: its current orbit never, ever approaches the Earth. The nearest approach was in May, and the comet was still at more than 100 million km. At the time of the impact, it will be at 130 million km. While the 370kg impactor will move very fast (38 000 km/h), and cause a big explosion, this is negligible compared to the mass and energy of the comet. The effect of the impactor on the comet's orbit will be less than that of a fly hitting a plane. The orbit will not be changed, so there is still no risk of impact between Comet Tempel 1 and the Earth. Note for those who really like to worry: there are many, many unknown comets and asteroids around - these are the ones one should worry about.

What will happen to the comet:
We don't know for sure, that's why the experiment is interesting. The most likely scenario is that the impactor will create a crater about the size of a large football stadium, dumping in space all the ice and dust that occupied that volume. Much less plausible scenarios indicate that the comet could swallow the impactor, like a golf ball falling in fluffy snow. We would not see much. Even less likely is the complete breakup of the comet nucleus in pieces. That would be really interesting (we would really see the interior of the comet, and we would get a much better understanding on how comets are held together), but it is very unlikely.