Reading Rosetta

Imagine picking a moving target, an icy nucleus of a comet at great distances from the earth. Imagine that it rotates and has an irregular shape with unknown icy, rock surface features. That is the daunting picture posed by the first mission to land on a comet–a mission slated for launch in February 2004.

Hubble Space Telescope in orbit. Credit: ES

The rendezvous between a spacecraft and such a tiny, distant object will try to do something no one has ever done before – to chase and land on a comet. The Lander science will focus on the in situ study of the composition and structure of the nucleus material. Results from the NASA/ESA Hubble Space Telescope played a major role in preparing the European Space Agency’s (ESA’s) ambitious Rosetta mission for its new target, comet 67P/Churyumov-Gerasimenko. Hubble has been the critical tool in measuring precisely the size, shape and rotational period of the comet. Information that is essential if Rosetta is to rendezvous with a comet and then drop down a probe, something never before attempted and yet a major step to elucidating solar system origins.

Hubble’s observations from March 2003 revealed that comet 67P/Churyumov-Gerasimenko (67P/C-G) is approximately five-by-three kilometers, and shaped like a rugby ball on which it is possible to land. ESA Mission scientists needed to know the exact size of the solid nucleus to adapt the mission to the comet’s gravity.

"Although 67P/C-G is roughly three times larger than the original Rosetta target, its highly elongated shape should make landing on its nucleus feasible, now that measures are in place to adapt the lander package to the new scenario," says Dr. Philippe Lamy of the Laboratoire d’Astronomie Spatiale in France.

The approximately 4 kilometer cometary nucleus to be landed upon by the Rosetta and its harpooning landing hooks
Credit: Hubble

Lamy is presenting the Hubble results on comet 67P/C-G at the annual meeting of the Division of Planetary Sciences of the American Astronomical Society in California, USA, on 5 September 2003.

Mission scientists began considering the new target when the Rosetta mission’s launch date was postponed. The delay made the original target comet, 46P/Wirtanen, no longer easily reachable. But scientists did not have enough information on the backup comet, 67P/C-G, and sought data from the largest telescopes.

Using a technique developed over the past decade by Lamy, Imre Toth (Konkoly Observatory, Hungary), and Harold Weaver (Johns Hopkins University Applied Physics Laboratory, Laurel, USA), the team snapped 61 Hubble images of comet 67P/C-G over an interval of 21 hours between March 11 and 12, 2003. Hubble’s Wide Field Planetary Camera 2 isolated the comet’s nucleus from the coma, the diffuse cloud of gas and dust surrounding it, and quickly provided the missing figures. The telescope showed that the nucleus is ellipsoidal and also measured its rotation rate of approximately 12 hours.

Rosetta’s launch is currently planned for February 2004, with a rendezvous with the comet about 10 years later. Measurement goals on Comet Churyumov-Gerasimenko include the determination of the elemental, molecular, mineralogical, and isotopic composition of the cometary surface and subsurface material. Highest priority is given to the elemental and molecular determinations as it is believed that some mineralogical and isotopic measurements can be carried out adequately by orbiter science investigations. In addition properties like near-surface strength, density, texture, porosity, ice phases and thermal properties will be derived. Texture characterization will include microscopic studies of individual grains.

Hampered by rocketry concerns, the landing phase presented planners with another set of challenges altogether. "Firstly, we don’t know anything about how rough the surface is," said Rosetta Project Scientist Gerhard Schwehm. "It could be covered with fluffy snow like the Alps or it could be hard rocks and craters. We can, however, be sure that it will not be smooth and flat resembling parking lots."

Scientists designed Rosetta’s landing gear to cope with most nasty surprises as soon as it is to touch down on its selected target. Two harpoons will anchor the probe to the surface. The self-adjusting landing gear will ensure that it stays upright, even on a slope. The lander’s feet will drill into the ground. These devices will help counteract the fact that there is no appreciable gravity on a comet.

What’s Next

close-up of Eros as it rotates.
Video: NEAR Shoemaker flyover of Eros. Credit: Johns Hopkins Univ. AP

On Valentine’s Day, 2001, the Near-Shoemaker spacecraft successfully landed on the asteroid, Eros. Its remarkable journey–to soft-land on a peanut shaped asteroid about 176 million kilometers (109 million miles) from Earth, prompted Andrew Cheng, NEAR Project Scientist, to note: "On Monday, 12 February 2001, the NEAR spacecraft touched down on asteroid Eros, after transmitting 69 close-up images of the surface during its final descent. Watching that event was the most exciting experience of my life."

On Jan. 2, 2004, another NASA spacecraft called Stardust will fly within 75 miles of a cometary main body (called Wild-2)–close enough to trap small particles from the coma, the gas-and-dust envelope surrounding the comet’s nucleus. Stardust will be traveling at about 13,400 miles per hour (mph) and will capture comet particles traveling at the speed of a bullet fired from a rifle. Its main camera, built for NASA’s Voyager program, will transmit the closest-ever comet pictures back to Earth. Launched in February 1999, Stardust was designed to capture particles from Wild 2 and return them to Earth for analysis. The spacecraft already has collected grains of interstellar dust. It is the first U.S. sample-return mission since the last moon landing in 1972.

In the next 5 or so years, there will be several encounters of spacecraft with comets and asteroids.

2004 Jan. 1 Comet Wild 2 Stardust (coma sample return)
2005 July 3 Comet Tempel 1 Deep Impact (big mass impact)
2005 Sept. Asteroid 1998 SF36 Muses-C (sample return)
2014 Nov Comet Churyumov-Gerasimenko Rosetta (simple flyby)

The team is composed of P.L. Lamy and L. Jorda (Laboratoire d’Astronomie Spatiale, France), I. Toth (Konkoly Observatory, Hungary), and H.A. Weaver (Johns Hopkins University Applied Physics Laboratory). The movie simulation of the Hubble results is provided by Mikko Kaasalainen (University of Helsinki, Finland) and Pedro Gutierrez (Laboratoire d’Astronomie Spatiale, France).