Rosetta has already begun to deliver data from other comets as it travels toward comet C-G. Rosetta has already observed comets LINEAR and Tempel-1. It has also tested its instruments on the atmospheres of Earth and Mars during its flybys for gravity assists. Next, it will observe the asteroids, Steins and Lutetia.
Here are some brief overviews of what Rosetta has explored so far:
Mars Flyby - February, 2007
The Mars flyby was the second gravity assist for Rosetta, but quite different from the first flyby of Earth, which also provided a gravity assist This Mars flyby was designed to slow the spacecraft down to a more effective approach speed for rounding Earth a second time in November, 2007. In the Mars flyby, the planet's gravity put the brakes on Rosetta, slowing the spacecraft down by about 4,900 miles per hour, relative to the Sun. It was an amazingly close flyby (only 250 km above the planet!).
During its approach to Mars, Rosetta's Standard Radiation Monitor (SREM) observed Mars’ radiation environment for 48 hours, and its RPC spent 48 hours measuring the properties of the charged particles and the magnetic field within Rosetta’s trajectory. During the approach, OSIRIS camera system captured Mars’ moon Phobos as it emerged from behind the planet, and VIRTIS and ALICE performed a spectroscopic analysis of the Martian atmosphere.
This was the first opportunity for ALICE to function in something like a comet’s environment. In ALICE’s spectral band, Mars’ upper atmosphere is similar to the ultraviolet spectrum of a comet, emitting primarily carbon monoxide, hydrogen, carbon, and oxygen, as well as some of their ions. ALICE observed Mars’ “dayglow,” the emission of ultraviolet light from its sunlit upper atmosphere, the first in situ observation of this phenomenon on Mars. Dayglow tells us much about how carbon dioxide, the main element of Mars' atmosphere, behaves at high altitude. ALICE also mapped the nightside spectrum, studying the far ultraviolet “nightglow” and searching for auroras. VIRTIS mapped Mars, and OSIRIS and ALICE searched Mars’ equator for a dust ring. OSIRIS took pictures of Phobos crossing Mars.
Rosetta shut down its solar-powered instruments before executing the flyby because it Mars was about to begin an eclipse just as Rosetta rocketed around it! Without sunlight, Rosetta's onboard instruments would have to consume valuable battery power, and this power needed to be saved for comet C-G. The Martian eclipse was an unfortunate side-effect of re-targeting Rosetta at comet C-G instead of its original target, comet 46P/Wirtanen. The different timing of the launch and trajectories caused this unusual coincidence of events, but the Rosetta team improvised to get pictures anyway. They turned on the Rosetta Plasma Consortium (RPC) — a suite of plasma sensors that includes the U.S. instrument IES. Installed on the lander Philae, the RPC has its own independent source of battery power, so it was able to observe the complete flyby for three hours. Philae's camera captured spectacular images of Rosetta and Mars, one of which you can see here.
After Rosetta finished observing Mars and departed through the planet's magneotail, it turned its instruments toward Jupiter. From an extremely long distance, ALICE watched as the New Horizons spacecraft flew past Jupiter. ALICE observed Jupiter for over two months as New Horizons flew down the Jovian magnetotail. ALICE also observed Jupiter’s aurora and its moon Io’s plasma torus.
Comet Tempel-1, July 4, 2005
Scientists used Rosetta’s camera system, OSIRIS, for the first time to observe comet 9P/ Tempel-1 as it impacted a probe sent by the NASA Deep Impact spacecraft into its path. OSIRIS observed the comet from five days before impact until ten days afterward. Rosetta was in a good position to observe comet Tempel -1. It was about 46 million km from the comet and the solar elongation was greater than 90 degrees. The Narrow Angle Camera (NAC) observed the cometary dust through five different filters, and the Wide Angle Camera (WAC) observed emissions of OH, CN, NA, and OI.
During observation by OSIRIS, the comet’s overall brightness dimmed by about ten percent. During this period, OSIRIS NAC’s clear filter was used to produce a “light curve” that shows how the Tempel-1’s intensity varied in time, beginning with the impact. The light curve shows that about 30 minutes after impact the brightness of the comet had increased by five times. Before and after the impact, the comet varied regularly in intensity consistent with its rotational period.
Scientists analyzing the impact ejecta discovered no new coma structures. The icy grains ejected from the comet must have been accelerated rapidly by their collisions with gas molecules. They apparently sublimated and dispersed quickly within the first hour after impact. The rapid dissolution of the ejecta prevented scientists from using ballistic models to study the density and tensile strength of Tempel-1’s nucleus.
Earth Fly-by 1 - March 4, 2005
Rosetta executed its first flyby of Earth for a gravity-assisted catapult to Mars. Rosetta used the Earth’s magnetic field and the Moon to test and calibrate its instruments onboard.
The Rosetta Plasma Consortium (RPC) instruments measured the Earth’s magnetic field strength, temperature and plasma density. Rosetta entered the Earth’s magnetosphere from the deep geomagnetic tail on the dark side of the Earth, opposite the Sun, crossed the Van Allen radiation belts and entered deep into the plasmasphere, before exiting through the magnetopause and bow shock. The Van Allen belts, which bow around the Earth’s equator, caused no radiation problems for any of the RPC suite’s instruments.
During the flyby, Rosetta’s minimum altitude above the Earth's surface was about 1955 km. A few hours before this point, the spacecraft was turned toward the Moon, and the remote sensing instruments were switched on for calibration. After the flyby, one of the two navigation cameras was switched to asteroid tracking mode, using the Moon as the “asteroid.” This was in preparation for the future flybys of asteroids Steins and Lutetia in 2008 and 2010, respectively.
Comet C/2002 T7 (LINEAR) - April 30, 2004
On April 30, 2004, Rosetta’s OSIRIS camera system took images of Comet C/2002 T7 (LINEAR), which was traveling for the first and only time through the inner Solar System. Later that day, three more Rosetta instruments (ALICE, MIRO and VIRTIS) took measurements of the comet. Because LINEAR appeared during Rosetta's commissioning, MIRO was the only one of these four instruments whose pointing had been confirmed by the commissioning process. Therefore, all four instruments used MIRO's field of view to observe the comet. The four instruments took images and spectra of Comet C/2002 T7 (LINEAR) to study its coma and tail in different wavelengths, from ultraviolet to microwave. MIRO successfully measured the presence of water molecules in the tenuous atmosphere around the comet.
This MIRO data can be used to determine the rate at which the comet produces water at that point in its orbit. Along with measurements by other ground-based and spacecraft instruments, this data will help define how LINEAR's rate of water production varies as it moves around the sun.
The OSIRIS camera produced high-resolution images of Comet C/2002 T7 (LINEAR) from a distance of about 95 million kilometres. An image showing a pronounced nucleus and a section of the tenuous tail extending over about 2 million km was obtained by OSIRIS in blue light:
ALICE successfully observed LINEAR on both 30 April 2004 and 14 May 2004. ON 30 April, ALICE acquired the following spectrum of the comet’s nucleus region from a distance of 16M km. ALICE detected H Lyman and (emission lines of hydrogen) at wavelengths 1216 Å and 1025 Å, respectively. ALICE also detected oxygen (O) at 1301 Å, and carbon (C) at 1561 Å and 1657 Å.