Comet Cruiser Glimpses Earth

The departing Galileo probe took the Earth-moon system in silhouette.
Credit: NASA/Galileo

The banner image, taken by ESA’s Rosetta comet-chaser spacecraft, shows the Earth-Moon system from a distance of 70 million kilometers (42 million miles). This is close to the maximum distance reached by the spacecraft so far this year.

However, this is a tiny distance compared to Rosetta’s epic journey when, in 10 years time, it will have travelled distances of over one thousand million kilometers (600 million miles) from Earth, and about 800 million kilometers (480 million miles) from the Sun, to meet Comet 67P/Churyumov-Gerasimenko.

This image was taken by the Navigation Camera System (NAVCAM) on board the Rosetta spacecraft, activated for the first time on July 25, 2004. This system, comprising two separate independent camera units (for back-up), will help to navigate the spacecraft near the comet nucleus. The cameras perform both as imaging cameras and star sensors, and switch functions by means of a refocusing system in front of the first lens.

At the comet, extremely high-precision measurements of the relative distance and velocity (between spacecraft and nucleus) will be needed. These are not achievable with the ground-based methods normally used with all other spacecraft or for normal Rosetta trajectory determinations.

The doppler effect
Earth as seen by the departing Voyager spacecraft: a tiny, pale blue dot. Credit: NASA


History of Looking Backwards

The history of such deep-space views looking backwards towards the home planet have captivated the imagination for a generation of astronomers. As Carl Sagan’s widow, Anne Druyan , described this perspective image to Astrobiology Magazine, such earth views make "us look at this tiny planet, at the pale blue dot, and to see it in its real context, in its actual circumstances, in its true tininess. I don’t know anyone who’s able to really see that one-pixel Earth and not feel like they want to protect the Earth; that we have much more in common with each other than we’re likely to have with anyone anywhere else."

The evocative phrase describing the Earth as a ‘pale blue dot’ was coined by Carl Sagan after seeing our planet as a single pixel. The view was taken from the departing Voyager spacecraft. The entire earth could be encompassed as a flicker of light. The first image of Earth ever taken from another planet that actually shows our home as a planetary disk was captured by the Mars Orbital Camera on May 8th, 2003. The first image of Earth from the current generation of surface rovers on Mars was snapped on March 11, 2004. The most bewitching image of Earth was taken by the Galileo flyby on its way to Jupiter, because the question of whether a passing satellite could distinguish that the Earth was full of life, while the moon was not, did not appear obvious using just remote sensing instruments.

Rather than seeing the Earth as an obvious candidate for life, the Galileo pictures gave surprisingly few clues of the biological potential of our own planet.

Comparison of Mars, Venus and Earth in water bands, showing the clear presence of water on Earth uniquely
Credit: NASA Workshop, Pale Blue Dot

From afar, how Galileo missed the obvious signs of terrestrial life as we would have expected to see them, was at first disconcerting to the scientific community, because future missions aim to observe more distant extrasolar planets and detect what would be visible in the spectra–the ‘pale blue dot’ scenario.

One answer may lie in the fact that the spacecraft made its observations while still quite close to the Earth. "The spectrograph was designed to look at small areas of Jupiter, so the field of view of the spectrograph was quite small," said Nick Woolf of Arizona, in earlier discussions with the Astrobiology Magazine.

"Also, since the surface brightness of Jupiter [the Gaileo’s intended visual target] is far less than the Earth, the spectrograph detectors saturated except when the spectrograph was pointed at the darkest area of Earth – a cloud-free section of sea," Woolf noted. The cloud-free sea is considered very dark relative to the dominance of bright clouds in a global picture of Earth. Thus it should come as no surprise that Galileo was successful in only imaging a relatively dark and lifeless planet, mainly because its design was not intended to look at Earth, but to probe Jupiter instead.

A spectroscope that might detect infrared or visible light looking back on Earth or outwards to other planets might focus mainly on four gases that are found in Earth’s atmosphere and linked to life:

  • Water vapor A baseline sign, indicating the presence of liquid water, a requirement of known life.
  • Carbon dioxide Can be created by biological and non-biological processes. Because it is necessary for photosynthesis, it would indicate the possible presence of green plants.
  • Methane Considered suggestive of life, it also can be made both by biological and non-biological processes.
  • Molecular oxygen (O2) – or its proxy, ozone (O3). The most reliable indicator of the presence of life, but still not conclusive.


Illustration of Rosetta sitting on comet surface
Credit: ESA

Unless molecular oxygen in the atmosphere is constantly replenished by photosynthesis, it is quickly consumed in chemical reactions, in the atmosphere, on land and in seawater. So the presence of a large amount of oxygen in an extrasolar planet’s atmosphere would be a sign that it might host an ecosystem like present-day Earth’s.

An additional oxygen-related biosignature is the possibility of detecting green plants that make oxygen. Chlorophyll reflects near-infrared light very strongly, a phenomenon known as the "red edge" because the light is just beyond the range of colors human eyes can see. (If humans could see the red edge, plants would look red instead of green.) Near-infrared cameras would have no trouble picking up this distinctive signal.

What’s Next

Comets may have played a major role in the origin of life on Earth, delivering a significant share of the Earth’s water as well as carbon-rich organic compounds. Because comets are composed of ice, dust, and gas – the building blocks of the solar system – particles collected from a comet may be able to tell us something about how the solar system formed. When NASA Stardust’s Sample Return capsule containing the comet particles arrives on Earth in 2006, it will be sent to NASA’s Johnson Space Center in Houston for analysis.There are two more comet missions currently planned. NASA’s Deep Impact mission will visit the comet Tempel 1 on July 4, 2005. ESA’s Rosetta mission launched in March of this year and will reach the comet Churyumov-Gerasimenko in November 2014. In the meantime though, Rosetta’s cameras can also be used to track automatically the two asteroids that Rosetta will be visiting during its long cruise, Steins and Lutetia.

Related Web Pages

Stardust, JPL
JPL Photojournal
Stardust’s Success
Early Wild Success for Stardust
Telescopes for Stardust
Harpooning a Comet
Two-Way Asteroid Trip Takes Off
Tale of a Comet
We Are All Made of Stars
Winter Boon From Deep Space
Museum of the Galaxies
The Earth as Seen by Galileo
Crescent Moon and EarthShine–courtesy Chris Cook, astrophotography