After more than 12 years in space and two years at comet 67P, the European Space Agency has said goodbye to its historic Rosetta spacecraft. The first craft to enter orbit of a comet and deploy the first probe to land on a comet, Rosetta and its Philae lander greatly improved humanity’s understanding of comets, their environments, and the conditions under which the solar system initially formed.
Mission background and firsts:
Regular, predictable, comet passes close to Earth are exceedingly rare – especially short-interval comets.
And yet, ESA has a tremendous track record when it comes to these bodies of our solar system
Following the success of its 1986 Giotto mission to Halley’s Comet – through which ESA became the first space agency to successfully send a probe to a comet for up close observations – the space agency began developing the Comet Nucleus Sample Return (CNSR) mission.
In 1993, however, ESA budget constraints forced a cancellation of CNSR, and ESA began development of a canceled NASA Comet Rendezvous Asteroid Flyby mission to conduct in-situ examinations of and land a small probe on a comet.
The mission eventually gained the name Rosetta, with its associated lander taking the name Philae.
Rosetta itself was named after the Rosetta Stone, a stele from Egypt that allowed for the decoding of Egyptian hieroglyphs because the stone presented the same block of text in three different languages.
The Philae lander likewise took its name from the Philae obelisk – which included a transcription of text in both Greek and Egyptian hieroglyphs.
Over the course of their mission, Rosetta and Philae both made numerous firsts for not just ESA but for spaceflight in general.
Rosetta became the first European (and non U.S. spacecraft) to pass through the asteroid belt and became the first European spacecraft to examine – through flyby close encounters – asteroids 21 Lutetia and 2867 Steins in the asteroid belt.
Rosetta was also the first spacecraft with solar cell power technology as its main power source to operate at a distance close to Jupiter’s orbit.
Moreover, in 2014, Rosetta became the first spacecraft to enter orbit of a comet and the first spacecraft to fly with a comet as it began its transit through the inner solar system toward the sun.
The mission therefore became the first to perform in-situ observations of the frozen exterior of a comet as its exterior began to sublimate as the comet progressed closer to the sun.
Moreover, the Philae lander became the first probe to soft land for surface operations on a comet.
Philae’s instruments also provided the first images from the surface of a comet and made the first in-situ examination of a comet’s composition.
Once engineers determined the final specifications for Rosetta, the spacecraft was built in a clean room to COSPAR (Committee on Space Research) rules and regulations.
However, since comets are not regarded as objects where living microorganisms are found, sterilization of Rosetta – as the Mars rovers are subjected to – was not carried out for the spacecraft.
Following construction, Rosetta was set for launch on 12 January 2003 for a rendezvous with comet 46P/Wirtanen in 2011.
This plan, however, was abandoned after the Ariane 5 rocket on which Rosetta was scheduled to launch suffered a failure on 11 December 2002 – resulting in the grounding of the rocket until the cause of the failure could be determined and corrected.
During the Ariane 5 stand down, scientists and mission planners at ESA retargeted Rosetta to comet 67P/Churyumov-Gerasimenko, and set a new launch date of 26 February 2004.
Retargeting to comet 67P resulted in a need to modify the landing legs of the Philae lander due to the larger mass of comet 67P over the original target.
After two scrubbed launch attempts, the Rosetta mission lifted off aboard an Ariane 5 rocket on 2 March 2004 at 07:17 GMT from the Guiana Space Centre in French Guiana.
One year after launch, Rosetta returned to Earth for a gravity assist flyby on 4 March 2005 before moving on for a close encounter, low-altitude flyby and gravity assist maneuver of Mars on 25 February 2007.
This close encounter with Mars brought the craft within 250 km (160 mi) of the red planet and required Rosetta to pass through Mars’ orbital shadow – thus making its solar panels inoperative for 15 minutes and causing a significant shortage of power for the craft.
The Mars flyby was a complete success, with the spacecraft returning detailed photographs of the surface of Mars as part of a campaign to test various instruments and systems before it arrived at comet 67P.
Rosetta then began a return trajectory toward Earth for another gravity assist on 13 November 2007.
During the spacecraft’s approach to Earth on 7 and 8 November, an astronomer looking for Near Earth Asteroids (NEAs) mistakenly identified Rosetta as a NEA that would pass extremely close to Earth.
With this identification, Rosetta was given a provisional designation of 2007 NV84, and some speculation erupted that the “NEA” could impact Earth.
It wasn’t until another astronomer realized that the trajectory of this newly discovered NEA matched that of Rosetta that the Minor Planet Center confirmed the “discovery” was in fact Rosetta on its scheduled approach to Earth.
Rosetta then performed a close flyby of asteroid 2867 Steins on 5 September 2008, using its onboard cameras to allow for an approach trajectory to a minimum distance of 800 km (500 mi) before moving on to complete its third and final gravity assist of Earth on 12 November 2009.
The spacecraft then encountered asteroid 21 Lutetia on 10 July 2010 before enjoying a near four year cruise.
Rosetta then arrived for rendezvous at comet 67P in August 2014.
However, unlike planetary missions where probes can simply fire an engine and slow down to be captured into the planet’s gravity, Rosetta had to maneuver and create a series of two successive triangular hyperbolic escape trajectory profile passes with the comet with alternating thruster burns to slow itself down and place itself into a proper trajectory to enter the low mass gravity field around the comet.
Prior to achieving a stable orbit, Rosetta successfully mapped the surface of comet 67P, allowing mission scientists to identify five potential landing sites for the Philae lander by 25 August 2014.
Five days after entering orbit, mission managers announced the selection of landing site “J”, name Agilkia – located on the head of the comet – as the site where Philae would land.
Initial contact with the surface of 67P was registered at 15:33 UTC but was intermittent as Philae bounced twice before finally coming to rest at 17:33 UTC.
The two bounces – and the soft, granular material coating the surface of the comet – prevented Philae’s two landing harpoons – which were designed to secure it to the surface of the comet – from firing.
The subsequent bouncing ultimately led Philae to come to permanent rest in the shadow of a cliff, with the lander canted at an angle of 30 degrees – making it unable to adequately collect solar power and significantly shortening its mission to just two days.
While not able to complete its primary scientific mission because of the unusual landing, Philae nonetheless returned valuable scientific data from the surface of comet 67P during its brief operational period.
Moreover, as the comet rotated and the shadowed region came into direct light from the sun, intermittent contact was reestablished with Philae on both 13 June and 9 July 2015 before all communication attempts ended in July 2016 ahead of final End Of Mission (EOM) operations.
Uniquely, controllers of Rosetta were not able to identify the exact location in which Philae came to rest.
In fact, it wasn’t until September 2016 that the Rosetta team, examining high resolution photographs sent back from Rosetta itself, finally pinpointed the exact location of Philae – allowing scientists to finally put into context the images and data the lander had returned.
Findings and End of Mission:
As Rosetta nears the end of its historic mission at comet 67P, one thing is certain – the data it has acquired and transmitted back to Earth will be investigated and reviewed for years to come as scientist use the information to better understand cometary formation, composition, and dynamics.
Of particular note for the scientific understandings Rosetta has already revealed, the first was the discovery of a magnetic field around comet 67P in the 40-50 mH range created by the comet’s interaction with the solar wind – making its magnetic field one that does not originate from its nucleus of the comet itself.
This discovery was made possible by using both the Rosetta spacecraft – which detected the magnetic field – and the Philae lander, which returned definitive evidence that the comet’s nucleus had no magnetic field.
In this way, the dual use of both the lander and the orbiter were proved highly beneficial despite Philae’s much shortened operational tenure on the surface of the comet.
Another fascinating and important insight for scientists came with Rosetta’s analysis of the isotopic signature of water vapor coming from the comet.
As Rosetta found, Comet 67P carries water that is substantially different from that found on Earth, with a ratio of deuterium to hydrogen nearly three times that of Earth’s water’s hydrogen to deuterium ratio.
The implication of this discovery is that Earth’s water could not have come from comets like 67P.
While cometary delivery of water to Earth is a leading theory for how Earth gained its signature liquid, the discoveries made by Rosetta lends evidence to a scientific theory that not all comets are the same when it comes to water – and not all comets could have delivered the water we have on Earth.
Moreover, and fascinatingly, Rosetta revealed something surprising about the electrons that surrounded comet 67P.
As revealed by the spacecraft, the comet’s electrons were produced not from photons from the Sun as previously thought but rather by the photoionization of water molecules by solar radiation.
But perhaps most importantly were the Rosetta spacecraft in Philae lander’s contributions to the field of astrobiology and the hunt for organic compounds on comet 67P.
Prior to Rosetta’s launch, it was widely understood that comets contained complex organic compounds – the elements needed to make up nucleic acids and amino acids, which are essential ingredients for life on Earth.
During its very short operational period on the surface, Philae was able to detect organic molecules in the comet’s atmosphere.
Moreover, Rosetta itself was able to provide solid evidence for the presence of “non-volatile organic macromolecule compounds” on the surface of 67P in areas where “little or no water ice was visible.”
Analysis of these data points strongly suggests that comet 67P contains carbon – as a polyaromatic organic solid mixed with sulfites and iron-nickel alloys – on its surface.
Moreover, Rosetta has also detected one amino acid from its observations of the comet: glycine.
Now, with comet 67P on the outbound trajectory of its orbit from the Sun, the Rosetta mission came to an end on Friday as controllers monitored the craft’s slow landing onto the surface of the comet.
While some refer to this as a crash landing, controllers are quick to point out that Rosetta was to actually approach the comet’s surface and touch down at a speed slower than what the Philae lander did two years ago.
Nonetheless, the maneuver gave Rosetta its closest look at the comet and gave its suite of instruments an unprecedented up-close and detailed examination of various structures as it made its final approach.
It was estimated that Rosetta’s cameras would be able to discern surface features at less than 1 centimeter in diameter as it approaches. The shots gained during the final moments proved that theory.
Rosetta’s controllers carefully fine-tuned its slow orbital descent so that the vehicle landed near a 130 m (425 ft) wide pit called Deir el-Medina.
The pit contains elements that scientists believe are the building blocks for the comet, and the hope is that Rosetta will be able to deliver an immense amount of data about this region as it descends toward its final resting place – a fitting end for a historic mission for ESA.