26 August 2016

Catching Some Comet Dust: Giotto II (1985)

Giotto 1 liftoff. Image credit: European Space Agency
On the overcast morning of 2 July 1985, the eleventh Ariane 1 rocket to fly lifted off from the Centre Spatial Guyanais in Kourou, French Guiana, an outpost of the European Community located a few degrees north of the equator on the northeast coast of South America. The last Ariane 1 to fly, it bore aloft Giotto, the first European Space Agency (ESA) interplanetary spacecraft. Giotto's destination was Comet Halley.

A "dirty snowball" containing materials left over from the birth of the Solar System 4.6 billion years ago, Halley needs about 76 years to revolve around the Sun once. Its elliptical orbit takes it from the cold emptiness beyond Neptune to the space between the orbits of Venus and Mercury. Halley travels around the Sun in a retrograde orbit, meaning that it orbits "backwards" relative to the eight planets and most other objects making up the Solar System.

Comet Halley has passed through the inner Solar System 30 times since its first verified recorded apparition in 240 B.C. In 837 A.D., it passed just 5.1 million kilometers from Earth; during that apparition, its dust tail must have spanned nearly half the sky, and its bright coma – the roughly spherical dust and gas cloud surrounding its icy nucleus – may have appeared as large as the full moon.

Shortly after its bright apparition in the year 1301, Italian artist Giotto di Bondone was inspired to add Comet Halley to his painting The Adoration of the Magi. The Giotto spacecraft was named for him.

Comet Halley appears near the top of Giotto di Bondone's The Adoration of the Magi. 
Throughout most of its known apparitions, Comet Halley was not understood to be one comet repeatedly passing through the inner Solar System. Not until 1705 did English polymath Edmond Halley determine that comets seen in 1531, 1607, and 1682 were probably one comet orbiting the Sun. He predicted that, if his hypothesis were correct, the comet should reappear in 1758 (which it subsequently did).

The Ariane 1's third stage injected 980-kilogram Giotto into a 198.5-by-36,000-kilometer orbit about the Earth. Thirty-two hours after launch, as it completed its third orbit, flight controllers in Darmstadt in the Federal Republic of Germany commanded drum-shaped Giotto to ignite its French-built Mage solid-propellant rocket motor. The motor burned 374 kilograms of propellant in 55 seconds to inject the spinning 2.85-meter-tall, 1.85-meter-diameter spacecraft into orbit about the Sun.
 
Two months before Giotto's launch, Americans P. Tsou (Jet Propulsion Laboratory), D. Brownlee (University of Washington), and A. Albee (California Institute of Tech) proposed in a paper in the Journal of the British Interplanetary Society that a second Giotto mission be launched to fly close by one of 13 candidate comets between 1988 and 1994. They proposed that the new spacecraft, which they dubbed Giotto II, might launch on an Ariane 3 or in the payload bay of a Space Shuttle. Giotto II's "free-return" trajectory would take it as close as 80 kilometers from the target comet's nucleus, then would return it to Earth.

Near the comet, Giotto II would expose sample collectors to the dusty cometary environment. Near Earth, it would eject a sample-return capsule based on the proven General Electric (GE) Satellite Recovery Vehicle (SRV) design. The capsule would enter Earth's atmosphere to deliver its precious cargo of comet dust to eager scientists.
 
Tsou, Brownlee, and Albee pointed out that the Mage solid-propellant motor had not been required to boost Giotto into interplanetary space; that is, that the Ariane 1 could have done the job itself. Giotto was, however, based on a British Aerospace-built Geos magnetospheric satellite design, which included the Mage motor. Re-testing the design without the motor would have cost time and money, so ESA elected to retain it for Giotto. After noting that the GE SRV could fit comfortably in the space reserved for the Mage, they proposed that, in Giotto II, the reentry capsule should replace the motor.

Giotto included on its aft end a "Whipple bumper" - named for its inventor, planetary astronomer Fred Whipple - to protect it from hypervelocity dust impacts. During approach to Comet Halley, the spacecraft would turn the bumper toward its direction of flight. The bumper comprised a one-millimeter-thick aluminum shield plate designed to break up, vaporize, and slow impactors, a 25-centimeter empty space, and a 12-millimeter-thick Kevlar sheet to halt the partially vaporized, partially fragmented impactors that penetrated the aluminum shield.
 
In the case of Comet Halley, dust would impact the bumper at up to 68 kilometers per second. Tsou, Brownlee, and Albee noted that the 13 candidate Giotto II target comets were all less dusty and would have lower dust impact velocities than Halley. Because of this, Giotto II would need less shielding than Giotto.

Comet dust would, nevertheless, create challenges for Giotto II. Tsou, Brownlee, and Albee devoted much of their paper to a description of how the spacecraft might successfully capture dust for return to Earth. One proposed capture system, based on the Whipple bumper design, would use a shield made from ultrapure material to vaporize and slow impacting dust particles. The vapor from the impactor and the impacted part of the bumper would then be captured as it condensed. Scientists would disregard the bumper material when they analyzed the condensate.
 
Tsou, Brownlee, and Albee also noted that thermal blankets from the Solar Maximum Mission (SMM) satellite, launched into Earth orbit on 14 February 1980, had demonstrated that intact capture of high-velocity particles was possible. The multilayer Kapton/Mylar blankets, which were returned to Earth on board the Space Shuttle Orbiter Challenger at the end of mission STS 41-C (6-13 April 1984), were found to have collected hundreds of intact meteoroids and human-made orbital debris particles.

The three scientists described preliminary experiments in which gas guns were used to fire meteoroid and glass fragments at "underdense materials," such as polymer foams and fiber felts. The experiments suggested that such materials could capture at least partially intact comet dust particles.

Giotto's encounter with Comet Halley spanned 13-14 March 1986. At closest approach the spacecraft passed just 596 kilometers from Halley's nucleus. The comet's 15-by-eight-by-eight-kilometer heart turned out to be extremely dark, with powerful jets of dust and gas blasting outward into space.

Artist's concept: Giotto at Halley. Image credit: European Space Agency
Halley's hot heart as imaged by ESA's Giotto spacecraft. 
The intrepid probe suffered damage from dust impacts – one large particle sheered off more than half a kilogram of its structure – but most of its instruments continued to operate after the Comet Halley flyby. ESA thus decided to steer Giotto toward another comet.

On 2 July 1990, five years to the day after its launch, Giotto flew past Earth at a distance of 16,300 kilometers, becoming the first interplanetary spacecraft to receive a gravity-assist boost from its homeworld. The gravity-assist flyby put it on course for Comet Grigg-Skjellurup, which it passed at a distance of 200 kilometers on 10 July 1992.
 
After determining that Giotto had less than seven kilograms of hydrazine propellant left on board, ESA turned it off on 23 July 1992. The inert spacecraft flew past Earth a second time at a distance of 219,000 kilometers on 1 July 1999.
 
By that time, a comet coma sample return mission was under way with two of the Giotto II proposers playing central roles. In late 1995, Stardust became the fourth mission selected for NASA's Discovery Program of low-cost robotic missions. Brownlee and Tsou, respectively Stardust Principal Investigator and Deputy Principal Investigator, designed the mission's sample capture system.

Artist's concept of the NASA Stardust spacecraft at Wild 2. Image credit: NASA
The 380-kilogram Stardust spacecraft left Earth on a free-return trajectory on 7 February 1999, and flew past Comet Wild 2 (one of the 13 Giotto II candidates) at a distance of about 200 kilometers on 2 January 2004. Stardust captured dust particles in aerogel, a silica-based material of extremely low density that was invented in the 1930s. Tsou, Brownlee, and Albee had apparently been unaware of aerogel when they proposed Giotto II in 1985.

Stardust returned to Earth on 15 January 2006. Its sample capsule streaked through the pre-dawn sky over the U.S. West Coast before parachuting to a landing on a salt pan in Utah.

When opened on 17 January 2006 at NASA's Johnson Space Center, in the same lab that received the Apollo moon rocks, Stardust's 132 aerogel capture cells were found to contain thousands of intact dust grains captured from Wild 2. Subsequent analysis indicated that some probably formed close to other stars before our Solar System was born.
 
Sources

"Comet Coma Sample Return via Giotto II," P. Tsou, D. Brownlee, and A. Albee, Journal of the British Interplanetary Society, Volume 38, May 1985, pp. 232-239

ESA Remembers the Night of the Comet, European Space Agency, 11 March 2011 (accessed 26 August 2016)

Stardust: NASA's Comet Sample Return Mission, NASA Jet Propulsion Laboratory (accessed 26 August 2016)

More Information

A 1974 Plan for a Slow Flyby of Comet Encke

Cometary Explorer (1973)

Missions to Comet d'Arrest and Asteroid Eros in the 1970s (1966)

4 comments:

  1. Thanks for this great post, David! It looks like a chapter about cometary exploration is building up for a future book :-) I was extremely pleased to see a European mission discussed in your blog. I think that JBIS is a great source of historical info and could add a lot of international diversity w.r.t. the NASA documents which you use most often.
    Cheers,
    Simon

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    1. Simon:

      This is a subject I have thought about for a long time. I labor under some restrictions, which limit what I can accomplish.

      I wish I could read French, German, Chinese, Japanese, and Russian as well as I can English, and that space archives in those languages were as readily accessible to me as are space archives in the United States. I can manage some French, some German, some Russian, but not with sufficient reliability to write about what I read. Spanish is my best non-native language; it was the only foreign language my tiny rural high school offered, and our lone Spanish teacher was non-native and not fluent. Unfortunately, Spanish-speaking countries are not generally heavily involved in spaceflight advance planning, though there are some great Spanish-language space blogs out there (Daniel Marin's Eureka leads the pack) which I read frequently.

      I have a fair number of JBIS papers in my files, and I've written posts based on them. Unfortunately, no library within 100 miles receives JBIS or SPACEFLIGHT. I keep meaning to drive to Embry-Riddle in Prescott, which has a whole lot of interesting things.

      I've contacted the BIS directly a couple of times to try to obtain copies of articles. Most recently, I contacted them while researching my little "Making 2001 Real" post series. Unfortunately, nothing came of my article request, though they did respond to my emails. I do not fault them, for I am not a BIS member and they appear to rely heavily on volunteers.

      I plan to write a post on Hermes/Columbus/Ariane 5. I have collected a fair bit of material. I am still sorting out how that evolved. I have not tried to contact ESA or CNES. I probably should.

      Spaceflight has long had an international component, and I mention the space programs of other nations frequently. One cannot avoid this unless one seeks willfully not to give other spacefaring countries and consortia their due credit. I like the fact that spaceflight is international and do my best to describe international involvement in NASA-sponsored missions.

      This paper, in fact, describes a proposed joint NASA/ESA mission. Its authors are all Americans frustrated by their inability to get a NASA comet mission.

      All that being said, I appreciate your comment very much. It reminds me to keep looking for those international connections and to write about them.

      dsfp




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  2. Wow, I had no idea Giotto was the first to use Earth for a gravity assist, on 7-2-1990.... one day before the arrival of myself into this world! From what I've read 1990 was quite a time for interplanetary & other missions (Hubble, Pale Blue Dot pic,etc.)

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    1. You make me feel old. :-) BTW, Galileo would become the second spacecraft to use Earth for a gravity-assist on 8 December 1990. It had been meant to fly directly to Jupiter after deployment from the Space Shuttle in May 1986, so would have already been in the Jupiter system when you were born. The Challenger accident grounded Galileo and, with the cancellation of the Centaur-G' upper stage, robbed it of its means of a direct Earth-Jupiter flight. So it did a Venus grav-assist and then two Earth grav-assists to make up for the loss of Centaur-G' following its launch from Earth orbit in October 1990.

      dsfp

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