Icarus 222 (2013) 421–423

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Icarus

journal homepage: www.elsevier .com/locate / icarus

Editorial

Introduction: Low-budget extended missions to comets

In late 2010 and early 2011 we witnessed flybys of two cometsby two different missions – EPOXI at Comet 103P/Hartley 2 andNExT at Comet 9P/Tempel 1. Both flybys were extended missionsfor spacecraft that had completed their prime missions under NA-SA’s Discovery Program. The total cost for both missions, includingthe cost of the unrelated EPOCh (Extrasolar Planet Observation andCharacterization) investigation of the EPOXI mission, was roughly$60 M, a bargain by any standard given the scientific return fromthe two missions. The EPOXI mission (EPOXI = EPOCh + DIXI),which included both the EPOCh investigation and the DIXI (DeepImpact eXtended Investigation) investigation, was carried out withthe Deep Impact Flyby spacecraft, which was undamaged in flyingthrough the coma of Comet 9P/Tempel 1 after delivering theImpactor spacecraft to the comet. The NExT mission (New Explora-tion of Tempel 1) was carried out with the Stardust spacecraft afterit returned its sample container to Earth. This issue contains manyof the papers that resulted from both encounters, both papers bythe mission teams themselves and papers by many other remotesensing observers who obtained unique data to provide both con-text for the in situ data and scientific results in their own right.

During its prime mission, the Deep Impact Spacecraft (two ofthem – a flyby spacecraft and an Impactor spacecraft) excavateda crater on Comet 9P/Tempel 1. The key goals of the mission wereeasily achieved, notably the conclusion that the volatiles excavatedfrom depths to 25 m had the same relative abundances as seen inthe ambient outgassing. However, the grains were much finer thanexpected, leading to a larger cross-section to mass ratio than ex-pected. Because these ejecta were optically thick over the impactsite until after the flyby spacecraft had passed the nucleus, the cra-ter produced by Deep Impact was never seen by the DI Flyby space-craft (DIF).

While the Stardust spacecraft was returning to Earth (bringingback the samples that had been collected from Comet 81P/Wild2) it was realized that the spacecraft could be retargeted toencounter Comet 9P/Tempel 1 a full orbit plus 1 month after theDI encounter. NASA approved this extended mission for the Star-dust spacecraft (SDU) as a mission of opportunity under the Dis-covery Program and renamed the mission Stardust-NExT, or SNfor short. The key scientific goals were, in order, to look for changesin the nucleus over a full orbital period, to observe parts of the nu-cleus that had not been observed by the DIF, and to observe thecrater produced by DI. To reach Tempel 1 the spacecraft performedan Earth flyby on January 14, 2009 and some dozen subsequentTCMs or Trajectory Correction Maneuvers (Fig. 1 from Veverkaet al., 2013).

One of the most challenging aspects of the mission was tounderstand the rotational state of Tempel 1 in sufficient detail topredict whether the impact site would be visible from the space-craft at the time of closest approach. Extensive work went into

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studying the rotation of the nucleus using different models and dif-ferent datasets to understand by how much the rotational periodchanged at each perihelion passage and to predict when the impactcrater would be on the sunlit hemisphere and visible from the SNspacecraft. Initially there were large disagreements between themodelers but in late 2009 results began to converge and in January2010 the decision was made to execute a maneuver that would de-lay the arrival time by 8.5 h to put the DI crater in the window ofvisibility for the SN spacecraft. This was designed to use a largefraction of the remaining fuel on the spacecraft, leaving only en-ough for some small TCMs and for attitude control.

The encounter with Tempel 1 took place on 14 February 2011(UTC Feb 15) at a range of 178 km nearly directly sunward of thenucleus and at a speed of 10.9 km/s. The comet was at a heliocen-tric distance of 1.548 AU, 34 days past its perihelion passage. Thephase angle varied from 81� on approach through 15� near closestapproach and back to 98� on departure, with imaging throughoutthe encounter.

All scientific objectives were achieved. SN imaged the DI impactsite under excellent viewing and lighting conditions at a scale of12 m/pixel revealing a subdued impact scar some 50 m across, con-sistent with surface mechanical properties similar to those of loose,dry snow. In the region overlapping DI and SN coverage most of thesurface remained unchanged between 2005 and 2011 in albedo,photometric properties and morphology. Significant changes tookplace only along the edges of a prominent smooth flow estimatedto be 10–15 m thick, the margins of which receded in places by upto 50 m.

In several cases, the sources of prominent jets can be tracedback to apparently retreating scarps. Perhaps the most significantresult of the extended coverage of the nucleus provided by SN isthat about a third of the surface of Tempel 1 is covered by smoothflows which are strongly restricted to gravitational lows on the nu-cleus consistent with the view that they represent materialerupted from the subsurface and date from a time after the nucleusachieved its current shape.

It is also noteworthy that during the Tempel 1 flyby, the SN dustinstruments detected bursts of impacts consistent with a processby which larger aggregates of material emitted from the nucleusfragment into smaller particles within the coma, a situation similarto that observed previously during the flyby of Wild 2 in 2004.

Having successfully completed all aspects of the original Star-dust and the extended Stardust-NExT missions, the spacecraftwas shut down with its fuel exhausted on March 24, 2011. Thespacecraft is in a 1.5-year solar orbit and will not come closer toEarth that 1.7 million km for at least 100 years.

EPOXI was first approved by NASA as a flyby of Comet 85P/Boe-thin in late 2008, but at the time of selection the comet, which hadbeen behind the Sun at its last apparition (a careful study had

Fig. 1. DIF Trajectory in Rotating Coordinates. The left panel, looking down on the ecliptic, shows the relatively large, Earth-centered orbit after the Earth flyby in December2007, through TCM 12, to the Earth flyby in December 2008. The orbit from December 2008 through June 2010 is the very small ellipse centered on Earth. The light blue arcshows the trajectory from the Earth flyby in late June 2010 through the encounter with Comet 103P/Hartley 2. The upper right panel is the same orbit seen looking parallel toEarth’s heliocentric velocity vector and the lower right panel shows the orbit looking directly toward the Sun. The two right-hand panels show the Earth flybys (five totalbetween December 2007 and June 2010) that were used for calibrations and for additional observations of Earth in the EPOCh program.

422 Editorial / Icarus 222 (2013) 421–423

shown that it should not have been seen, even by SOHO), had notyet been recovered. An intensive search showed that it was defi-nitely not recoverable, at which point NASA approved a requestto retarget the spacecraft to the previously identified backupcomet, 103P/Hartley 2, in late 2010. This was just in time to designthe flyby of Earth, with a TCM on 12 November 2007 to set up therequired trajectory. The flyby of Earth occurred on 31 December2007, putting the DI Flyby spacecraft into an eccentric orbit withperiod 1 year and close flybys of Earth each year until encounter.Early in 2008 an overheating problem developed in the telecom-munications system at perihelion. To correct this, a TCM (#12)was executed in mid-June 2008. This maneuver targeted the nextEarth flyby to increase the heliocentric inclination, while alsoincreasing the perihelion distance of the spacecraft almost to1 AU and maintaining the orbital period of almost exactly 1 year.This relieved the thermal problems and it also allowed a muchmore favorable phase angle on approach to Comet Hartley 2. Thetrajectory is shown in Fig. 1.

The Earth flybys were used for lunar observations in order toobtain flat-field data, primarily for the IR spectrometer. Theseobservations led unexpectedly to the clear confirmation of lunarhydration (Sunshine et al., 2009) that had been first suggested bythe M3 team on Chandraya’an (Pieters et al., 2009). Two of theencounters also provided approaches to Earth at high northernand southern latitudes providing polar views of the Earth to com-plement the EPOCh observations of equatorial views (Livengoodet al., 2011).

The encounter with Comet 103P/Hartley 2 took place on 4November 2010 at a range of 694 km and a speed of 12.3 km/s.The comet was at a heliocentric distance of 1.064 AU, one weekafter its perihelion passage. Because of the very different designof DIF and its instruments compared to SN and its instruments,the encounters were very different. Whereas the Stardust space-

craft passed sunward of its target and used the camera’s periscopeto track the cometary nucleus, the DI Flyby spacecraft with body-fixed instruments flew ‘‘under’’ the target (in ecliptic coordinates)with a sunward offset, resulting in only a small variation of phaseangle from 86� on approach down to 80� at closest approach andthen up to 93� on departure. As with the NExT mission, all scientificobjectives of the EPOXI mission were accomplished.

Preliminary results from EPOXI were presented by A’Hearn et al.(2011) and more detailed results are presented in this special issue.The lunar observations at Earth flybys led to such an improvementin the understanding of the calibration of the IR spectrometer thatmany parts of the calibration pipeline were redesigned. This re-sulted in delays in processing and the results from the spectrome-ter, therefore, were not ready in time for this special issue butresults from the cameras are presented here. Key results fromEPOXI, most of which were briefly presented either in the refer-ences below or in meeting presentations, include the existence ofa large cloud of icy grains in the coma, a high relative abundanceof CO2 that apparently drives the activity by entraining icy grains,a nearly axisymmetric, bi-lobed nucleus with most of the activitylocated on the end of the smaller lobe, a smooth waist from whichthere is apparently water vapor emerging but very little else, andsignificant heterogeneity in the outgassing from place to place onthe nucleus. The comet is likely a prototype for the group of cometscharacterized as hyperactive due to measured outgassing rateshigher than can be explained by standard sublimation models.Continuing the trend in results from other cometary missions,there is surprising diversity from one cometary nucleus to another– in overall shape, in topographic features, and in volatile compo-sition. The results in this special issue go into much more detailabout these and other properties of Hartley 2.

DIF is still healthy and in a 14-month heliocentric orbit withperihelion near 1 AU. After the flyby, it was used for observations

Editorial / Icarus 222 (2013) 421–423 423

of Comet Garradd, where abundant CO2 was observed as well asextremely high CO, when the comet was at 2.0 AU (Feaga et al.,in preparation). Continuing observations are limited by funding.

References

A’Hearn, M.F. et al, 2011. EPOXI at Comet Hartley 2. Science 332, 1396–1400.Livengood, T.A. et al, 2011. Observed properties of an Earth-like planet orbiting a

Sun-like star. Astrobiology 11, 907–930.Pieters, C.M. et al, 2009. Character and spatial distribution of OH/H2O on the surface

of the Moon seen by M3 on Chandrayaan-1. Science 326, 568–572.Sunshine, J.M. et al, 2009. Temporal and spatial variability of lunar hydration as

observed by the Deep Impact Spacecraft. Science 326, 565–568.Veverka, J. et al, 2013. Return to Comet Tempel 1: Overview of Stardust-NExT

Results. Icarus 222, 424–435.

Michael F. A’HearnDepartment of Astronomy, University of Maryland, College Park,

MD 20742, USAE-mail address: ma@astro.umd.edu

Michael R. CombiDepartment of Atmospheric and Oceanic & Space Sciences,

University of Michigan, Ann Arbor, MI 48109, USAE-mail address: mcombi@umich.edu

Joseph VeverkaDepartment of Astronomy, Cornell University, 312 Space Sciences Bldg.,

Ithaca, NY 14853, USAE-mail address: veverka@astro.cornell.edu