1 Introduction

1.1 Purpose of the Earth Gravity Assist

On October 16th 2022, exactly 1 year after launch, the Lucy spacecraft performed the first of three Earth flybys in its nominal mission. This flyby was designated Earth Gravity Assist (EGA) 1. EGA1 was designed to raise the aphelion of Lucy’s orbit from 1.16 AU to 2.26 AU. The second flyby, EGA2 on Dec. 13 2024, will further raise aphelion to 5.71 AU, sufficient to reach the Trojan asteroids that are the mission’s primary science targets. Had the spacecraft missed its primary October 2021 launch opportunity, it could have been launched directly into its post-EGA1 trajectory during a backup launch window in October 2022, thus allowing the spacecraft to execute an identical science mission regardless of the launch window used. EGA1 offered an early opportunity to test and demonstrate the performance of Lucy’s science instruments and systems, and a series of observations were developed to take advantage of this event.

1.2 EGA1 Geometry

The flyby geometry for EGA1 is shown in Fig. 1. The spacecraft approached the Earth from close to the direction of the sun, viewing both Earth and Moon at low phase angles. Closest approach was at 11:04:26 UT, shortly after Earth shadow ingress at 11:02 UT, at an altitude of 359.7 km off north-west Australia. The flyby was originally designed for a lower altitude of 300 km, but concern over Lucy’s aerodynamic stability in the Earth’s upper atmosphere, given the partially-deployed state of the one of its solar arrays, resulted in the decision to raise the flyby altitude to reduce aerodynamic forces, at the cost of a modest expenditure of fuel. Shadow egress occurred at 11:26 UT, over the Eastern Pacific. The spacecraft passed the Moon at a minimum range of 56,100 km at 5:37 UT on October 17th, though lunar observations were made much earlier, a greater range (Table 1). For more details on the EGA trajectory and execution, see Olkin et al. (2024).

Fig. 1
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Lucy Earth flyby trajectory for EGA1, looking down from the north. UTC times are for October 16th 2022. Tick marks are shown every 10 minutes along the trajectory

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Table 1 Summary of Lucy EGA1 Observations
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1.3 Observation Priorities

EGA1’s highest priority was a successful and accurate gravity assist. Remote sensing observations of the Earth and Moon were lower priority, and were limited to three short observing windows at roughly -3, -1.3, and +0.3 days from closest approach (Table 1). Within those windows, most observations were designed for instrument calibration, taking advantage of the Earth and Moon as well-characterized targets with large angular diameters. One observation, however, was designed primarily for public engagement.

The only observations of the Earth or Moon taken primarily for scientific purposes were two thermal emission spectroscopic observations of the Moon, taking advantage of the L’TES instrument’s ability to obtain high SNR lunar spectra at wavelengths inaccessible from Earth. In addition, the Double Asteroid Redirection Test (DART) spacecraft’s impact with Dimorphos, the moon of the near-earth asteroid Didymos, fortuitously occurred just 20 days before the Earth flyby, and science observations of the DART impact were included in the EGA1 mission plan.

Observations taken by Lucy’s science instruments during the EGA1 period are listed in Table 1. Observations are identified with informal designations in the first column, which are referred to in the text.

2 Calibration Observations

All of Lucy’s remote sensing instruments (L’LORRI, L’Ralph (MVIC and LEISA), TTCAM, and L’TES) obtained calibration data during EGA1. Data saturation was an issue for L’LORRI, MVIC, and LEISA, which were not designed to operate at 1 AU, but observations were designed to minimize this saturation, and all instruments obtained useful calibration data.

2.1 L’LORRI

The Lucy Long-Range Reconnaissance Imager (L’LORRI), a panchromatic long focal length camera, is the highest-resolution imager on Lucy (Weaver et al. 2023a,b,c). Both the Earth and Moon were so bright on EGA approach that L’LORRI images were saturated even at the fastest possible exposure time (1 millisecond). However, the brightness of the Earth made it a good target to characterize off-axis scattered light within the instrument, important for instance for assessing L’LORRI’s ability to detect faint objects, such as potential small satellites, near its primary targets. To this end, L’LORRI obtained a series of images pointed up to 3.4 degrees from the Earth’s limb, to determine the brightness of scattered Earth light, and its distribution across the detector (observations E1, E6, Table 1).

The most important L’LORRI calibration activity during EGA was to use images of the lunar terminator to determine L’LORRI’s effective angular resolution under flight conditions (observation M4, Table 1). This information will be used to optimize L’LORRI imaging of its primary Trojan science targets, ensuring that spatial resolution is sufficient to meet Lucy’s primary science goals. L’LORRI pointed at an inertially fixed location while the Moon’s terminator drifted across the field of view, obtaining 400 images with alternating 2 msec and 5 msec exposure times. These images covered the center of the Moon’s Earth-facing hemisphere at a scale of 1.3 km/pixel (Fig. 3c). Additional images covered the northern and southern parts of the terminator. Images of a nearby starfield (observation S1, Table 1) characterized the point spread function for the lunar images. Comparison to higher-resolution images from other spacecraft determined that impact craters can be identified in raw images with 90% reliability down to diameters of ∼18 pixels (Robbins et al. 2023a, 2023b). The same study showed that image subsampling and stacking, followed by deconvolution using the point spread function obtained from the nearby starfield images, improved the 90% crater detection limit significantly to ∼13 pixels, though the specific numbers vary considerably with details such as solar incidence angle and the crater measurement technique.

Because the radiometric properties of the Moon as a function of wavelength, surface location, and geometry are well known (e.g. Keifer and Stone 2005, Sato et al. 2014), the lunar data will also be valuable for refinement of the absolute radiometric calibration of L’LORRI for resolved sources.

One other calibration activity involved using the moon to test L’LORRI trigger mode (observation M5, Table 1). Trigger mode, which saves images only when a target is autonomously detected in the camera field of view, was developed for the original New Horizons LORRI instrument. However, it has never been used on science targets on New Horizons, because it proved impossible to validate trigger mode in flight on a resolved target in advance of attempting to use it on a high-priority science target. In the Lucy LORRI test, performed by scanning L’LORRI across the Moon, trigger mode performed perfectly, saving a series of images when the Moon appeared in the L’LORRI field of view. The mode can now be used with confidence if needed for future science observations.

2.2 L’Ralph MVIC

The Multicolor Visible Imaging Camera, MVIC, is part of the L’Ralph instrument (Reuter et al. 2023), and shares optics with the LEISA instrument described below. As with L’LORRI, Ralph color images of the Earth (observations E4, Table 1) were partially or entirely saturated, as expected. However, panchromatic images taken near the Earth (observation E5, Table 1) were used, as with L’LORRI, to characterize off-axis scattered light within MVIC, verifying that scattered light levels were low enough to meet requirements at the Trojans. In addition, images of the Moon (observation M3, Table 1) were obtained in all MVIC color filters. While parts of the Moon were saturated in several filters, all contained useful unsaturated data near the terminator, and three filters were almost entirely unsaturated, yielding a three-color image (Fig. 3b). As with L’LORRI, these data are valuable for refinement of the absolute radiometric calibration of MVIC for resolved sources, by comparison with the known wavelength-dependent photometric characteristics of the Moon (e.g. Keifer and Stone 2005). As reported in Reuter et al. (2023), there was good agreement between the absolutely calibrated radiances derived from the MVIC pipeline, and published radiance data at comparable geometries.

2.3 L’Ralph LEISA

The Linear Etalon Imaging Spectral Array (LEISA) is the other component of L’Ralph, providing infrared spectral mapping from 1.0 to 3.8 \(\mu \)m (Reuter et al. 2023). However, LEISA is designed to operate at the Trojan asteroids, > 5 AU from the sun, and cannot be cooled to normal operating temperature at 1 AU. Useful LEISA scans of the Earth (observation E3, Table 1) were nonetheless obtained during EGA1, by pre-cooling the detector before the observations, and reading out narrow swaths of pixels to minimize integration time. Due to the warm instrument temperature, some of the data were saturated, though unsaturated data were obtained at several wavelengths and Earth locations, including areas where nearly complete spectra were observed (see Fig. 4, and Reuter et al. 2023 for an example). Scans of the Moon were not attempted due to unfavorable geometry, which resulted in instrument temperatures even further from nominal than was possible for the Earth observations.

2.4 L’TES

The Lucy Thermal Emission Spectrometer (L’TES) obtains thermal IR spectra from 6 to 75 microns, with a single detector covering a 7.3 mrad FWHM field of view (Christensen et al. 2024). A raster scan of the Moon during EGA1 provided an invaluable opportunity to characterize the spatial response across the L’TES field of view (observation M1, Table 1, and Fig. 5), because the Moon, seen at low phase on approach, provided a very bright, relatively uniform, source that was smaller than the L’TES field of view (Fig. 6). Simultaneous L’LORRI and TTCAM imaging also provided the location of the L’TES aperture relative to the boresights of the L’LORRI and TTCAM apertures- essential information for accurately targeting L’TES science observations at the Trojan asteroids.

In addition, spectra of the Earth and Moon provided checks on the wavelength calibration and absolute sensitivity of L’TES (Observations E2, M1, M2, M7, Table 1, and Fig. 5) (Christensen et al. 2024).

2.5 TTCAM

Lucy’s Terminal Tracking Camera (TTCAM) is primarily used for automated tracking of Lucy’s Trojan asteroid targets during flybys, ensuring that the primary science instruments are pointed at the targets (Bell et al. 2023; Zhao et al. 2023). However, the cameras also address Level 1 Lucy science goals, in particular by providing high-cadence global imaging of the Trojan targets in the minutes surrounding closest approach. As with L’LORRI and MVIC, EGA1 imaging of the Earth and Moon with a wide range of exposure times (observations E6, M6, Table 1, and Figs. 3a, 8) provided an opportunity to calibrate instrument radiometric performance and assess image quality on extended targets. Derived radiance values for lunar images were within 15% of expected values (Zhao et al. 2023). TTCAM also provide support imaging for L’TES scans of the Earth and Moon (observations E2, M1, M2, M7, Table 1, and Fig. 6).

3 Science Observations

3.1 DART Impact

The DART spacecraft’s impact with Dimorphos, moon of near-Earth asteroid Didymos, occurred at 23:14 UT on September 26th 2022, as a test of the effectiveness of spacecraft impacts in changing the trajectory of potentially hazardous asteroids (Cheng et al. 2023; Daly et al. 2023). The impact was observed directly by telescopes on or near the Earth, including the Hubble Space Telescope (Li et al. 2023), and multiple ground-based facilities (Graykowski et al. 2023). L’LORRI also obtained extensive observations of the impact from the vantage point of the Lucy spacecraft (observation D1, Table 1). A total of 1549 images were obtained, using L’LORRI’s 4x4 pixel binning mode to minimize data volume. The L’LORRI observations provided both backup for ground-based observations of the impact, in case of bad weather at terrestrial observatories on the right side of the Earth to observe the impact, and a unique viewing geometry. Range (0.126 AU) was larger than for Earth observations (0.0757 AU), but solar phase angle (31.5 degrees) was significantly lower than the view from Earth (53.2 degrees). Images showed the dramatic brightening of Didymos due to the impact ejecta, and spatially resolved some detail of the impact cloud (Weaver et al. 2022, 2023b, 2023c). The lower phase angle of the Lucy observations provided unique information on the scattering properties of the ejecta cloud.

3.2 Lunar Thermal Spectroscopy

In addition to its calibration observations of the Moon, obtained when the Moon was much smaller than the L’TES field of view, L’TES observed the moon twice more for scientific purposes, first later during the approach, when it filled more of the instrument field of view (observation M2, Table 1), and second after the Earth flyby, when the moon was slightly larger than the field of view (observation M7, Table 1, and Fig. 2). These data provided higher SNR lunar observations, with some modest spatial resolution in the case of post-EGA images. The resulting spectra are scientifically valuable, because there are very few available well-calibrated thermal IR spectra of the moon, in particular at wavelengths outside the 10 and 20 \(\mu \)m atmospheric windows observable from the Earth’s surface (Salisbury et al. 1995; Greenhagen et al. 2010).

Fig. 2
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Examples of observations made during EGA1. a: A L’Ralph/LEISA scan of the Earth (observation E3, Table 1, and Fig. 4). The red, green, and blue rectangles show different wavelength channels of the LEISA instrument, which were swept across the Earth using the L’Ralph instrument scan mirror. b: A L’LORRI observation of the lunar terminator (observation M4, and Fig. 3c). The green rectangle shows the L’LORRI field of view. The yellow circle is the L’TES field of view, though L’TES data were not obtained during this particular observation

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Fig. 3
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Images of the lunar near side taken October 16th 2022 from similar range by Lucy’s three visible-wavelength cameras, providing comparison of imaging capabilities. The TTCAM image (a) was taken at 18:14 UT from a range of 241,000 km (observation M6, Table 1). The MVIC image (b), which uses MVIC’s violet, green, and near-infrared channels to create a color composite, was taken at 17:15 UT from a range of 261,000 km (observation M3, Table 1). The white box shows the location of the L’LORRI image. The L’LORRI image (c) of the lunar terminator was taken at 17:37 UT from a range of 254,000 km (observation M4, Table 1). The resolution has been improved by Lucy-Richardson deconvolution

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Fig. 4
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A LEISA false-color composite spectral image of the Earth at 1.95 (blue), 2.0 (green) and 3.5 (red) \(\mu \)m wavelengths, from observation E3 (Table 1). To enable short exposures to limit saturation, only narrow imaging swaths could be obtained, in 3 separate scans. Despite some image artifacts, many details are visible, including bright clouds in blue, the coastlines of the Arabian Peninsula and Iran on the far left in green, and Australia in yellow on the right

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Fig. 5
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Ten consecutive L’TES spectrum of the Earth, plotted in different colors, obtained during EGA1 (observation E2, Table 1), with prominent spectral features identified. The spectra overlap almost perfectly

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Fig. 6
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Cartoon illustrating the use of the Moon to map the shape and location of the L’TES field of view (FOV) relative to the L’LORRI field of view, 2.96 days before Earth close approach (observation M1, Table 1). The observation is shown in instrument-fixed coordinates, with the moon moving three times across the field of view in each of two orthogonal directions. Imaging by L’LORRI during the scans, showing background stars and/or the Moon itself, established the location of the Moon relative to L’LORRI as a function of time, which could then be compared to the signal seen by L’TES to determine the L’LORRI/L’TES alignment. Simultaneous TTCAM images were also used, but are not shown (the TTCAM field of view is larger than the figure)

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4 Public Engagement Observations

While many of the observations described above produced scenic images that were the subject of public releases after the flyby, only one observation (EM1, Table 1) was made purely for public engagement purposes. This was an approach view of the Earth and Moon in a single TTCAM frame (Fig. 7), effectively showing the relative sizes, separation, and albedos of the two bodies. Unlike LORRI and MVIC, TTCAM was able to obtain unsaturated images of the Earth, due to its ability to use exposure times shorter than 1 msec. Figure 8 shows TTCAM’s best full-frame image of the Earth, taken during the TTCAM Earth radiometric calibration observations (E6, Table 1).

Fig. 7
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The Earth and Moon seen by TTCAM, taken at 11:08 UT on October 13th 2022, with an exposure of 0.1 msec, during observation EM1 (Table 1). Contrary to the usual practice with similar images, the Moon (far left) has not been brightened relative to the Earth, preserving the true relative albedos of the two bodies

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Fig. 8
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Lucy’s best image of the Earth from EGA1, a 0.1 msec exposure taken with TTCAM at 04:54 UT on October 15th 2022 from a range of 624,000 km, during observation E6 (Table 1). Image scale is 46 km/pixel. The image is centered on the Indian Ocean and includes, near the left limb, the Ethiopian discovery site of the Australopithecus fossil “Lucy” that gave its name to the mission

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5 Summary

EGA1, Lucy’s first planetary encounter, was a success from an engineering standpoint, delivering Lucy into the next phase of its orbital journey to the Trojans. But the flyby also provided valuable calibration data for all of Lucy’s instruments, including radiometric, spectral, and geometric calibrations using both the Earth and the Moon. Valuable science data, and images for public engagement, were also obtained during the flyby. Because EGA1 met all the desired calibration and other goals, and because of other demands on the flight team, EGA2, in December 2024, will likely include a much smaller set of observations of the Earth/Moon system. The EGA1 data provide a small appetizer for the wealth of science results anticipated when Lucy reaches its primary Trojan targets, starting in late 2027.