Observation of Cassini's Entry into Saturn: No Detection, and Lessons Learned

To assure planetary protection of the possibly habitable moon Enceladus, Cassini's epic 20 yr mission (Lorenz 2017) was terminated by disposal into Saturn's atmosphere. This attracted much public interest, and inquiry as to whether the spacecraft's demise would be observed. Planning for such (speculative) observations were made rather late, and to avoid raising unrealistic expectations, were not extensively publicized. Here we report on efforts to observe the Cassini entry.

Insofar as spacecraft entries are of known trajectory, mass and composition, they serve as useful reference events in meteor studies in order to measure luminous efficiency and other physical parameters. The entry of sample return capsules from interplanetary space have enabled verification of thermal protection systems performance (Jenniskens 2010). The destructive reentry of controlled spacecraft in Earth's atmosphere were studied to determine the process of fragmentation and the impact risk on the ground from falling debris (Jenniskens & Hatton 2008).

The optical emission during Earth entry is often characterized by emission lines from metal atoms (aluminum, titanium) and metal oxide molecular bands from the ablating vehicle materials, as well as emissions from atmospheric oxygen and nitrogen (Winters et al. 2019). Persistent (minutes) emissions are sometimes observed from chemiluminescence involving the recombination of oxygen atoms and ozone, as well as from scattered light from dust particles left in the path of the meteoroid.

Observations of entries in extraterrestrial atmospheres are of particular interest because the composition and density of these atmospheres differ from those of Earth. Efforts to observe the entry of the artificial Huygens probe (Lorenz et al. 2006) were unsuccessful. The much larger energy (0.245 kT) of the Cassini entry, however, offered some modest hope that it might be detectable, since bolides of energy 40 kT (a factor of ten smaller than the Chelyabinsk bolide of 2013) are detectable without prior warning on Jupiter even with small amateur telescopes (e.g., (Hueso et al. 2018)).

Saturn is about 10 million times further (10⁹ km) than a typical meteor seen from Earth's surface (100 km), making the entry 18 magnitudes fainter. Assuming the luminous efficiency would be 5% an 0.245 kT impact would radiate for about 5 s with about −16 magnitude peak visible brightness when seen from 100 km distance. That suggested a + 2 magnitude emission for Cassini against the bright Saturn disk, predominantly in sharp emission lines. Because that emission is essentially a point source seen against the bright Saturnian disk, large telescopes would be required to resolve the source as best as possible from the background.

The epoch of the Cassini entry on the hourly timescale was unfortunate, in that it occurred when Saturn was out of view of the major observatories of the Americas and Hawaii, which not only feature the world's largest telescopes, but have affiliation with the USA and ESA member states behind Cassini which would motivate special arrangements to observe. The campaign timing of the SOFIA observatory precluded its consideration. However, a number of Asia-Pacific observatories were able to respond with requests to observe.

The primary goal of Hubble observations was to determine the luminous efficiency, since this is poorly known and would facilitate interpretations of Jupiter impact flashes. A secondary goal was to use the flash timing to constrain models of Saturn's atmosphere at 5–100 μbar. Groundbased observations were hoped to provide lightcurves in a number of spectral lines, that might inform models of ablation and fragmentation. The event would have peak brightness for a period of only seconds, so high time resolution was desirable.

The most promising groundbased observations were spectra from the WiFes Integral Field Spectrograph with the 2.3 m ANU telescope at Siding Spring, Australia (observer: Bessell). 1.5 s exposures with about 13 s readout time, yielding red and blue spectra with R = 3000, were acquired from 11:30 to 12:22, but no anomalous signals were detected. Other observations include Macquarie University (Australia) 140 mm/CCD Hydrogen-Alpha (Goodwin, Baković) and 16 inch CMOS video (Spitler, Arnison) and I-band and visible CMOS imaging with 24 inch remote telescopes operated by the National Central University of Taiwan. Observations at rather high airmass (2.7 and 3.2) were attempted with a 2.16 m telescope in Xinglong, China (Ge) and at the 1.6 m PIRKA telescope in Nayoro in Hokkaido, Japan (Imai, Ono) but seeing was poor. Again, no signatures were noticed. 2.0 m observations at Hanle, India (Baug) yielded no useful data. 1 Hz EMCCD observations with a 2.4 m telescope (Zhang) in Lijiang, China and at Mount Abu, India (1.2 m, S. Ganesh) were frustrated by clouds.

The last Saturn image taken by Cassini itself was taken on 2017 September 14 at 19:59 UT. Data continued to be collected by the spacecraft's other instruments. The predicted altitude for loss of signal was 1500 km above Saturn's cloud tops, at which time the spacecraft was expected to tumble, and then ablate and create an ionized envelope that would block any further radio signals, before breaking into fragments. The entry angle in Saturn's atmosphere was 30° and entry speed 31.1 km s⁻¹. As of 2017 September 11, the tumble time was predicted to be 11:55:14 UT on September 15, but 2 days later was revised to 11:55:02. Cassini's actual final transmissions were received by the Canberra Deep Space Communication Complex, at 11:55:46 UT. The peak optical emission would have been perhaps 100–250 s later.

Observations with HST were frustrated by timing evolution, causing Saturn to be occulted by Earth at the time of entry. For HST, variability in the Earth's upper atmosphere makes orbital phase uncertain more than a few months in advance, which is why the final scheduling of observations is not done until that time. For Cassini, since the final five Saturn periapsides were at altitudes where drag was significant but poorly known in. The atmospheric density was roughly double predictions, resulting in a reduction in the spacecraft's 9370 minute orbital period of 2 minute per periapsis. The time of the entry flash progressively advanced to be while Saturn was still below the Earth horizon as seen from HST.

Figure 1 shows the observation obtained once Saturn came into view. While aurorae are visible near the northern pole, no intensification could be detected near the entry site. A refined analysis of the STIS time-tagged signal within the circle yielded the same conclusion. No confident signatures were noted in any of the groundbased observations.

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R.L. and F.C. acknowledge the support of the Cassini project. P.J. acknowledges NASA grant NNX14AR92G. G.L. acknowledges the support of the Xinglong observatory and partial financial support from the Open Project Program of the Key Laboratory of Optical Astronomy, CAS. X.L.Z. acknowledges the Lijiang telescope, jointly operated and by Yunnan Observatories and the Center for Astronomical Mega-Science, CAS, and funding from Civil Aerospace D020302. M.R.A. and L.S. acknowledges the support of David Monaghan, Paul Stewart and Adam Joyce. We thank Mike Bessell for performing the ANU/WiFeS observations, Shashikiran Ganesh, Padma Yamanadra-Fisher, Henry Throop and many colleagues who assisted with observations, attempted observations, and coordination.