Comet Siding Spring will pass within 134,000 kilometers of Mars on October 19. More importantly for us on Earth, the comet will pass that close to seven Martian spacecraft. Now that we have reasonable confidence that our orbiters will be safe, we are free to be excited about the opportunity that the encounter represents. Siding Spring is a long-period comet on its first trip into the inner solar system. And it will be passing within a few hundred thousand kilometers of a veritable fleet of scientific spacecraft. But they're not comet missions; they're Mars orbiters. Will spacecraft designed to study a planet up close be able to get good science on a tiny world that's significantly farther away?
Spacecraft safety is still the first and most important concern. Comets may be small, but they're surrounded by a vast cloud, or coma, of dust and ice particles shed from their active nuclei. For the rovers on the surface of Mars there's no problem: Mars' atmosphere is thick enough that such tiny particles burn up without reaching the ground. Our worry has been about the orbiters. We think they're probably safe, but out of an abundance of caution, all three orbiters currently at Mars are shifting the timing of their orbits just slightly so that they will wind up on the far side of Mars from the comet when Mars' path through the tail reaches the region of highest dust density, about 100 minutes after the closest approach of the comet. The two orbiters on their way to Mars will also perform adjustments to keep on the safe side at the right time. Here's a summary of recent and near-future maneuvers planned to keep spacecraft safe:
- Mars Reconnaissance Orbiter: two maneuvers, one on July 2 and one on August 27
- Mars Odyssey: one maneuver on August 5
- MAVEN: will phase its orbit as part of its third Period Reduction Maneuver on October 9 during its transition from initial to science orbit
There was a workshop at the Applied Physics Laboratory on Monday to gather together people working around the world on observing Siding Spring to discuss plans for comet observations from Earth and space. Most of the workshop concerned observations from telescopes on Earth and above it, but the last few hours of the workshop were devoted to the plans of Mars spacecraft, and I listened in. You can watch it here if you want all the details, but I'll do my best to summarize it below.
Every spacecraft plans to observe, some more than others, according to what their unique array of instruments can accomplish. Most of the work will focus on the comet's coma -- its extent, its composition, how it varies with time, how big are the particles in it, how it's supplied by jets from the nucleus. Another major area of study will be the comet's effect on the Martian atmosphere -- we've never studied a comet's coma impinging on the atmosphere of any planet before, and the results of that work will be fascinating. And a single spacecraft may possibly be able to photograph the tiny nucleus of the comet as it passes by.
The comet will be moving extremely fast, at a relative speed of 57 kilometers per second. Its nucleus is likely only 1 or 2 kilometers across, but its neutral gas coma extends for more than 100,000 kilometers in all directions from the nucleus. Ions may be farther away than that, and the tail stretches for millions of kilometers. So while most of the instruments won't even detect the comet's nucleus, most will be able to see the coma or the coma's effects on the atmosphere.
Here's a brief summary of the spacecraft at Mars, and the instruments they carry. The resolutions reported for the cameras are what they achieve on Mars, roughly 300 kilometers away; even near closest approach, the comet will be approximately 1000 times farther away than that.
- Mars Reconnaissance Orbiter, arrived 2006. Has 3 cameras: HiRISE (30 cm/pixel on Mars), CTX (6m/pixel), and MARCI (1km/pixel) plus an imaging spectrometer (CRISM, 18m/pixel on Mars). Also has SHARAD radar sounder.
- Mars Express, arrived 2004. For Siding Spring, will use HRSC camera (somewhat lower resolution than CTX), and SPICAM ultraviolet/infrared atmospheric spectrometer.
- Mars Odyssey, arrived 2001. For Siding Spring, will use THEMIS thermal emission imaging system (100 m/pixel on Mars).
- MAVEN, arriving 2014. Has a suite of instruments devoted to Mars' upper atmosphere (no camera).
- Mars Orbiter Mission, arriving 2014. Has a varied instrument suite but unknown if it will be performing Siding Spring observations.
The rovers are also planning to observe but have unique constraints that means the plans won't be set in stone until the day before they're executed; Mark Lemmon has promised me a separate post about that.
Enough preamble; here is a collated summary of all the orbiters' plans for comet observations. (All times UTC.) These plans are a work in progress, so may change before the encounter.
Aug 6: Mars Reconnaissance Orbiter CRISM and HiRISE successfully tested their targeting of the region of the sky where they'll be observing the comet. Deputy project scientist Leslie Tamppari reported that they have high confidence in their ability to target correctly.
Sep 22 02:00: (approx.) MAVEN orbit insertion. MAVEN observations of the comet will only occur if orbit insertion goes off without a hitch. Although they will not be in their science mission yet, MAVEN has already acquired experience in attempting comet observations: they tried to study ISON. They didn't succeed in detecting it, but it gave them valuable practice for the Siding Spring campaign.
Sep 24: Mars Orbiter Mission orbit insertion. Specific plans for Mars Orbiter Mission observations were not announced. At the meeting, Padma Yanamandra-Fisher -- who was presenting a set of slides sent by Anil Bhardwaj (PI of the MENCA instrument on the mission) -- said that they are not yet certain of their final orbit, because it depended in part upon MAVEN's orbital plans. MAVEN had been considering delaying their transition from initial orbit into science orbit because that would have allowed them to hide behind Mars longer. MAVEN has decided to stick with their initial plan. Yanamandra-Fisher hoped that there would be more detail about ISRO's plans at an upcoming Siding Spring meeting on September 19.
Oct 9: (possible) Mars Reconnaissance Orbiter CRISM and HiRISE will shoot photos 10 days before closest approach to reduce the uncertainty in our knowledge of the comet's trajectory.
Oct 14: 5 days before closest approach, MAVEN will perform its first ultraviolet observations of the comet; it should be able to get images of the coma in ultraviolet wavelengths and measure the ratio of deuterium to hydrogen.
Oct 17: Mars Express plans observations of the comet on 10 orbits around closest approach, orbits 13706 to 13715. (Mars Express orbits are 6.7 hours long, so this is about 5 days.) I don't have a detailed breakdown of when these activities will take place. In all, Mars Express HRSC plans 12 observations of the comet. It will also use SPICAM to study the response of the atmosphere to the comet by observing 6 stellar occultations and 4 limb surveys. SPICAM will watch one stellar occultation by Siding Spring's coma.
Oct 17: 2.5 days before closest approach, Mars Reconnaissance Orbiter will turn into a Mars Comet Observer for five days, devoting all of its data-gathering time to observations designed to study the comet and Mars' amtospheric response to it. It orbits Mars once each 112 minutes. Every other orbit will be devoted to imaging the comet with HiRISE, CRISM, and CTX cameras, with two comet observations in each orbit. They mostly won't see the nucleus directly, but they hope to follow the variability of the comet's activity and determine its rotation rate. CRISM, especially, should be able to map the distributions of neutral gas molecules in the coma, things like C2, C3, water, CN, NH2, and oxygen -- anything abundant that has spectral features in the range of 0.4 to 1.0 micrometers. On the alternate orbits, MARCI, MCS, and SHARAD will study Mars' atmosphere, gathering a data set on the background state of the atmosphere before the comet reaches it. The comet encounter happens to occur at the peak dust storm season for Mars, and the comet effects may be hard to tell apart from dust effects; these pre-encounter observations will help to tease the two apart.
Oct 18: 1 day before closest approach, MAVEN will operate in its normal science mode, gathering data on the atmosphere. Since MAVEN will still be in its initial checkout phase, this will be some of the first science data MAVEN will get at Mars with its entire instrument suite operating simultaneously. It will look for changes in the temperature, density, circulation pattern, and composition of the atmosphere, as well as for changes in the magnetosphere.
Oct 19: Roughly one orbit before the predicted maximum dust peak, MAVEN will be placed into a safe mode where no instruments requiring high voltages will be operating. However, three of its instruments (its magnetometer, Langmuir probe, and solar energetic particle detector), will be able to continue gathering data safely. MAVEN's orbits are much longer than Mars Reconnaissance Orbiter's, about 4.5 hours.
Oct 19 16:42-17:12: On Mars Reconnaissance Orbiter's last orbit before closest approach, it will perform 2 CRISM and 4 HiRISE scans of the nucleus with different exposure times as CTX rides along, roughly 1 image every 5 minutes, then briefly point to Mars to observe with MCS, MARCI, and SHARAD. Two of the three exposure times for HiRISE are chosen so that the nucleus will be visible whether it's bright or dark. The comet will be roughly 300,000 kilometers away. Most comet nuclei that we've seen up close are extremely dark, with an albedo around 3%. But we don't know whether comets start out so dark, or if they get dark on repeated passes around the Sun. Since this is likely Siding Spring's first passage into the inner solar system, it may not be as dark as other comets we've visited. How dark the comet turns out to be could be one of the most significant findings of the Siding Spring campaign, Alan Delamere said. A third, longer exposure time will hopefully silhouette the comet against its coma, in the same way Giotto was able to pick out Halley's shape from its silhouette, something that Delamere called "a major enabling capability of the Halley mission." However, he tried to temper people's expectations of the HiRISE images. The nucleus is only estimated to be about 1 or 2 kilometers across. Delamere showed rescaled Giotto images of Halley to simulate what HiRISE would see on a 1-kilometer nucleus. With these pictures, Delamere said, "We're not going to turn the public on the way Rosetta has. We're going to have pretty lousy pictures. Only people in this room and people in phone will appreciate them." I think a few more than that will appreciate them, but the point is well made.
Oct 19: On Mars Odyssey's last orbit before closest approach, it will perform repeated THEMIS scans across the coma, which could theoretically be assembled into a mosaic. Odyssey THEMIS images are the only thermal infrared observations currently planned by any Mars orbiter, and could be crucial for identifying the size and distribution of dust particles in the nucleus.
Oct 19 18:14-19:58: Throughout closest approach, Mars Reconnaissance Orbiter will perform 2 CRISM and 6 HiRISE scans at different exposure times, roughly 1 image every 5 minutes, with CTX riding along, followed by more MCS, MARCI, and SHARAD. Neither CRISM nor CTX will be able to resolve the nucleus, although HiRISE will. Dave Humm reported for CRISM that they think they might at least be able to detect the nucleus in one pixel at closest approach, but not if it's very dim. Even if CRISM does detect the nucleus, it will be hard to separate the spectrum of the nucleus from that of the coma.
Oct 19 18:32: Closest approach to Mars at a distance of 134,000 kilometers and a relative velocity of 57.4 kilometers per second. It will be Opportunity sol 3817 at 06:37 LTST, and Curiosity sol 783 at 16:09 LTST.
Oct 19 18:47-20:16: 2 CRISM and 4 HiRISE scans of the nucleus with CTX riding along. The comet will be roughly 300,000 kilometers away, so the images will have lower resolution than the close-approach ones by a factor of about two. The MCS and MARCI observations that follow the comet imaging are the ones most likely to show effects of the coma's interaction with Mars' atmosphere. These might include heating of the middle atmosphere due to dust flying into it. In response to that heating, the atmosphere should inflate, causing vertical motion within the atmosphere. And the increased amount of dust in the middle atmosphere could seed clouds that weren't there before.
Oct 19: 20:07 +/- 10 minutes: Center time for coma particle encounter with Mars. The center of the disk of Mars that will receive dust from the coma is at 8 degrees south, 319 east, but the effects spread out across much of one hemisphere of Mars, within a great circle of radius 60 degrees centered on that spot.
Oct 19: An orbit or two after its closest approach, Odyssey will again acquire multiple THEMIS images across the comet, scanning in a direction perpendicular to its previous scan, in order to image as much of the coma and tail as possible. The cartoon below shows the difference between the closest-approach THEMIS mosaic (green), which prioritizes the coma, and the later, lower-resolution mosaic focused on both coma and tail (blue).
Oct 20: As the comet recedes, Odyssey will scan from space down to Mars to capture a single image that includes both the comet and Mars, signified by the red boxes in the diagram above. The scientific purpose of this observation is to gather data on Mars -- whose properties are well known to the THEMIS team -- to help them calibrate their comet observations. But it should also generate a pretty picture. I'm as excited about this photo as I am about the hoped-for HiRISE images of the nucleus. Again, though, the THEMIS team was careful not to promise too much. THEMIS was designed to look at Mars, which is substantially warmer than the comet will be. They've done the math and think they will be able to detect a decent signal from the comet, but the uncertainty is very large. "We're just going to have to take a shot at it and see what we come up with," Jeff Plaut said. Even if they don't catch the comet, the image of Mars should be quite nice -- just look at this limb view, which Odyssey took on the day of the Phoenix landing.
Oct 20: Mars Reconnaissance Orbiter will return to its cadence of alternating orbits of comet imaging and atmospheric measurements for 2.5 more days (see October 17 entry for details).
Oct 20: One orbit after the predicted maximum dust peak, MAVEN will return to normal science mode for two days.
Oct 22: MAVEN will return to its planned checkout of its scientific instruments, preparing for the Mars phase of its mission.
Oct 22: Mars Reconnaissance Orbiter and Mars Express will take their last images directed at the comet, but both will continue to study Mars' atmosphere as part of their normal science operations.
After this workshop I am enormously excited for the visit of Siding Spring to Mars. We have flown spacecraft past comets only about 10 times in our spacefaring history (depending on how you count various distant encounters). In one fell swoop, one comet is about to visit seven more spacecraft.
There will be another community meeting on September 19; stay tuned for much more!