The year in astronomy: a deeper look

One Sedna mystery Brown would like to solve soon is “that whole, crazy, 40-day rotation thing.” Back in March, he announced a possible rotation period of 40 days, and his team speculated Sedna rotated so slowly because it had a moon. Available data are consistent with a rotation of both 20 and 40 days. “We won’t know the real answer until a few 20- or 40-day periods have passed,” Brown says. “It’s very difficult to get data spanning that length of time,” he says, because his project has to compete with others for telescope time.

Asteroids typically rotate in a range from a few hours to a few days. While astronomers are less sure about the full range of rotation in Kuiper Belt objects, these bodies also seem to spin in less than a few days’ time. With a 20- or 40-day spin, Brown argues, Sedna is practically at a standstill. While it’s possible the last impact to occur on Sedna struck in just the right way to nearly cancel its asteroid-like spin, the odds of such a perfect hit are extremely low.

How big is Sedna? “We have upper limits now from the [Hubble] Space Telescope, which didn’t resolve the disk, and from the Spitzer telescope, which didn’t detect Sedna’s thermal emission,” Brown says. “Both of those limits are consistent with a size of about three-quarters that of Pluto.” Another argument also puts Sedna in the same size range. If Sedna reflected all the sunlight falling on it, calculations show, it would be about 1,120 miles (1,800 kilometers) across. Yet nothing in the outer solar system reflects more than 20 percent of incident light, which brings the size down to 75 percent that of Pluto, or a little over 1,000 miles (1,610 km). Brown and his colleagues are trying to obtain new observations to constrain the size still further.

Brown’s group has initiated new programs fine-tuned to locate objects moving through the sky at a Sedna-like pace. “Almost all of the searches for Kuiper Belt objects that are currently taking place stop at about 100 AU. And the reason is that, as things get farther and farther away, their motions in the sky are slower and slower,” he notes. An object moving slower than a couple of arcseconds an hour will evade detection. So, the team’s new programs look exclusively for such slow-moving objects, like Sedna. “We are quite excited to see if we can find things that are moving much more slowly than that.”

Finding such a population, he argues, is where the biggest scientific payoff is going to be. Objects like Sedna appear to be in a special place in the solar system, held to the Sun more strongly than the distant comets of the Oort Cloud but more loosely than the planets. Simulations show that placing Sedna in its orbit without disrupting the comet cloud requires unusual circumstances, such as the close passage of a Sun-like star early in the solar system’s history.

“When we find, you know, tens or dozens of objects like Sedna on orbits like Sedna’s, we will be able to quite definitively read this fossil record and tell you almost exactly what happened 4.5 billion years ago,” he says. “You just don’t get those opportunities very frequently.”

To hear Brown speak of it, Sedna and its companions are a fossil record of circumstances at the solar system’s birth. But this scenario won’t hold if a distant, unknown planet affects Sedna and its relatives the way Neptune scatters Kuiper Belt objects. Still, Brown thinks such an object is increasingly unlikely.

“If you had asked me 2, 3 years ago, I would have said, ‘Not only is that high on the list, but I’m going to bet that’s the most likely thing.’ But we’ve spent the last 3 years diligently searching the outer solar system, and we’ve covered a lot of territory,” Brown explains. “Nothing like that has shown up yet. There’s always room to hide things, but the hiding places are getting smaller and smaller.”

What’s the most personally satisfying aspect of discovering Sedna? Says Brown: “The high points are the letters from the kids. There’s no comparison. … I have a pile of mail I’m still going through from school kids.”

Learn more about Sedna at Mike Brown’s web site

“What you’re seeing in press releases is just a skimming of some of the cream. We’re so busy operating the spacecraft, and getting the data down, and creating a plan of what to do next, the teams don’t have enough time or work force to get all the cream out of the data, let alone all the discoveries,” he says.

Matson believes one reason for Cassini’s success was the mission’s rigorous testing program. “We had the philosophy that, whenever you’re doing anything this complicated, you’ve got to … find every single error that was made,” Matson recalls. That philosophy continues today in spacecraft operations. “When we went into orbit around Saturn, there was nothing new that we were doing — every command, every piece of software, everything that was done had been exercised in some way prior to orbit insertion,” he says.

Conservative design is one reason Cassini has fared so well during its 7-year voyage to Saturn. All key spacecraft systems have backups — even the engine. “It took weight and mass, but it was felt that, cruising for so long, all the key systems on the spacecraft had better be redundant. So we have a completely redundant control system for the spacecraft, all the way from the central computer … through to the main engine,” Matson says. So far, Cassini has used only one of its 2 engines, and it has “performed spectacularly.”

Yet, due to budget and scheduling constraints, there was no pre-launch end-to-end test of communications between the Cassini spacecraft and Huygens, the European Space Agency’s probe that will drop into the atmosphere of Saturn’s haze-shrouded moon Titan. In February 2000, controllers finally ran the test, using NASA’s Deep Space Network tracking antenna in Goldstone, California, to simulate a data stream from Huygens. But what Cassini sent back wasn’t what they expected. Matson calls that discovery “a real white-knuckle thing.” Had the problem gone unnoticed, he says, “We would have received only about 20 percent or less of the data from Huygens.”

Fixing the problem required changing Cassini’s approach to minimize the line-of-sight speed — and thus the Doppler shift — between the two spacecraft. So, instead of arriving on Titan November 2004 as originally planned, Huygens will be deployed December 24 and encounter Titan January 14, 2005.

This has all been tested on the ground with additional simulations from Goldstone to the spacecraft. “We get the right answer now,” Matson says.

Software on the web site collected the observations and made a first pass at the calculation, but the error associated with this computation was surprisingly large. On November 2, 2004, ESO released the results of a more extensive data analysis, done in cooperation with the Institut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE) in Paris. The final result:

Here’s another way to think about this calculation. Radar observations tell us that 1 AU equals 149,597,871 km. Using this value as a known quantity, astronomers can compute the radii of Venus and the Sun from the transit timing observations. The results:

Despite a Europe-heavy distribution of observing sites, and relative inexperience among reporting observers, ESO says the results are more accurate than any of the transit-based calculations of past centuries.

According to IMCCE astronomers Patrick Rocher and Jean-Eudes Arlot, who wrote explanatory notes detailing their computations, many factors helped make the project so successful. Telescope optics, transit timings — in many cases assisted by digital image recording and image-processing software — and knowledge of each observer’s location on Earth were much improved over historical transit observations.

In fact, says ESO, if astronomers had not known the distance to the Sun before the 2004 Venus transit, they would have been able to measure it with an accuracy of 0.007 percent through this project alone. For more on the project, including notes on the calculations, see the VT-2004 web site.