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Planetary News: Vulcanoids (2004)

Small, Faint, and Elusive: The Search for Vulcanoids

The fog was unusually thick in the predawn hours in White Sands, New Mexico, on the morning of January 16, 2004. So thick that engineers who have been working there for decades said they hadn’t seen the likes of it in 20 years. As a result, observers on the ground could hardly distinguish the outlines of the flame shooting out from behind the long, thin Black Brant rocket. The fog itself appeared to glow and the engine’s roar filled the air as the rocket lifted into the sky at 5:29am.

Black Brant is not one of NASA’s heavy lifters, like the Delta and Atlas launchers. It is a relatively small, low-cost rocket, used to send scientific payloads on short suborbital flights. On this morning the Black Brant was carrying a combination of payloads knick-named "Big Dog." It included a spectrometer, designed to study the planet Mercury, and a highly sensitive video camera designed to search for “Vulcanoids” – space rocks thought to inhabit the region between the Sun and Mercury. Although scientists have long suspected that Vulcanoids exist, they have proven to be some of the most elusive objects in the solar system.

Why Vulcanoids?

No one has ever observed a Vulcanoid. Being small faint objects, orbiting in the immediate vicinity of the Sun, they are easily lost in the blinding Solar glare and cannot be viewed with ordinary Earth-based telescopes. But, explained Dr. Alan Stern, Director of the Space Science department at the Southwest Research Institute (SwRI) in Boulder, Colorado, and the Principal Investigator of the mission, there is good circumstantial evidence that they exist. Between the orbit of Mercury and the Sun there is a gravitationally stable region,” he pointed out. This means that objects circling the Sun in that region would continue to do so indefinitely, in stable orbits.

As it happens, gravitationally stable regions are few and far between in the solar system. In most regions, the gravitational pull of neighboring planets would ultimately destabilize the orbits of smaller objects. A space rock in such an orbit might end up being hurled into the Sun, heading to interstellar space, colliding with a planet, or simply set in a different orbit. The end result is that over time a gravitationally unstable region would be cleared of rocky debris, whereas a stable region would likely retain its occupants. Significantly, explained Stern, “all the gravitationally stable regions in the solar system, are occupied by some type of rocky debris – asteroids in the asteroid belt, Trojans near the orbit of Jupiter and Neptune, Kuiper Belt Objects (KBO’s) and comets near the orbit of Pluto, and so on.” The only gravitationally stable region that is not known to be occupied is also the one most difficult to observe – the Vulcanoid region inside the orbit of Mercury. It stands to reason, explained Stern, that this region too could have its own population of space rocks, hard though they are to detect.

The other strand of evidence for the existence of Vulcanoids comes from their closest planetary companion – Mercury. Anyone who has seen an image of Mercury will recall the innumerable impact craters which pockmark the planet’s surface. Where there are impact craters, the reasoning goes, there must also be space rocks that caused them – in other words, Vulcanoids. Furthermore, Mercury has a giant iron core and a relatively thin mantle– much thinner than the mantle of the other three rocky planets of the inner solar system. According to Stern, Planetary Scientists suspect that Mercury has been the victim of a massive impact, which shattered its crust and sent its rocky fragments into space. Some of this debris was eventually recaptured by Mercury, but it is very likely that some rocky relics of this ancient catastrophe still orbit the Sun inside Mercury’s orbit.

Oddly enough, even though scientists are not sure whether Vulcanoids exist, they nevertheless think they know quite a bit about them. The fact that all efforts to detect Vulcanoids with Earth-based telescopes have failed, places serious constraints on what kind of bodies these objects might be. Vulcanoids, according to Stern, are almost certainly small, and even the largest of them cannot be over a few tens of kilometers in length. There cannot be many of them – perhaps several dozens or hundreds at the most larger than a few kilometers across– a fraction of the number of space rocks found in the asteroid and Kuiper belts. If they were larger, they would certainly have been discovered already; if there were more of them, computer modeling demonstrates that they would certainly collide with each other repeatedly until their population was reduced to a sustainable number.

Small, Faint, and Elusive

Detecting small dark objects near the Sun, which are also few and far between, is certainly a tall order for planetary scientists. Being so close to the Sun, Vulcanoids should theoretically be visible from Earth just before Sunrise or just after Sunset. With the Sun just below the horizon, it should in principle be possible to detect some Vulcanoids just above the horizon. But looking at the skies at a very shallow angle at twilight or dawn is just about the worst possible way to observe celestial objects. The red and pink shades which color the horizon at these hours may be admired by lovers and vacationers, but to astronomers they are unwanted atmospheric phenomena that get in the way of telescopic observations. The only way to actually observe Vulcanoids, they concluded, was to get away from beautiful sunsets and into dark skies. And that means, going up.

If one could situate an observation platform a few hundred miles above the Earth, explained Durda, one would no longer need to worry about atmospheric interference. Using the Earth as an occulting sphere to block out the Sun, it should then be possible to detect even those small and dark Vulcanoids. Stern and Durda had already tried this approach in the past, flying a dozen times on an F-18 fighter jets with a highly sensitive camera aboard designed to detect Vulcanoids. The technique worked well in principle, as the twilight sky was indeed much darker at high altitudes than it is in on the ground. But even an F-18, which flies faster and higher than almost any aircraft in existence, can only reach as high as 49,000 feet, where the atmosphere is thin but still very much present. If the camera could be placed in actual space, above the atmosphere altogether, then it would stand a much better chance of capturing the elusive Vulcanoids.

Flight of the Black Brant

And so on the morning of January 23, 2004, the Black Brant rocket lifted off from White Sands, New Mexico, carrying with it a Vulcanoid-detecting camera. This instrument, known, quite naturally, as VulCam, is essentially a very high intensity video camera with an 85 millimeter Nikon lens, which should excel at detecting dark objects in the blackness of space. Within 80 seconds of the launch, at a height of 370,000 feet, the payload’s shutter door opened, enabling the VulCam to look outside. Within 4 and a half minutes, the VulCam had reached its maximum altitude of 900,000 feet, began its controlled descent back to Earth. 7 minutes and forty seconds into the flight, at a height of 360,000 feet, the shutter door closed, ending the data-gathering segment of the flight. Less than 10 minutes after lift-off the payload parachuted safely back to Earth, and the suborbital flight was over.

As the flight was going on, Stern used a joystick to point the payload in the proper direction, David Slater, also of SwRI, kept track of the engineering data, and Durda monitored the mission’s progress on a video screen. “When the signal first came in” said Durda, “the screen was completely dark.” That was because although the camera was now operating, the shutter door in the rocket’s fuselage was still closed. “When it opened seconds later, the first things on the screen were bright flashes of light, like fireworks.” These, explained Durda, were the last remnants of the flame emanating from the lower stages of the Black Brant rocket, now tumbling away from the payload. At the controls Stern now pointed the VulCam away from the Earth, where city lights were visible, and towards the night sky and Mercury, with the curving edge of the Earth visible along the way. If there are Vulcanoids out there, that is where they would be seen and detected. Finally, Stern pointed the camera for a close look at the Moon, and a few seconds later the VulCam’s shutter door closed.

Stern and Durda will be pouring over the 50,000 plus video images from the flight of the Black Brant in the weeks to come, looking for objects that might be Vulcanoids. They do not, however, expect to definitely confirm or refute their presence based on a single short flight. To determine that an object is indeed a Vulcanoid it would be necessary to calculate its exact orbit on the basis of several observations within the space of a few weeks, explained Durda. The purpose of the flight was to confirm that this method of searching for Vulcanoids is indeed feasible, and hopefully to detect some objects in the sky that could be “Vulcanoid candidates.” Inevitably, further observations would be needed to confirm or refute their status.

Where to Go from Here

Stern, Durda, and their colleagues, are now planning the next stage in their search for Vulcanoids. It may be an additional NASA suborbital flight, or it may take the form of highly sensitive and targeted telescopic observations, based on the results of the Black Brant flight. The SwRI group is also looking closely at the X-Prize competition to design and build a reusable vehicle for suborbital flights. Such a vehicle may yet prove the ideal platform for the Vulcanoid search.