The Earth has one Moon, but it’s not the only rocky thing orbiting us…..

I spend far too much time at pub quizzes. Perhaps it’s because I’m an irritating know-it-all or I just like a vaguely intellectual pretense for going to the pub. One of the more geeky parts of it is correcting the quiz-master when they are wrong (Reykjavik is north of Helsinki and Blazin Squad did not do the original of Crossroads etc.). One such wrong answer was a week or two back when it was claimed the Earth has four moons. Additional moons of the Earth have long been claimed and were popularised a few years back when QI claimed that a co-orbital body called Cruithne was a second moon. As far as the definition of stable, natural bodies orbiting the Earth goes there is only one, although it would be entertaining if schoolchildren were taught about the wonderfully named Wahrhafter Wetter-und Magnet Mond (or veritable weather and magnetic moon). However there are sometimes other bodies that briefly orbit the Earth.

The Solar System is a crowded place. Besides the eight planets and numerous dwarf planets there are millions of asteroids. Some of these have orbits that bring them close to the Earth. While most of these whizz by us, some are in orbits which mean that they can gravitationally interact with the Earth and the Moon and go in to orbit around it. These orbits are not stable and the objects will eventually be kicked out of the Earth-Moon system.

To date only one known object has been discovered to have undergone such a process. Known as 2006_RH120 it is a small body, only 3-5m across. In 2007-2008 it undertook four orbits of the Earth at a distance more than twice as far away as the Moon. But how often do objects like this perform their temporary dance with the Earth? Well a new paper of has been looking in to the rate of capture and when such events happen.

The authors use a simulation of the how asteroids will pass through the Earth-Moon System. They select a series of objects with orbital elements in the range where they could possibly be captured and then examine how they would be affected by coming close to the Earth and Moon. Previously it was thought that a close encounter with the Moon gave objects a gravitational tug allowing them to be captured by the Earth. However the new model finds that while the Moon does play a role in the capture, none of their simulated near-Earth objects came close enough to the Moon to get a sufficient enough tug for capture.

The model also found that capture most likely at aphelion and perihelion (when the Earth is furthest and closest to the Sun during its orbit). The same capture probability peaks were previously noted for temporary satellites of Jupiter. It’s also possible that the Moon itself could capture asteroids and get its own temporary satellites. However no objects in the simulation managed to complete an orbit of the Moon.

Objects in unstable orbits around the Earth will of course have the possibility entering the atmosphere and becoming meteors. About 1% of objects in the simulation impacted on the Earth, none on the moon. This means that a temporarily captured object is 3.5 times more likely to strike the Earth than an near-Earth object in a similar orbit. In total the authors estimate that a tenth of one percent of objects striking the Earth were in temporary orbit around us.

In all the authors estimate based on their model and the fact there aren’t a large population of observable temporary satellites that at any one time there is one object of approximately one metre in size temporarily orbiting the Earth along with potentially other smaller bodies. So the Earth only has one Moon, but it’s not the only natural object orbiting us.

Granvik, M., Vaubaillon, J., & Jedicke, R. (2011). The population of natural Earth satellites Icarus DOI: 10.1016/j.icarus.2011.12.003

Lovejoy’s fiery farewell

Comet Lovejoy, seen here by the SOHO satellite, is on a final plunge towards the Sun. Later today it will get too close (perhaps as little as 140,000 km from the solar surface) and be destroyed by the extreme temperatures. The poor thing never had a chance of reaching its destination before it disintegrated; a ball of dirty ice couldn’t even cope with the heat of my kitchen! Oh, and before you complain that there’s no way I’d find a comet in my house, all I’d have to do is invite sometime-poster-on-this-blog Stuart round and provide him with some liquid nitrogen.

UPDATE: IT SURVIVED!! The Solar Dynamics Observatory saw it fly away:

A Christmas burst

Last Christmas something exploded in the constellation of Andromeda. Well, that’s not quite true. This gamma-ray burst (named GRB 101225A) went off a long, long time ago, but the resulting flash finally arrived last year and were picked up by the SWIFT satellite (which then probably interrupted several festive lunches with its Burst Alert alarm).

This is a cross posting with the Astronomy Twitter Journal Club who are going to be discussing this topic on twitter (search for the #astrojc hashtag) this Thursday at 20:10 GMT. If you’re interested please come and join in.

Artist's impression (with festive enhancement) of a supernova explosion and resulting gamma-ray burst. Photo credit: NASA with apologies

The majority of gamma ray bursts are thought to be massive stellar explosions in distant galaxies, but the Christmas event didn’t fit this picture; the initial burst of gamma rays lasted for an unusually long time and, despite the efforts of powerful optical telescopes, no convincing candidate for the host galaxy could be found.

Nearly a year on from the initial observation two groups of astronomers have come up with two different, but equally plausible, explanations for the odd GRB 101225A. The first team suggest that it was caused by something small, like a comet, breaking apart and then falling into a neutron star within our own Milky Way. The alternative theory, put forward by the second team, also involves a neutron star, but in this case it’s merging with a young red giant star in another galaxy.

Unless the host of this gamma ray burst is found, and its distance measured, there’s no easy way to choose between these two options. Hopefully deeper optical data with, for example, a telescope like Hubble will provide the answers and settle this debate.

ResearchBlogging.orgThöne CC, de Ugarte Postigo A, Fryer CL, Page KL, Gorosabel J, Aloy MA, Perley DA, Kouveliotou C, Janka HT, Mimica P, Racusin JL, Krimm H, Cummings J, Oates SR, Holland ST, Siegel MH, De Pasquale M, Sonbas E, Im M, Park WK, Kann DA, Guziy S, García LH, Llorente A, Bundy K, Choi C, Jeong H, Korhonen H, Kubànek P, Lim J, Moskvitin A, Muñoz-Darias T, Pak S, & Parrish I (2011). The unusual γ-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33. Nature, 480 (7375), 72-4 PMID: 22129726 (alternative link for the paper here)

ResearchBlogging.orgCampana S, Lodato G, D’Avanzo P, Panagia N, Rossi EM, Della Valle M, Tagliaferri G, Antonelli LA, Covino S, Ghirlanda G, Ghisellini G, Melandri A, Pian E, Salvaterra R, Cusumano G, D’Elia V, Fugazza D, Palazzi E, Sbarufatti B, & Vergani SD (2011). The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star. Nature, 480 (7375), 69-71 PMID: 22129725 (alternative link for the paper here)

In other astro-news: hello SCUBA-2, Hubble’s milestone, an anniversary & astroweather on the news?

One story is dominating the astronomy news this week – the announcement of exoplanet and, in the words of some sections of the media, potential ‘Earth twin’ Kepler-22b. I don’t want to talk about that though; I want to give some attention to a few other interesting bits of astro-news that are in danger of being eclipsed. If you do want to discuss the much hyped new planet however, it’s the topic for this week’s astronomy twitter journal club.

Firstly, a new era of submillimetre astronomy began at the James Clerk Maxwell Telescope in Hawaii with the unveiling of the SCUBA-2 camera. This large instrument is the successor to SCUBA, which I’ve written about here before. Indeed it’s its faster, more sensitivity son, and should hopefully prove an invaluable tool in understanding the dusty Universe. To do this it has to be cooled to within 0.1 degree of absolute zero which the press release confidently states makes it “…colder than anything in the Universe that we know of”. Except, as was quickly pointed out on twitter, the Planck satellite which also has to operate at these chilly temperature.

The 4.5 tonne SCUBA-2 instrument mounted on the James Clerk Maxwell Telescope. Photo credit: Joint Astronomy Centre

The next story on my list is an intriguing e-petition calling for astronomy weather reports to be included in the normal weather forecast:

We the undersigned request that The Met Office produces regular Stargazing / Astronomy focused weather information, to be shown as part of the BBC Weather reports. Not only would this be a boon to amateur astronomers, it will also help keep the study of Astronomy prevalent in the public consciousness, which in turn helps encourage the study of science, which should be a boon to the economy.

Onto the Hubble Space Telescope and the publication of the 10,000th refereed scientific paper using its observations. Reaching this milestone really reflects how Hubble has managed to remain a cutting-edge instrument throughout its long life. The paper in question reports the finding of the faintest supernova ever associated with a long duration gamma ray burst – you can read it here. I wonder if the Space Telescope Science Institute will send the authors a prize for their achievement?

I was going to end with this story reporting the discovery of the most massive black holes ever seen. However, I really want to devote a separate post to this (coming soon, hopefully). Instead, here’s one of the most iconic images of our own planet – the blue marble hanging in space – which was taken nearly 40 years ago today. Thanks to Galileo’s Pendulum for the tip.

Our Earth as seen by Apollo 17. Photo credit: NASA