Debris comes down to Earth
The 2.4-tonne German ROSAT satellite re-entered Earth’s atmosphere during the night of 22 to 23 October at a speed of 27,000 km/h.
During its descent, the satellite was subjected to the intense heat of re-entry and most of its components burned up at high altitude. However, certain elements like titanium or ceramics can resist the high temperatures there. DLR, the German aerospace agency, estimates that up to 30 pieces of the satellite weighing a total of 1.6 tonnes could have survived re-entry.
Re-entries like this pose very little danger to populations, since 70% of Earth’s surface is covered by oceans. “For ROSAT, the risk of getting hit was estimated at 1 in 2,000 for the entire global population; that’s 1 in 12,000 billion for any one person,” explains Fernand Alby, in charge of space debris activities at CNES in Toulouse. The debris from the satellite that survived re-entry finally came down in the Indian Ocean.
Trajectory tracking and prediction
The world’s main space agencies tracked the satellite’s trajectory and constantly refined predictions of its re-entry point and descent. Working together within the Inter Agency Space Debris Coordination Committee (IADC), they pool their results to track such events more effectively.
In France, the defence procurement agency, DGA, and CNES are responsible for tracking, under the direction of the French air force. The radars operated by DGA and the air force acquire the trajectory measurements that CNES uses to predict where a satellite will fall. But the degree of uncertainty on the exact spot is usually of the order of several thousand kilometres.
“There’s a lot of uncertainty surrounding a re-entering satellite because it is affected by many parameters, particularly the influence of solar activity on the atmosphere, which is very difficult to forecast, and the satellite’s orientation,” adds Fernand Alby.
The ROSAT satellite was launched on 1 June 1990 and deactivated in 1999 after 8 years of observations. Viewing in the X-ray domain of the spectrum, ROSAT completed a full-sky survey of X-ray sources Normal 0 21 false false false MicrosoftInternetExplorer4 /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Tableau Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:"Times New Roman"; mso-ansi-language:#0400; mso-fareast-language:#0400; mso-bidi-language:#0400;} — the first of its kind.
